HYDROGEOLOGICAL BULLETIN

FOR THE BUA CATCHMENT ,

WATER RESOURCE UNIT NUMBER 5

by

A.K. SMITH-CARINGTON

GROUNDWATER SECTION

DEPARTMENT OF LANDS .~LUATION AND WATER

PRIVATE BAG 311,

LILONGWE 3,

MALAWI.

MAY, 1983

HYDR~EOLOG!CAL BULLETIN FOR THE BUA CATCHMENT

WATER RESOURCE UNIT NUMBER 5

CONTENTS

List of Contents

List of Figures ,

List of Tables

Background

Summary

1 • I~RODUCTION

1.1 Location

1.2. Topography and Drainage

1.311 Geology

114, Climate

1.5 Soil

1.6 Land use

1.7 population

2. HYDROGEOLOGY

2.1 Occurrence of Groundwater

2.2 Aquifer Properties

2~3 -,

Groundwa ter Level Fluctuations

2.4 Groundwater Movement

2.5 Groundwater ChemistrY

3. C~TCEMENT WATER BALANCE AND GIOUNDWATER RESOURCE EVALUATION

3,.2

3t3 3 4

3.5

lbin[a:l1

Evaporation and Transpiration

Surface hydrology

Surface waler hydrograph analysis

water Balance and Groundwater Resource Evaluation

PAGE

1

12

28

4. G~OUNDWATER DEVELOPMENT 48 . I

4,1 Existi'rtg, Hater demands and supplies

4.2·; Groundwater abstraction methods

4.3 Scope for further groundwater development

~T OF FIGURES

1. Blla Catchment, drainage and topography

2. Geology

3. Idealised section of typical. catena sequence of plateau soils

4. Typical profile of \veathered basement aquifer

5. Groundwater level fluctuations' (seasonal)

6. Grounch.ater levels from borehole maintenance records

7. Map to show piezometric form

8. Electrical oonductivity survey in Hadisi area

9. 'l'rilinear plot of bydrochemistry

- A) Unit 5D

- B) Unit SE

10. Thiesocn polygons for estimating total catchment rainfall

11. River hydrograph, station 50 1, 1964/65

12. Dambo model

1. Soils of the plateaq area

2. Estimated population of the Bua catchment (1977)

4. Typical water quality of groundwater in plateau area

5. Estimates of evaporation and transpiration

6A. Estimates of actual evarotransp±ration dambo arelfs

6B. Estimates of actual evapotranspitation t interfluve area, cropped and fallo\!

6C. Estimates of actual eVapotranspiration area, trees

7A. Hydrograph analysis SC 'I

7C. Hydrograph ,analysis 5D 2

interfluve

8. Summary of averag'e hydro:logical components ,a,nd water balance for catchm·s-rif: t".", Si) i.

9. Urban water supplies

PAGE

6

11

19

24

29

32

33.,

34

38

39

40

45

50

BACKGROUND

This hydrogeological bulletin was completed as part of the evaluation

of the water resources of Malawi for the National Water,Resources

Master Plan and was prepared under the auspices of the Groundwater

Project.

The report represents a three month desk study and is largely

devoted to a presentation of hydrogeological conditions in the

weathered basement aquifer of a plateau area. As such it is a

pioneering work but it is clear from the available data that

little is known and more detailed research work is required to

further understanding of the complexities of the hydrogeoiogy.

There has been little published work which is of direct relevance

to the groundwater hydrology of the Bua catchment. The most

important texts giving the background hydrogeology are by Chilton

(1979) and wilderspin (1974); a summary of surface water hydrology

is given by Dray ton et al (1980) and a large volume of meteoroio·

gical data is presented by van der Velden (1979, 1980). ~ ,

,

. ,

SUMMARY

The Bua Catchment can be divided into three major hydrological zones:­

i) the flat plateau drained by dambos,

ii) the steep slopes of the uplands and rift valley escarpment and

~ii) the lakeshore plain.

The weathered basement rocks form the principal aquifer which Is

present over extensive areas of the plateau. Tbis bulletin is largely

devoted to a consideration of hydrogeological conditions in this area.

m the dissected escarpment and upland~ the ,weathered ~one is thin

and the bedrock rarely gives rise to Significant aquifers even where

it is fractured, as the availab.7e, storage is negligible. Tbere is

little scope for groundwater development in these areas. Tbe

~lluvial lakeshore deposits :cover a relatively small area, but form

potentially good aquifers where there are significant thicknesses

of sands and gravels,

fhe weathered basement aquifer of the plateau area :is relatively

thin (10 - 25 m) wlthgenerally low permeabilities and potential

yields of 0.5 - 3 l/sec. The aquifer material is variable, with

the most permeable material usually found towards the base of the

weathered profile. Tbese zones tend to be semi-confined by compacted

$urface clays. The aquifer yields are quite adequate for rural

domestiC'supplies and it forms an"important, extensive sourCe of

protected water supplies. Seasonal water level fluctuations

appear to be in the range of 2 - 4 m, and long term monitoring

does not show any evidence of declining groundwater levels. Tbe

groundwater quality is generally good with low mineralisation'; it

is usually quite potable though there are frequently high iron

concentrations' whicb m,: ... ke it unpalatable-. Hm\~eV'er, there are some

qery localised areas of saline groundwater, which are not fit for' , human consumption.

Surface water hydrographs have been'analysed and a catchment water

balance has been attempted for the pleateau area. Replenish' able

groundwater resources have been evaluated using a ~ambo model

which is considered to be more realistic than the traditiona1 method

of baseflowseparation.

Average annual recharge is likely to be 18 mm at minimum, and could

be significantly greater if the interflow contribution to baseflow

is negligible and the evapotranspiration from the dambo areas

throughout the dry season is maintained by grounawater rather than

storage in the dambo clays. However, the aquifer properties (e.g.

transmissivity, hydraulic gradient) and surface water drainage

pattern are such that the average annual groundwater discharge (and

by implication, recharge) is unlikely to exceed 40 mm.

There is considerable scope for further widespread development of

groundwater for rural domestic supplies without depleting

replenishable resources, as the yields required are relatively

small (0.25 - 0.5 llsec). Conditions are suitable virtually all

over the plateau and lakeshore plain. Groundwater abstraction

methods are discussed. There is also some scope for small town

supplies and small irrigated plots where aquifer properties and

recharge conditions are favourable in the weathered bedrock aquifer .•

Some potential for larger irrigation schemes may exist in the lake­

shore areas where the alluvium is relatively thick and sandy.

-,

,

1. INTRODOCTION

1 .1. LOCATION

The Bua River drains an area of some 10,000 km' and discharges into

Lake Malawi at a point 20 km north of Nkhotakota. The catchment lies

approximately between lonqitudes 32°35' and 34°15'E and latitudes

12°40'and 12°SS'S. It lies within the administrative districts of

Mchinji, Kasungu, Dewa, Ntchisi, Lilongwe and Nkhotakota. Under the

Water Resources Branch classification system (1979) the Bua River

catchment is numbered as resource unit 5 and is divided into four

sUb-catchments SC, 50, SE and SF (figure 1).

1 • 2. TOPOGRAPHY AND DRAINAGE

The Bua Catchment can be divided into three distinct hydrological

zones, based on topoqraph¥ (figure 1):-

(a) plateau

(b) steep slopes on the highland rising from the plateau

and the rift valley escarpment

(c) lakeshore plain.

The most extensive part of the upper catchment comprises a gently.un­

dulating plateau at an altitude of between 1000 and 1200 m. The val"

leys are broad, the slopes are mainlY less than 2° and there are large

level areas on the interfluves.~ This "African·Surface" is an ancient

late Cretaceous - early Miocene

peneplained (Dixey 1.937, Lister

sur.face which has been. extensively

1967) • The

the SW as a result of uplift along the rift

erosion surface slopes to

valley but the drainage

system has kept pace with these earth movements and drains ~I as a

result the valleys become more incised towards. the ~scarpment. The plateau

is largely drained by 'dambo. • which are broad, per iodically inundated,

grass-covered swampy depressions with poorly defined channels. The areas

which are liab~e to, flood cover 20 - 30% oftM plateau1 those in the flat­

test area towards Kasungu are most susceptible to inundation where there

are few major tributaries to the north bank of the RUlla River. The dambo

headWaters from different tributaries may even coalesce where they are

well-developed. There is a dendritic drainage pattern which may be con­

trolled by structural weaknesses along fracture traces. Aerial photographs

show major lineations trending SW - NE and minor ones NNW - SSE which . . '

are ~ommon~y preferentially followed by dambo· • The main rivers have

Figure 1: BUA CATCHMENT • DRAINAGE AND TOPOGRAPHY

Kasu~ ...

\ \

I I

I

I I

I

I I

I , ,."'-- .... \

\ \ \ \ \ \ \ \ I \ I \ +Dow~

N

t I~ 2.0\<m t

Malawi

Rift eS«W/'Ihent. 4< steel' sIoyoes

~/.~.\ lo~ pbi" .......

50 Water Resource Unit

2.

well-graded profiles and numerous meanders typical of a very old erosion

surface.

There are some areas of highland rising abruptly from the plateau where

the underlying strata are more resistant to erosion. In the SW of the

catchment the Mchinji Hills form a ridge with steep slopes grading into

pediment. This forms the headwaters of the Sua and its main tributary

the Rusa. The ridge rises to over 1700 m and represents a remnant of

the Post Gondwana Jurassic to mid-Cretaceous erosion surface. The

Namitete River rises in the northern end of the Dzalanyama Hills in the

South West of the catchment. The Dowa Hills on the crest of the rift

valley escarpment also have steep slopes and dissected valleys. Other

tiplandareas' are small inselbergs rising abruptly from the plateau

surface.

The escarpment falls steeply towards Lake Malawi in a series of fault­

.controlled steps down to 500 m a.s.l. The slopes are very dissected

with deep valleys and tributary gUllies and there are gorges with

rapids over the faul t sections.

On entering the lakeshore plain the gradient becomes very gentle and as

a result much of the river load is deposited. There is annual flooding in

the rainy season and the alluvium is spread over the plain. The Dzadeda

Swamps behind Bua Point are partly caused by floods and partly by sand

spits o~ lacustrine sands which I1ave become infUled formirig marshes.

1 • 3 • 1 • GEOLOGY'

Almost all of the Bua Catchment is underlain by Pre Cambrian.'- Lower

Palaeozoic gneisses, granulites and schists ass.igned.to the Malawi

Basement Complex (Carter and Bennet, 1973). In the 'east on the lakeshore

Plain these are over lain by Quaternary alluvial sediments (figure 2). , Bulletins 24,25,26, 27, 30, 31 and 32 of the Geol~ical Sur¥eyof

Malawi describe the geology in detail.

1.3.2. Structure

The Basement Complex forms part of the Malawi Province of the Mozambique

Orogenic Belt (Cannon et a1., 1969). There is evidence of polyphase

deformation 1 the major structural axes of folding trend NNW in the

Mchinji Ridge and over much of the plateau and NNE in. the escarpment zone.

\ :J:. 2. <> ." -p ,. ~

'"

rz-Jl8lJ L' ''0,.::<, ---:- - : ..... :!:;

'" .,- ", p p .';

lA m " ., m ~

<> n m

'" c p " '" '" "' '" '" '" ~ P t::l"

" .... :cl r.; .... "-n ~:'.\

" "' 3 El I ." '" r- e to ~

.., ~, x p

" c:: 3 El_ 3 <> .... P '0 '" :;.,- '" N n !2,

-------_ ...

"1'1

to c: .., I\)

N

Cl IT! e r­e Cl -<

3.

Fracture traces can be picked out on aerial photographs and occur on a

variety of trends; those aligning NW or NNH are particularly prominent

and exercise some control over orai.naqe patternso There is considerable

faul t ing j n th e (~~~c~u: };,m.c.;nt aJ~t:.',:;. :·· .. ·.;~·":)c ~_.;..< ~.Jci ,.,1. t;l "che DE:'v"e lopment of the

Malmd Rift Valley ",nd th" uplift of the eastern edge of the plateau

area. Most faults trend NI\J1v and do"\'!nthrow to the east often with promi­

nent scarps (Harr i.son and Chapusa, 1975).

1.3.3. .0!:b.?29SilX._~tr_aLigra!2l:y

'rh" crystalline metamorphic rocks of the Basement Complex are believed to

be of both sedimentary and igneous origin •. They comprise predominantly

fine to medium grained bioti te and hornblende gneisses over the plateau

area, with the occurrence locally of varying types of granulites and

schists and intrusion" of syenite or basic metagabbros. The Mchinji

Hills comprise mainly coarse grained quartzites and schists. The coarser

grained parent materials give rise to the best aquifers as they decompose

to a more sandy texture. Locally marbles occur, which can form important

aquifers when they an" \VeIl fractured. Where the rocks are more resistant

to erosion they remain a" -1.""01 h0ras or as uplands as in the Mchinji Ridge.

Over most of the plateau, oxcept in the east tOl.ards the escarpment, the

bedrock is deeply ;'eathered and largely covered by residual soils and

colluvium. It is th"se deposits '<lhich form the prinicpal aquifer. Towards

rnent of the Rift Valley ha" resulted in rejuvenation of the rivers ·and

increased erosion of the I{eathered material. On the escarpment area itself,

the l'I<'>athering products L \\7(' been la.rg-ely stripped away_ by erosion. The

lakeshore plain is underl}i.n by a variable sequence of Quaternary alluvium,

~.Thich comprises zdternati/lg layers of claysr silts and sands. The total

thickness of the alluvia.> sequEmce varies,but in general the 'depth to

bedrock increases towards Lake MalaNi with a maximum of over 30 m recorded.

'"xposed to the nor th of i :ua Poi nt:.

1. 4 • 9d.:,!!~

The climate is markedly seasonal and rainfall is largely associated with

the migration of the Inter-1'ropical Convergence zone. However climatic

ccnditions are complex due to the range in altitude of the catchment and

the influence of Lake Mal.dVlL

The rainy seaSOn usually extends from November to March, initially

intermittent and becoming more continuous in January. Rainfall over

the plateau occurs largely as hea\ry convectional tropical storms

'tlhich C.:ln b0 V(;f y lcc,:;)lised, how'over the total annua.l rainfall

(mean 800 - 1000 rm~) is suspected to be less spatially variable.

Over the esoarpment and uplands e~posed to the prevailing south

easterly Idnds, rainfall is also orographic and the annual total can

be three times that of adjacent areas (Dray ton et a1, 1980). These

areas may receive orograpilic rainfall and mists, ImOl1l1 as Chiperoni,

during the 'cool dry' season which extends from May to August.

P~infall along the escarpment is relatively high (1200 - 1500 mm)

dry with progressively increasing temperatures,and occurs from

September to Novemb"r, or. early December.

Temperatures are closely related to altitude (van der Velden,

1979, 1980). The mean monthly temperature ranges from 16° - 26° C

on the plateau with actual maximum temperatures over 3D· C in

No·,ember ana December. On the higher ground temperature falls with

monthly averages ranging from HO - n" C and on the lakeshore plain

it is h~gher, in the range of 20° 27° C.

The average annua.l pan evaporation ranges from 1600 - 1950 mm on

the plateau to arOl...m;'l ::::;.on 1[,(: Oil t.:~;(: 1.i.:kC.:::r..J':::0 (V':""ii (.;2r Velden,

1979). This is likely to be IllIlch greater than average annual

actual evapbtranspir'ation.

1.501. SOILS

Soils in tb" BCla catchment [all i.nto fol.l!:" main groups based on the

classification by Brmm an':i Young \ 1965) ,-

(a) Latosols are found on the gentle slopes of the plateau. , These are 'formed by prolonged weatherin,! .clays and .down-

t,<1ard leachirq of e7~cha.rigeable bases ann stlica.tes.

Strongly leached ferrallitic soils with an advanced

state of clay mineral weathering (mainly kaolinite)

are most common and generally have poor .nutrient status.

They often have associated laterite layers which

may restrict draina90" ?e .... ~H,~1.nlJus soils are

found where the ,,'ea thed ng of parent ma tor ials is less

advanced and the soils are relatively weakly leached.

(b) Lithosols are shallow, immature and stony soils found on the

steep slopes of the Rift Valley escarpment and highlands

rising from the plateau. There is little horizon differen­

tiation and any weathering of parent material is balanced

by losses through slope wash or soil creep. Drainage may',

be rapid but the shallow profile resUlts in low moisture

retention.

(c) Hydromorphic soils are waterlogged for all or most of the

year and are found in valley floor sites on both the plateau

and the lakeshore plain. These are swelling clays with

a very heavy texture and thus low permeability~ this

coupled with poor site drainage results in annual flooding.

(d) Calcimorphic soils are derived mainly from alluvial parent

material and found on the lakeshore plain. The texture

varies widely with alternating layers of clays, silts and

sands commonly occurring. The clays are stronglY swelling

but do not usually crack on drying out due to the high

proportion of silt. Drainage is impeded where the water

t.a.ble is high. -,

Physical characteristics" of soils in relation to recharge

On the plateau area the soils are predominantly fine textured and

the clay content usually increases with depth (Soil Survey Re~rts,

,'5.

1969 - 1973). The soils appear to have relatively low infiltration

capacities (Table 1.) although infiltometer measurements cannot be taken

as totally reliable. Surface runoff is high and often the lower soil

layers are dry llven following prolonged rainfall. Soakaway pits from

boreholes are commonly very poorly drained, confi.rming the low'perl\\ea­

bility. Most variations in texture' can be related broadly to ,topography

and are determined by the extent of erosion, leaching and relative

position of the water table. A distinct catena sequence occurs from 'the

interfluves to the drainage lines (figure 3). This sequence is modified

by geology and in detail tpe pattern of soil types is~()(j)plelC. In,

general more basic parent materials resul,t in more heavily textured,

darker red soils.

Figure: 3 IDEALISED SECTION OF TYPICAL (ATENA SEQUENCE OF PLATEAU SOILS

FerraUitic soils

large S110 after

Ferrallitic soils with taterite

dry season th~refore I-

little recharge i ~'f"",-41'()(I 10-1$' ........... "'>lo. '!If) F. " IQ; I{ I/, ~ --fa", ~

_RWL (max)I, "fil'f~/'i'(! Q(;fill'~" . - - - - - - _ _ 'if <Cl! ("et. "(j r("..... ........

------ ---__ -- ---- -!.o~0;g..!'-f,,--""

i

(olluvial soil

Hydro ril°rPhic soils

Vertisols

----- -~,~- ----. -- - . ----IQX::/~Q Jl~=---------- -~;,;:- ___ . _______ . RWl(max) -<T' :!{_~~ __ ,.~ -- - "e'''' ~ .. ;--____ _

- - - - - ...'..r,"@.']B .2':ertiC91 {~ 3 . -- _J _____ __ RWL!mm)

Soil

weathered bedrock

Table 1,

Soils of the Plateau Area*

Soil type

Latosols

a) Ferrallitic soils

b) Ferrallitic soils ,dth laterite

c) Ferruginous soils

<1) Colluvial soils

Hydromorphic soils

a) Dambo soils

b) Ver tisols

Lithosols

Estimated % Area

45

35

12

2

3.5

1.5

Bstimated Infiltration Capacity (cm/hour)

Based on infiltrometer measurements

7 - 23

o where massive laterite

3 - 12

2· - 30

2 - 10

0.2 - 9

2 - 25

* After Lowole (Soil Survey Branoh, 1982, personal oommunioation)

·6 .•

Latosols are found on the· ·crests, upper and middle slope positions and

range from loamy sands to sandy olays. These soils are relatively deep

(1 - 3 m) and are generally more permeable than the soils of the valley

bottoms (Table 1), though dra.inage may be impeded where the interfluves are . .

flat· andl or where massive laterite is present, associated with ferral-

li tic soils. ,.

On the lower slopes where the water. table is high, fluotua~

tions in water level may give rise to soft incipient groundwater laterite,

but this tends to be indurated in better drained sites. Fossil laterites

may be found anywhere, even on crests and are generally lower in the

profile on upper slopes. Where the laterite is massive and at shallow

depth (a feature which is quite common around Mchinji·and Kasungu) exoess

rainfall enters the soil :OUl'f2:::e but vertical flow is impeded and infil-.

tration to the ~ter table is likely to be neglible. Lateral throu9h­

flow along the top of the laterite directs the water,downslope where water­

logging may occur if satUl:ilti.(Y'. builds up to the surface. Nearer the

valley bottom, if the laterite is less developed, subsequent infiltration

to groundwater may occur. It the.laterite is nodular or fraotured vertical

drainage is possible but is significantly reduced. If there is no laterite

and the texture is sandy', ferrallitic soils have very high permeabilities.

Ferruginous soils, despite 'their heaviel:· texture, tend to have relatively

high infiltration rates as there is usually no impeded internal drainage

7.

anq the soils are moderat.ely "7ell structured. Ferruginous soils are

found mainly in the soo.th·ea.st of the plateau area towards the Dowa Hills

where the slopes ·~re gently rolling and the stage of weathering is less

aclvancedo Over lllU0/1 '~'Ji: th;"·' :(0St nf f:h~ plateal.1 Cir.ea ferrallitic soils

are dominant: t.hey are sandier 2r.'d most strongly leached and weathered

in the Rusa cat""ment and towards Kosungu !ihere the topography is extremely

flat, but the development of laterHe is widespread in these areas.

Very large soil. moisture Clerici ts can build up in the upland soils during

the dry season; li ttle penetration of recharge to the water table can occur

until the soil has been restored to field qapacity. It may take a major

pal:t of the rainy S0'C:",n 1:.0 Jati3fy this condition and some years it is

possible that no recharge to groundwater occurs at all.

Termites have a·considen,b;.e ,-nfluence on soils and have had for a very

long time, as . they continually worl, the soil over and break down orqanic

matter. Termite mounds are found all over the plateau except in the

centre of the dambos where water logging is permanent. They tend to be

large domes on the interfluves and low mounds at the dambo margins.

l;cti ve termi taria are generally found where the water table is shallow.

The surface of the mound tends to be a structureless clay with low

intrinsic permeability but the soils beneath have a network of burrows

which significantly increases permeability. Any infiltrating water

reaching these channels il!o8si;~ly vi" ~hrouglldow from upsill9pt!) will

be able. to take these flow routes.. Mixing and reworking of soil by

termi tee on the old erosion sur face .over a long ·period.is thought to

lead to the creation of 'stone lines' since large particles cannot be

moved and tend to accumulate at the base of the zone of termite activity.

These stone lines could provid, preferential flow routes divtlrting

infiltration water laterally. It is clear that termite activity is a

very important mechanism in the moulding of the Plateau landscape.

Colluvial soils' whicfJ have been transported short distances downslope

by gravi ty generally o"cupY a 1a.rrm7 transi Hon between upland and

hydromorphic soils. They may extend over the whole of the valley

floor where the dambo is immature, slopes are steep or waterlogging is

not perennial. These soils are deposited from upslope wash of material

into the valleys and are generally coarse loamy sands or sandy clay

loams with heavier texturep in the st!bsoil. In the absence of laterite

8.

colluvial soils are very permeable and drain rap~dly although they may be

waterlogged for short periods during the rainy season when the water table

is at ground level. 1'here may be localised development of laterite within

the zone of \<lateL~·tCJ.ble :f1 tlctu()tion as for e~ample on the Kasungu and

Mchinji Plains. If this is the case dOl-inward water movement is reduced

despi te high permeability of the surface layers, and infiltrating water is

directed laterally into t.he dambo where it either drains as surface runoff

(return flow), evaporates or infiltrates depending on the relative height

of the water table.

Hydromorphic black dambo clay soils are found in the valley bottoms where

water logging is pr,"lons,/; 2<1 t.hough they mE]' be some drying out in the

dry season. Low permeability clayey textures are most commom although

some sandy loams may occur. Interbedded clays and colluvial sands may

occur, marking changes in erosional and depositional processes and shifts

in the channel position. These layers are mostly restricted to the heads

and sides of dambos. C.ood examples of washed sandS in the dambo head are

found near Mchinji, where they are to be used for making glass. High water

tables in the dambo areas will prevent infiltration for most of the year

and recharge to groundwater will be very limited.

In the centre of wide dambos there may be vectisols which have high.

contents of montmorillonite clays. These expand when wet and contract on

drying resulting in heavy soii cracKing a" tne surface during the dry

season.· l<1i th the initial rains a very Hmi ted amount of water may

penetrate to the water table via the cracks but these quickly seal UP.

and infiltration rates rapioly decrease. Recharge is likely to Qe

negligible because of high Hater tables for most of the year.

Lithosols found surrounding inselbergs and on steep slopes of higher

land rising from t.he plateau are usually saprolites (i.e. developed in

situ). They have variable but generally low permeabilities, though any , surface runoff may infiltrate on reaching the deeper latosols.

It is clear that recharge to groundwater over the plateau area is complex

as·it is both spatially and temporally var iable; the influence. of

la teri te and the water table position are both very important in deter­

mining the extent of in flIt ration.

8.

colluvial soils are very permeable and drain rapidly although they may be

waterlogged for short periods during the rainy season when the water table

is at ground level. ~'here may be localised development OL laterite within

the zone of '\vate1:-table fJ uct.nRtion as for ~xampl€ on the Kasungu and

Mchinji Plains. If this is the case dOWlmaI:d water movement is reduced

despite high permeability of the surface layers, and infiltrating water is

directed laterally into th" dnnbo where it ei·ther drains as surface runoff

(return flow), evaporates or infiltrates depending on the relative height

of the ",ater table.

Hydromorphic black dambo clay soils are found il1 the valley bottoms where

dry season. Low permeability clayey textures are most comrnorn although

some sandy loams may occur. Interbedded clays and colluvial sands may

occur, marking changes in erosional and depositional processes and shifts

in the channel position. These layers are mostly restricted to the heads

and sides of dambos. Good examples of washed sandS in the dambo head are

found near Mchinji, where they are to be used for making glass. High water

tables in the dambo areas will prevent infiltration for most of the year

and recharge to groundwater will be very limited.

In the centre of wide dambes there may be vertisols which have high.

contents of montrnori1lonite clays. These expand when wet and contract on

drying reSUlting in heavy soil cracKlng ae toe surface during the dry

season." With the initial rains a very limited amount of water may

penetral!e to the water table via the cracks but these quickly seal uP.

and infiltration rates rapidly decrease. Recharge is likely to be

negligible because of high Nater tables for most of the year.

Lithosols found surrounding inselbergs and on steep slopes of higher

land rising from the plateau are usually saprolites (i.e. developed in

situ). They have variable but generally low permeabilities, though any , surface runoff may i'nfiltrate on reaching the deeper latosols.

It is clear that recharge to groundwater over the plateau area is complex

as"i t is both spatially and temporally variable; the influence. of

lated te and the water table position are both very important in deter­

mining the extent of infiltration.

9.

Soils on the escarpment are predominantlY 1ithoso1s with same associated

ferra11itic or ferruginous soils in areas which are less dissected. This

area is of little hydr.ogcological significanc".

On the lakeshore plain the Goils are mainly alluvial calcimorphic soils

commonly comprising alternating layers of clay, sand and silt. These

soils have generally high permeabilities on the old flood plain of the

Bua River, but infiltration may be restricted by high water tables.

Close to the river and in the Dzadaza S",amp area there are hydromorphic

soils where there is perennial waterlogging caused by high water tables

and 101< gradients. Along the lakeshore there are coarse lacustrine sands

,.,hich nw,y develop f0rrz.l.l.i.t.ic scils"

1.6. LAND USE

The natural vegetation of· the plateau area is Brachystegia-Julbernardia (Miombo)

woodland, though much of this has been recently cleared for cultivation.

The interfluveareas have only been populated since the 1930's with the

provision of water from borehbles. It is estimated that about two thirds

of the cultivable land in the Central Region is fallow or unused (National

and Shire Irrigation Study, 1980). Waterlogged areas in the dambos are

unsuitable for cultivation and remain as marsh·grasslan9. In the small

areas adjacent to dambos where water levels are always close to the ground

surface "dimba gardens" are found wltn crops grown all year roUnd.

-. Smallholder crops account for most of the agricultural-output. The

principal sUbsistance crop is maize with smaller amounts of beans and

cassava. Tobacco and groundnuts are the two main cash crops. There are

some large commercial estates, covering about 15% of the plateau in total,

where cash crops (mainly tob3cco) are grown for export. The growing

season is broadly speakIng November to May. Agricultural potential is

moderate but may be restricted where soils are highly leached of nutrients , or waterlogged (Land Husbandry Identification Reports, 1978 ~ 1981).

Kasungu Agricultural Development Division (KADD), which administers ··most

of the plateau area, is encouraging development within the Ntchisi and

Dowa West areas (Project Preparation Report, 1980) under the National

Rural Development Programme (NRDP); There are also plans for agricultural

development projects in the Mchinji and Rasungu South areas.

10.

On the steep slopes of the escarpment and the upland areas, soils are

thin and natural Brachystegia woodland remains. There is very little

cultivation and much of these areas are forestry reserves. Nkhotakota

Forest Reserve, Mchinji Ridge Forest Reserve, Dzalanyama Forest Reserve

and the western edge of Kasungu National Park are the main examples.

The lakeshore plain is moderately cultivated where soils are not water­

logged, the principal crops being maize, groundnuts and cassava. The

area is administered by the Salima Lakeshore Agricultural Development

Division (SLAnD); this was set up in the early 1970's and is one of

the oldest of the NRDP projects. There is .an irrigation scheme, estab­

lished by the Chinese Agricultural Mission in 1976 and now managed by

the Ministry of Agriculture and Natural Resources. This scheme takes

water from the Bua River by direct run-of-river abstraction, which

enables rice to be grown over sorne 200 hectares. The scheme is subject

to severe water shortages ·in the dry season and flood damage in the wet

season.,

1.7. POPULATION

The 1977 population of the Bua catchment is estimated to be about

530,000 (National Statistical Office, 1980). The census figures show

that settlement is mainly concentrated on the plateau and is predominantly

in rural villages (Table 2). The main towns are Mchinji and Ntchisi,

which "re district centres with ,~bornas" (administrative offices) and

Mponela, which has expanded rapidly. with the impact of-the tar road

running north from Lilongwe. The densest rural population (over 100/km')

is found in parts of Dowa District (Dzoole, Chiponda and Kayambe

Traditional Authorities) where agriculture is most producti~. The

population is relatively low in Mchinji and Kasungu Districts, where

settlement is predominantly recent, with the establishment of tobacco

estates.

Towards the top of the escarpment there are scattered settlements but

the lower slopes within the Nkhotakota Game Reserve are unpopulated.

The small area of the lakeshore plain has rural population densities of

25 - SO/km" but no large centres.

The national intercensal (1966 - 1977) population growth rate was 2.9%

per annum but it was much higher in Kasungu and Mchinji Districts (6.6%

and 5.8% respectively) with the growth of the estates. The urban growth

rate is high and growth has been parti.cularly rapid in Mponela. The

projected 1990 population is in the order of 800,000.

Table 2.

Estimated Population of the Bua Catchment (1977)*

Population

Rural population on plateau 513,000

Mchinji town 2,000

Mponela town 3,400

Ntchisi town 1,700

Escarpment 7,000

Lakeshore plain 4,000

Total 531,000

Area (km' )

9,500

10

8

5

1,000

100

flensity (per km')

30 - 140

19.6

420

331

0 - 20

25 - 50

* Based on 1977 Population Census figures (National Statistical Office, 1980)

'.

11.

12.

2. HYDROGEOLOGY

2.1 • OCCURENCE OF GROUNDWATER

The prolonged in-si tu weathering of the crystalline basement rocks of the

plateau area has resulted in a layer of unconsolidated saprolite material

which is commonly 15 - 30 m thick but locally may be over 30 m. The

degree of alteration and unconsolidation increases progressively upwards

from the fresh, unweathered· bedrock. A generalised profile is given in

Figure 4. Above the hard, fresh bedrock there is a zone of broken and

hydrated rock where the surfaces are chemically weathered and stained but

the centres of the blocks remain fresh and unweathered. This grades into

a zone of crumbling decomposed bedrock often of sandy or gravelly

texture which retains the original structurer these lower layers would

generally have the highest permeability and effective porosity. Above,

there are pale brown or buff sandy clays or clayey sands often with many

small quartz fragments. This whole sequence makes up the aquifer and is

commonly 10 - 25 m thick. The aquifer is then partly confined by an over­

lying thickness of 5 - 10 m of red-brown compacted clays and latosols at

the surface. In detail the character of the weathered zone varies with

parent rock type and texture, fracture patterns and topography. There is

considerable spatial heterogeneity even over short distances and the more

permeable horizons may have only limited lateral extent. Nevertheless

this relatively thin weathered zone of the basement shield forms the

important aquifer over most of tQe plateau.

The aquifer is partly confined by the compacted clay layers at the surface;

groundwater is first struck at the base of the clays and usually rises

(sometimes by several metres) before its static level is found. Despite

the semi-confining effect of the surface clays it is most likely that

recharge occurs regionally over the plateau. This i~ confirmed by the chemical

dominance of bicarbonate ions and the generally very fresh nature of the

groundwater. it is .probable that recharge is spatially very variable and

occurs preferentially along specific zones, for example, where fractured

quartz veins and pegmatites extend· to the ground surface, and to a lesser

areal extent around the bases of inselbergs and higher ground where the

talus or outwash material has a higher permeability. It is possible that

there is also some recharge via cracks in the dambo clays a.t the beginning

FIGURE 4

TYPICAL PROFILE OF WEATHERED BASEMENT AQUIFER*

'" r.: ....

Latosol

Red/brown compacted clays

:;: Pale brown or buff sandy clay or clayey

~ sand often with many quartz fragments

~ '0

<11 UJ

'" <11

" g ....

Crumbling decomposed bedrock, often

sandy or gravelly but original struc­

ture retained. Clay matrix common •

Broken, hydrated bedl:ock u ~ ::.:'~ ~~'eathered

and stained surfaces but hard "core"

blocks

Fresh oedroc1<

* After Chilton and Grey, 1981

confining layer (5-10 m)

Aquifer (10 - 25 m)

13.

of the rainy season, but this must be of limited extent before the clays

swell and seal. The interfluves, where still wooded, may only allow

limited recharge since the evapotranspiration demands are very high and

tap roots can draw moistu.t:e dirf;ctl:i from the W6.ter table from depths of

15 m or more. With cultivation, it is suspected that the permeability

of the soil is increased by the tillage, the transpiration losses are

reduced and as a result there may be more recharge.

Infiltrating water may be directed laterally by laterites or flow with

least resistance along the more permeable zones associated with stone

lines and thus become concentrated in certain areas. Preferential flow

routes may also be found in the network of burrows. in termite mounds.

Although potential borehole yields are relatively low (less than 5 llsec

and often less than 1 l/sec) the aquifer is more or less continuous and

an important source of rural domestic water supply. Yields are generally

greatest where the bedrock is coarsest and the weathered zone is thickest.

However there is considerable lateral variation which partly reflects the

heterogeneous weathering profile. yields are also affected by borehole

design, which has in the past been very poor. Towards the rift valley

escarpment the aquifer is thinner where rejuvenated drainage has partly

stripped away the weathered layers, thus yields in unit 5D are generally

lower.

The weathering profiles in the dambo areas themselves are largely unknown , and thought to be highly variable depending locally on the various .~

of aggradation and erosion in the genesis of the drainage syst~. Bed­

rock is seen to outcrop in some incised dambos with well defined channels,

yet other dambos are thought to be underlain by relatively t~ick weathered

zones ..

In the pediment to the Mchinji Ridge coarse quartz sands are found in

the river beds. ' These are derived from the quartzites of the uplands

and have been transported downnlcpe. The thickness and laterial eK­

tent of the deposits is not well known but the potential groundwater

yields and recharge are likely to be relatively high due to the coarse

nature of the sediments.

. ~-'

14.

The underlying fresh bedrock is rarely a significant aquifer even

where fractured as the available storage is negligible. Although

there are many boreholes drilled to considerable depths into fresh

rock (often 50 - 60 m deep) these will rely on storage in the over­

lying weathered zone. Outcrop of fresh rock is found where insel­

bergs project above the plateau surface and on the escarpment. In

these areas the soil cover and weathered material are thin and

aquifers are poor and discontinuous. Occasionally high yielding

boreholes occur where they tap a system of interconnecting fractures

but in general yields are unreliable even where fracture traces can

be located. Runoff is high on the steep slopes and recharge is nOt . -dependable. As a result there are few existing bore holes in these

regions and scope for further groundwater development is limited.

There may be significant recharge via the runoff into the more

permeable material of surrounding pediment areas.

The alluvial lakeshore deposits in the Bua catchment comprise a

variable sequence of clays, silts and sands, though lithological

records from boreholes give little detailed information on the

succession. The yields appear to be generally low, but this probably

reflects poor borehole design. However, records from adjacent areas

(Mauluka, 1983) indicate that groundwater yields can be high (over

15 l/sec) where boreholes have been properly designed, especially

where there are significant thicknesses of sands in the alluvial

sequence. The thickness of th~.alluvium increases towards Lake

Malawi and may exceed 20 m. Near the base of the escarpment where

the alluvium is thin, groundwater will be derived mainly from the

underlying weathered basement aquifer with which it is in hydraulic

continuity.

2.2. AQUIFER PROPERTIES

There have behn ve,y few aquifer tests with detailed monitoring of

groundwater levels so estimates of aquifer properties are necessarily

rather crude and it is suspected that conditions are extremely variable

even on a very local scale, depending on the structure and lithology

of the bedrock and on the depth and nature of the weathered zone.

Poor borehole design further complicate~ the estimation of aquifer

properties. Conditions tn the .weathered bedrock aquifer are broadly

discussed below.

2.2.1. Borehole Yields

Records of bore hole yields are at best the result of short drillers

pumping tests (commonly five hour but sometimes 12 hour tests) but

for the older boreholes the only indication may be a driller's recom­

mended yield. These records are more meaningful where the drawdown

is measured too but in many cases the yield gives the only idea of

aquifer performance. The records are of the discharge rate at which

the borehole was tested and may reflect the pump capacity rather

than the aquifer capacity. Over much of the plateau area the bore­

hole test yields appear to be low to very low (mostly less than 1.5

l/sec) and in general are lowest in Unit 50 where the weathered zone

is thinner, however the yields are usually adequate for rural domestic

supplies. The low test yields at least in part reflect the very poor

borehole construction (see Section 4.2) and it is likely that many

of the boreholes are very inefficient due to high well losses. With

improved borehole designs, yields of 1 - 3 l/sec might PQssibly be

obtained over much of the weathered plateau, with even higher yields

where the weathered zone is thickest which could be important for

agricultural development. On the escarpment, yields are usually low

and unreliable because of low storage.

2.2.2. SpeCific capacity(S.C.)

The Specific Capacity (yield divided by drawdown) can be determined

where water levels were measured during the drillers pump tests.

Records are only available for ~ limited number of boreholes and

are not strictly comparable since they are not for a ~niform time

and the specific capacity is likely to be severely non linear for

different yields. Nevertheless they give a better indication of

aquifer and well performance than the records of yield on eheir own.

The Specific Capacity appears to be very low (mostly less than

0.1 l/sec/mr. The very large drawdowns (often 20 - 40 m during five

hour tests) are thought to largely reflect poor borehole design and , inefficiency as well as the aquifer characteristics.

Multiple rate step tests have been carried out for some of the urban

supply boreholes in Mponela. These show that at lower discharge

rates the incremental drawdown for an increase in pumping rate is

relatively constant or gradually increasing. However at high

pumping rates there is cQffimonly a substantial increase in well

losses and incremental drawdown and a reduction of bore hole efficiency.

Pumping rates should be chosen so that they are less than that where

such a "breakaway situation" arises.

16.

2.2.3. Transmissivity (T)

The fel< aquifer tests v/hich have been carried out are on the boreholes

at Mponela. They are mostly of too short a duration for detailed

analysis and only have limited water. level Measurements taken within

the pumping borehole itself. These are subject to inaccuracies due

to surging of water, fluctuations in discharge rate and well storage

effects at the start of the test. In addition it is suspected that

the weathered zone is largely cased out or lined with screen of very

low open area and the borehole is only open in the hard bedrock below.

The heterogeneity of the material (both vertically and laterally)

will result in variable contribution of f,low from ~i££erent layers

in the aquifer 0 The f:I1i::J.lysis of data should therefore be treated

with caution since the basic assumptions for conventional pumping

test analysis are not satisfied.

The corrections of water, level measurements for dewatering are

difficult to estimate because of the variable contributions from

different layers. Approximately the first ten minutes are affected

by well storage in the pumping borehole, and later data appears to

have a component of delayed gravity storage typical of water table

aquifers so straight line methods of analysis would be misleading.

The data is difficult to match with accuracy to Boulton or

Neuman Log-Log Type Curves because of the shallow gradients, so the

estimates of Transmissivity ate only a guide as to the order of

magnit.ude. '.

Seventy-two hour tests were carried out on boreholes at Mponela in

1980 and the transmissivity appears to range from 5 - to m'/d)

however it must be emphasised that the data is very suspect,' and is

likely to give an estimate of Transmissivity for the underlying

bedrock. Other tests on these boreholes at Mponela were carried out

in 1979 (Howard Humphries) but the tests were too short, at variable

discharge rates and, the analysiS is somewhat suspect,.

The dangers of estimating transmissivity from measurements within

a poorly designed pumping well itself cannot be overstated. It

should be noted that detailed aquifer tests carried out with obser­

vation boreholes at Lilongwe Airport (to the south of the Bua

Catchment but likely to b~ typical of the weathered bedrock aquifer

of the plateau) are difficult to analyse, using conventional methods.

17.

This is despite the fact that t,he boreholes were known to be well

designed and with linear specific capacity. The complexities appear

to be due to hydraulic bounderies, aquifer layering and dewatering.

It is likely that there could be considerable variation in transmis­

sivity over the plateau area even within short distances, depending

on the nature of the weathered zone and the extent of residual

fracturing which is likely to retain a Significant influence on the

permeability.

2.2.4. Permeability (K)

On the basis that the estimated transmissivity of the weathered

aquifer in the plateau area is 5 - 10 m'/d and that the aquifer

thickness is typically 10 - 20 m, the average permeability is likely

to be in the range 0.3 - 1 m/d. This can only be a rough approxima­

tion and the permeability is likely to vary considerably both late­

rally and vertically in different layers of the aquifer. Highly

variable water quality over very short distances (see Section 2.5)

suggests low permeabilities.

2.2.5. Storage.Coefficient (S)

Since there have been no aquifer tests with water levels monitored

in observation wells the storage coefficient cannot be easily

determined. For a semi-confined aquifer with a granular, though

often·poorly sorted and clay r~ch matrix, the'storage coefficient

is likely to be in the range of 5 x 10-' to 10-'.

2.3. GROUNDWATER LEVEL FLUCTUATIONS

Groundwater levels have been monitored at three sites within the

Bua Catchment with autographic recorders since 1980 (Figure 5).

These give an indication of the seasonal changes in the volume of

stored groundwater., and with continued measurements the long term

effects of groundwater abstraction can be evaluated.

Observation wells 5E325 X (grid reference WV651849) and SF 153 X

(grid reference WA055180) are sited in on the lower slopes of valleys

and show seasonal fluctuations of 2 - 3 m. The slightly larger

fluctuations of the latter site could reflect lower storage co­

efficient of higher recharge at this site, which is near a dambo.

Figure 5~ GROUNOWATER LEVEL FLUCTUATIONS ( seQsonQI)

2 MllllbUIIQSE 32SXCSM 284) .

3 ,

.... --. , ,. I ,I' I

I 10 I , 5 ---- -----

Chimwonikllngo 5 F153)IGK121l

1-[1 '" -",,--. / ir2 , I ,

~-,

lt3 ,.' "11' , ~I:

, "..f\ ----!.R ~- --,. --~ Kotondo Estllte 5ESXlL 158J II -

,

MAY. JUN AUG SEP OCT

1981 1982

18.

Water levels do not begin to rise until two to three months after the

beginning of the rainy season; much of the early rainfall is used to

satisfy large moisture deficits which have built up in the soil and

unsaturated zone during the dry season and little infiltration can

occur until these have been made up. Maximum groundwater levels occur

around March/April, towards the end of the wet season implying that

percolation to the water table is relatively slow. A gradual recession

of water levels follows reaching a minimum bet\~een December and

February. Water levels at these two sites are relatively close to

the ground surface Cl - 5 m) and remain within the semi-confining clays.

The percolation rates will vary temporally with the rainfall occurence.

and intensity, moisture conditions in the unsaturated zone, and

spatially with the nature'of the weathered profile. The possibility

of lateral throughflow along laterite layers could cause considerably

different responses within small distances.

The observation well 5E 5X (grid reference WA 45220'1) is sHed on an

interfluve. The hydrograph shm,'s " ,.,,,,,ti nued rise in groundwater level

Over the monitoring period. One possible explanation for the lack of

seasonal variation is that the direct recharge is minimal at this site

(and possibly over crest sites generally). The gradual rise in water

level ("'er the '11('m'toeing period could be caused by higher than average

rainfall and recharge over the region during che antecedent couple of

years. ·The result could be a baeking up of groundwater with overall

annual discharge being less than the recharge. Also c~nges in land

use in the surrounding area, from woodland Or smallholder farms to

tobacco estates, may have led to reduced evapotranspiration and better

structured soils thereby increasing recharge. The effect coold have

been transferred upslope causing the general rise in.groundwater levels

seen at the recorder site. This is only a tt>nta f:h,,, explanation and

further investigation is necessary.

It would be desirable to install a series of piezomaters across an

interfluve and into a dambo to monitor grourtdwater levels and thus further

understanding of the spatial variability of recharge and local patterns of

groundwater movement.

19.

Diurnal fluctuations of water level associated with atmospheric changes

in barometric pressure are observed in all boreholes with autographic

records (Table 3). The larger fluctuations on the ',interfluve site may

indicate a higher barometric efficiency anq thus a thicker, more

impermeable confining layer than at the boreholes nearer the dambc.

Alternatively it may reflect larger changes in pressure or a lower

Storage Coefficient.

The daily variations in atmospheric pressure have peen measured at Madisi

(3 - 6 mb which is approximately equivalent to 30 - 60 mm water). It is

not known how variable pressure changes a're over the plateau, but assuming

Madisi to be representative for all water level recorder sites the baro­

metric efficiency has been calculated to be 25 - 80% (Table 3).

Table 3.

Analysis of Diurnal Fluctuations of Groundwater Level

Site

Grid Reference

Borehole Number

Geological ,Survey Number

Aquifer thickness (rn)

Estimated Effective porosicY

Average diurnal water level fluctuation (mm)

Barometric efficiency (%)

Elastic storage (Se)

"

Dambc

WA05518(j

5F 153 X

GK 127

12

10 - 20

33

4 x 10~·

Near Dambc

WV651849

SE 325 X

SM 284

32

0.25

10 - 15

25 - 35

1 x 10--

Interfluve

WA452201

5E 5 X

L-158

?35

O.2S

2S - 30

49 - 8" 6 x 10-0

This can be used to estimate the elastic storage (Se) of the aquifer

using the equation,-

l'1here Cl =

y =

b =

l E = w

, Se = " y b E B w

linked porosity (estimated to be 0.25)

specific weiyht of water (1,00v kg/m')

saturated thickness of, aquifer (m)

bulk modulus of compression of water (4.7 x 10-9m2 /"l)

B = barometric efficiency.

The elastic storage calculate( by this'method is around 10-', but it must

be emphasised that this can 0: 1y be a rough estimate.

Figure 6: GROUNOYJATER LEVELS FROM

.. After Chilton (1979)

BOREHOLE MAINTENANCE RECORDS

OOREHOLE GEOLOGICAL NUMB£II SURVEY depths in metres below ground level

SE 27

5E47

5E41

5F3

5080

508

5E6

5031

NUMBE~~ ________________________________________ -r ____ ~ ______________ ~

([154)

(W1541

('11241

5

10

5

10

15 0

5

10

10--...... --- , ..... -----... --__ __.4----------- ...... ------.-_. ___ -:-4'- -, ....... -----.-----..----- - .----. --\ .... -- \ 1'-------\ ..--------.::0_-- 0--- _*_ __ ..... ..... _0

.. ..... f" ---------~-------" , , , '01

.. ___ -... __ - ---·0 ... ---- .... - \ .,..------ ........ ...,.:.-,-------------.~-_--0_._______ --..... __ ---.

~o 0.------"--./)'\ P'>... /.----,~----------.-- -----\ I ..... - ,/- --. -..... ~,,/ \. J -_ '

\ 1 -__ ,," (W231 15 \/ - ~ .. .....- ........... /

(0.2191 15 2()'

,,--- . ......... ___ ---~------..... ;" -- ..... ~---__ ~" ---0-_____ ~ • __ -04-'"

5 (K65) 10

15

c..-- --_.

~ ---'..... _--....----0-" ---.o"'1I!! _---, -.--- ... - _ .... --- -- ...... ----------... -------" ---- ........

(H07) 5 ... ---"-- .-------l.._ ... __ ,/ -__ _0 __

1()."'·------ JI--........ ---..... -- ./ -.-----.--- ----- __ e

1 -----" .. -0--_-0"

fE110) 10 1"11:- -------- -_ -0 _______ -- ---0-- --0--- -- -0- -0-- -0_ l;;r ,~~- --__ -..

1971 1972 1973 1974 1975 1976 1977 1978 ' 1979 1980 1981

20.

It is considered thllt the surface clays are only semi-conffning and

that there must be some component of gravity drainage with the decline

of groundwater levels. Thus the determination of Se canno): be used

to approximate the Storage Coefficient (S) which could r~alistical1y

be in the range of 5 x 10-' to 10- 2 where the water level remains

within' the clays. Thus at SE 325 X and SF 153 X, the seasonal water

level fluctuations of 2.5 to 3.5 m [including an extrapolation of dry

season recession rates (Figure 6)) could result from annual recharge

ranging from 13 mm to 35 mm. The Storage coefficient is only crudely

estimated so the recharge cannot be determined with any greater

precision. If the groundwater level falls, below the confining clays

there will be increased yield as the aquifer passes from semi­

confined to water-table conditions; the storage coefficient will

thus vary temporally and this method of determining recharge cannot

be used with any confidence.

Groundwater levels have been measured in boreholes by Borehole

Maintenance Units since 1971 but readings are irregular as they lire

only taken when handpumps are removed Ilnd in addition many of the

measurements are suspect. They do'however show that in general the

piezometric levels (liS shown by water levels in boreholes) over the

platellu are relatively close to the ground surface, being commonly

shallowest towards the dambos, in the range of 2 - 10 m. Boreholes

with the most frequent records (Figure 6) show that there is no evidenae of dealing water levels. over the period 1971 - 1981 despite

an increase in abstraction, and there is no suggestion- of aquifer

depletion.

Piezometric measurements suggest that seasonal fluctuations.'of water

level in the alluvial material at Bua Point ar,!! in the order of 0.5

- 1 m but the record is only of six months duration. A longer period

of data collection is required before any analysis to determin!! the , Storage Coefficient. can be made.

2.4. GROUNDWATER MOVEMENT

A generalised form of the regional piezometric surface has been ob~

tained using estimates of minimum qroundwater levels for the plateau

area (Figure 7). Groundwater flow is generally radially towards the

basin centre with discharge to the Bua River and its tributaries and

with an outflow to the North East. There are insufficient data

21.

points to aonetruct more detailed groundwater level contours but it

is likely that on a small scale they are much more ,crenulated with

localised flow routes to the dambo headwaters.

TO the south-west of the Mchinji Ridge the groundwater flow is

structurally controlled by the upland rising from the plains.

Groundwater flow in the upper valley of the Bua River is along the

edge of the ridge as far as Mchinji Town then it is diverted to the

east onto the main plateau area.

The average regional hydraulic gradient appears to be extremely low

(0.001 - 0.005) especially upstream of the conf1uence'of the Bua and

Rusa Rivers, and on a local scale will be considerably lower where

the flow routes towards the dambos are very tortuous,

gradient increases towards the escarpment (approaching

The hydraulic

0.01) where

the slope of the ground surface increases, ,the plateau is more

dissected and there is a smaller percentage of area covered by dambos,

for example in the Kasingadzi and Mtiti catchments.

A flow net analysis has been attempted using the equation:-

Q .. nf T h where

Q ., discharge

nf .. number of flow tubes

T .. transmissivity -, h .. water level contour interval

Using an estimated transmissivity of 10 m'/d and a contour interval

of 50 m the groundwater discharge from the catchment to the ,gauge at •

5D I in 40 flow tubes is estimated to be 7.3 x 10' m·/yr. This would

represent an average of 0.8 mm recharge over the catchment (9410 km') which is considered to be a large underestimate for the reasons which

follow. ,

This method is clearly inappropriate for calculating the groundwater

discharge from catchments on the plateau and the analysis could be

very misleading. This is largely because it is possible that there

is significant evaporation of groundwater discharged into the dambos

(see Section 3.5). Also although a generalised contour map of

piezometric form can be drawn because the slope of the ground/surface

is so gentle, the water level data is insdequate to draw sufficiently

o ,

FIGURE 7; MAP TO SHOW PIEZONETRIC FORM

",

...... ,,' ,~;j..';'o-, ', ..

10 2.01<", ,

" .....

(/ at; ,

.'\ .... " 0°

~ , 0'

... ,~ . , 0°

,'I' ,

~: : ,: $' " , , " .p

", .' " : ~ " ~,'"

, ,,' <\~Zl' : .. " . .

: " "

: "

.. ,1150 .. , pie'Zomcl;y,c. -iO~ ...... line rne.tn?s ",loo"", c:I"bJ,.,."

05E.5)< si!:.e.. of a....t."'~ .... p"'ic. wa.le..- le~\ re<».-c>le.r

22.

detailed contours for meaningful flow net analysis. On a local scale

the direction of groundwater movement is likely to be very variable,

with flow towards, and discharge along, each dambo tributary. Thus

it is likely that considerably too few flow tubes have been considered.

In addition the estimated transmissivity of the weathered basement

aquifer could be in error, although this is difficult to quantify.

Detailed monitoring of piezometric levels over a single interfluve

would allow a better calculation ·of groundwater flow through the

aquifer to>lards a dambo area and could be extrapolated to cover larger

simila.r areas.

Using the equation

O· = TiW where

Q = discharge

T = transmissivity

i = hydraulic gradient

W " width

a consideration of the maximum expected hydraulic gradient (0.01)

together with a maximum transmissivity (10 mO/d) would give a maximum

annual groundwater discharge of 36,600 m' through a one kilometre

wide section of the aquifer. In the long run, the annual groundwater

discharge will be balanced by recharge over the area; This assumes

that there is no evaporativG loss directly from the water tabl~ because

of th~ semi-confined nature of ~he aquifer, and that downward leakage

into the bedrock beneath is neglib~ble. The surface ~ater drainage

pattern is such that the lateral separation between the interfluve

crest and the dambo margin is commonly about one kilometre. The

implied maximum recharge over the area or 1 km' is thus 37 mm.

2.5. GROUNDWATER CHEMISTRY

The existing 6ata 9n groundwater quality is, ·for the most part, major

element chemical analyses (some only partial· analYses) carried out

by the Geological Survey during the 1970's. This archive is considered

to provide a useful indication of water quality, but it cannot be

taken as completely reliable. This is evident as the ion balance is

often poor, frequently greater than 5%. Caution must be t;aken in.

interpreting the analyses as the samples were probably collected

without filt.ration, unstable parameters (pH and bicarbonate) were not

measured in the field, Some of the analytical techniques may not be

23.

reliable and sampling/storage conditions are likely to have been poor.

Nevertheless the records are valuable in the absence of any other

analyses.

The Department of Lands valuation and water are now constructing a

water quality laboratory with facilities to make more accurate and

reliable water anlayses. As yet sampling in the Bua Catchment has

been restricted mainly to a small area near Madisi.

Electrical conductivity (EC) of groundwater has been measured in

samples from many boreholesl this gives ,an indication of total

mineralisation of groundwater over the plateau area: The EC is

generally very low, usually less than 1,000 ~S/cm and commonly below

500 ~S/cm. This indicates that the weathered zone is highly leached

of soluble minerals and that the groundwater is likely to be derived

from relatively recent recharge. In units SE and SF the EC is

generally in the range 100 - 600 ~S/cm. The groundwater quality is

generally slightly more mineralised in unit 50 with higher average

concentrations of ions and locally BC greater than 3,000 ~S/cm,

which could be a function of the thinner, less leached zone of

weathering. It is suspected that there is quality layering within

the aqu ifer •

The water quality can be very variable even over short distanCes.

For example a survey in part at the Powa West· Agricultural Project

Area showed saline water with EC approaching 4,000 ~S/cm at Madisi

with fresh water (EC < 1,000 ~S/cm only one kilometre away (Figure 8).

There appear to be some areas where the water quality is worse regard­

less of whether the water comes from a borehole, dug well br surface

water source, for example along the Chawawa Dambo. It is clear that

there can also be considerable variation in conductivity at different

water points within one village. This is evidence of low aquifer

permeabilitil!s an4 slow groundwater movement. The EC of water in

the Lithembwe and Nkalalo Rivers is high (greater than 1,000 ~S/cm),

however nearby protec.ted water sources appear to be less mineralised.

The groundwater is classified predominantly as calcium (Ca) - bicar­

bonate (HCO,) (Table 4 and Figure 9) although there are cases where

magnesium (Mg) and/or sodium (Na) are the dominant cations and in

some areas there are frequently high concentrations of sulphate (SO,).

Figure ~:J ELECTRICAL CONDUCTIVITY SURVEY IN MADISI AREA

KEY

o Borehole v Open dug well

a Protected dug well

A Stream river dam 350 Electrical conductivity (y sIc m)

o 1 2 3 4 5 Km • ! , , ••

MPONELA

24.

The distinctive hydrochemical facies and generally very low minera­

lisation is thought to be the result of silicate alteration reactions

as recharge water moves through the weathered zone. The pH measure­

ments in the old records are unreliable since they are not field

measurements and samples are likely to have 'been stored for sometime.

The few more reliable measurements show that the groundwater is

usually slightly acid (pH 5 - 7/'.

The dominance of the HCO, ion (mainly in the range 100 - 500 mg/l)

suggests that the infiltration is recent and that the water quality

is controlled by solution processes in the, soil and weathered profile.

TABLE 4.

Typical Water Quality of Groundwater in the Plateau Area

Electrical conductivity (EC)

Total Oisolved solids (TDS)

calcium (Ca)

Magnesium (Mg)

Sodium (Na)

Potassium (R)

Total Iron (Fe)

Bicarbonate (HCO, l

Sulphate (SO.)

Chlor ide (Cl)

Nitrate (NO,-N)

Fl.uor ide (F)

'.

100 ~ 1,000 pS/cm

60 - 600 mg/l

10 - 100 mg/l

5 - 25 mg/l

5 - 70 mg/l

1 - 6 mg/l

1 - 5 mg/l

100 - 500 mg/l

5 - 1000 mg/l

< 20 mg/l

< 1 mg/l

< 1 mg/l

Sulphate (SO.) levels are generally low (less than 20 mg/l).'although

there are some local areas with very high levels (greater than 1,000 mg/ll,'

It is thought that in those areas where relatively high concentration~

of SO,occur in groundwater these are produced by a progressive , oxidation of sulphl:de-rich parent material (Bath, .1980). In the

Dowa West Agricultural Project area, in Unit 50, high SO. concentrations

appear to be linked with the occurrence of high Mg and ca concentrations.

In Units 5E and 5Fthere appears to be high total iron (Fe) associated

with high SO. levels, some of which could have ~en released into

solution by the acidic conditions produced by sulphide oxidation .

although this is not seen in Unit 50.

25.

concentrations of Fe are very variable but high levels are widespread,

commonly up to 5 mg/l,\whiCh is far in excess of the WHO advised limit

of 0.1 mg/l and maximum permissible limit of 1 mg/l. This leads to

problems of acceptability of water because of the bitter taste and

discolouration of laundry and food. It must be noted that the Geological c

Survey records are for total Fe (i.e. dissolved and colloidal) as the

samples were not filtered. However measurements on both filtered and

unfiltered samples taken in the Lilongwe Plain to the South of the Bua

Catchment suggest that Fe is initially present as soluble complexes and

subsequently precipitates out due to oxidation after, prolonged standing

or boiling (Bath, 1980) • The iron is most likely to be derived from

ferromagnesian minerals during weathering and the presence of organic

fulvic acids may result in the ccmplexing and increased mobilisation of

Fe. Corrosion of borehole casing, pump or rising main by acidic ground­

water may also contribute to the iron problem. This corrosion is

commonly encountered, and can result in the need to replace rods and

pipes as often as every couple of years. The causes of high iron

concentrations and their apparently random occurrence are not yet

fully understood.

Chloride concentrations are relatively low (mostly 20 mg/l), concen­

trations in rainfall are being determined in order to estimate the

recha]:ge from their relative c •. l concentrations.

The low chloride levels together with the generally low nitrate'

(NO,-N) concentrations (mostly < 1 mg/l) indicate that groundwater

pollution is usually minimal. It is likely that the surface clays

offer considerable protection to the aquifer from surface contamination

derived from sewage and/or fertilisers by absorbing NH.. In water­

logged clays conditions are likely to be anaerobic, thus nitrification

would not ocbur. There are a few sites where there may be a pollution

risk, which is most likely to occur from the surface where the bore­

hole surrounds are poorly constr)lcted or maintained.

Calcium (mainly 10 - 100 mg/l), magnesium (5 - 2S mg/l) and sodium

(5 - 70 mg/l) all show considerable scatter but'the Ca ion is most

often the dominant cation. This may reflect the variation in

weatherable minerals in the basement complex and also the possibility

26.

of some cation exchange on clay surfaces. Potassium concentrations

are low (1 - 6 mg/l) as are fluoride concentrations (less than 1 mg/l).

The trilinear plot of bydroc)wllliGtL'y for Unit 5D ("'igure 9.A) shows

that there is considerable variation in the' composition of the water

quality. Ca is often but always the dominant anion. The position

of the samples on the "diamond field" of the trili,near plot show

that carbonate harness is dominant (i.e. weak acids and alkali earths

are the main controls) >lhere the TDS are low, and non carbonate

hardness is considerably more in high SO, water (strong acids) which

tend to have higher TDS. The trilinear Pfot of anions for Unit SE

(Figure 9.B) also sho>ls scatter'which appears to be mainly related, to

the occurrence of occasional sulphide rich parent material. Where

these are present, SO, concentrations are relatively high and tend

to dominate the anions. The proportion of ReO, is then corres­

pondingly reduced although the actual Heo, concentrations are not

always lower. The overall mineralisation of these water samples

appears to be greater than those from other sites within Unit SE.

The trilinear cation plot sho"m considerable variability in ground­

water type. This would appear to be related to the underlying bed­

rock minerology which controls the ions that are available for

leaching, rather than any progressive evolution of groundwater types

across the ;'cater resource unit. The position of the samples from

SE on the "diamond field" of the trilinear plot show that alkali

earths are dominant: The twe.of hardness var,ies depending on whether

weak acids or strong acids are the, main control (carbonate hardness

resulting from the former and non carbonate hardness from the latter).

The generally low mineralisation of groundwater on the pla~au area

renders it perfectly potable in most cases. There,are, however,

small localised areas wnHe the conducti vi tv is too high (> 3 ,000 ~S/cm)

for domestic consumption. The only other drawback for domestic use is

the occurrenc~ of l;1igh Fe in SOme sources f there is no danger on

grounds of health but the wo,ter may be rejec",ed as unpalatable or

because of the staining it causes', resulting in the return to

traditional and more polluted water 'sources. The water would appear

to be suitable for irrigation, although minor element concentrations

are not known, but the yields which could be supplied are ,small.

Figure 9A. TRlllNEAR PLOT OF HYOROCHEMISTRY 1

UNIT SO (after Bath J 1980)

Ca

___ -_'0 .... (a

Figure 96 TRILlNEAR PLOT OF HYDROCHEMISTRY UNIT 5 E (after Bath, 1980)

(1 + NO~

27.

The groundwater quality in the Bua lakeshore plain is largely unknown,

but in adjacent lakeshore areas the electrical conductivity is higher

(commonly over 1,000 ~S/crn) especially nearest to the escarpment.

28.

3. CATCHMENT WATER BALANCE ANO GROUNOWATER RESOURCE EVALUATION

3.1. RAINFALL

Long-term measurements of rainfall have been made at several sites.

Using area weightings for the Thiessen polygons (Figure 10), average

annual rainfall over the plateau

50 1 and 50 2 has been estimated

to the river gauqing

{Tables 7B and 7C).

stations at

This can only

be an approximation at best because of the uneven distribution of

rainfall stations over the catchment. There are no long term rep­

resentative rainfall measurements in the RUBa Catchment to the north­

west, although this is suspected to have relatively low rainfall OVer

the plains. and the uplands of the Mchinji Ridge can receive rainfall

throughout the year. Rainfall is generally intermittant at the

beginning of the wet season with storms becoming more frequent and

heavier in January with rainfall intensities of over 20 mm/day being

quite common. Storms then decrease in number and intensity towards

the end of March (figure 5). There is very large spatial variability

in torpical·storms even over short distances but the annual total

rainfall at any site is suspected to be reasonably representative of

the local area.

3.2. EVAPORATION AND TRANSPIRATION

Measurement or estimation of evaporation and transpiration is commonly

subject; to large error margins which is particularly Significant in

the tropics where this is a relatively large factor in-the water

balance. There is no long term data for any stations within the

Bua Catchment.

Open water pan evaporation measurements are available from 1951 for

Lilongwe in the Linthipe Catchment to the south. This is the most

representative station for the. plateau although rainfall is sig-, . . nificantly lower. Estimates of open water evaporation (Eo) have also

been made using the empirical Penman method for Lilongwe from 1972/73

- 1977/78 (Table 5). These are· significantly lower (Pan factor of 0.9)

which confirms the generally accepted fact that pan evaporation

measurements are overestimates. Both of these estimates of open

water evaporation are well in excess of precipitation

much greater than average actual evapotranspiration.

evapotranspiration (Et) has also been estimated using

and are obviously

Potential

the Penman

Figure 10: THIESSEN POLYGONS FOR ESTIMATING TOTAL CATCHMENT RAINFALL

, " " " " "

A Streclm gauging station

lit .. KASIYA ( 944)

I(ASUNGU

(i) Rainfall stations with mean o.nnLlal ruinfall (1959160-197415) ( 884) ""-, /' \ Thiessen polygon

,/ lI. / \ /'

v

29.

met:lOd "for a short green crop with no shortage of water". The Et

rat',s vary from 2.0 mm/d in June to 5.5 mm/d in October. The method

of ',omputation of Penman estimates is satisfactory but the empirical

rel.,tionship used to estimate radiation from the number of hours of

sun:;hine (Glover and McCulloch, 1958) may not be totally reliable.

Act' .• al evapotranspiration will be less than potential wherever large

soil moisture deficits build up, but no measurements have been made.

Datd for Nkhotakota is representative of the lakeshore.

In:he dambo areas it is possible that water will continue to be lost

to .. he atmosphere for most of the year at ~east at the potential rate.

The:'e is a general presumption that evapotranspiration from dambo

veg •. 'tation is much greater than evaporation from a free water surface

of ':he same size. Actual evapotranspiration measurements from a dambo

wer,- shown to be higher than potential in the rainy season and much

reduced in the dry season· (Ba1ek, 1977). It is clear that there must

be considerable spatial variation of evapotranspiration over short

dis·cances.

Lil,ongwe

Nkho)takota

Table 5

Estimates of Evaporation and Transpiration

Water Year

1972/73

1973/74

1974/75

1975/76

1976/77

1977/78

Mean

1972/73

1973/74

1974/75

1975/76

1976/77

1977/78

Mean

Rainfall (mm)

600

1,090

900

900

1,150

780

903

1,800

1,790

1,350

1,980

1,340

2,060

1,720

-,

Pan EVdporation {mm)

2,000

1,800

1,890

1,850

1,880

1,882

2,060

2,460

2,280

2,500

2,325

E (Penman) o (mm)

1,850

1,530

1,640

1.650.

1,700

1,560

1,655

2,080

1,820

1,900

1,910

1 ,950

1,820

1,913

Et (Penman) (mm)

(potential)

1,440

1,170

1,270

1,270

1,320

1,200

1,278

1,650

1,420

1,500

1,500

1,540

1,410

1,503

JO.

The Morton method (1978) of estimating actual evapotranspiration from

observations of temperature, .humidity and sunshine was used for data

from Lilong>le by Chilton (1979). Estimates for the period 1965/66 -

1974/75 .were, shown to be outside the 10% accuracy limits for the method,

in excess of the preoipitation and clearly too high.

It is felt that the most reliable eaSily obtained estimate of average

actual evapotranspiration for the plateaU area as a whole might be

obtained by subtracting the total runoff from the rainfall (Section 3.5

and Tables and 6 and 7) despite the errors involved in determining both

these parameters. The method also assumes .that there are no changes

in groundwater storage from year to year and that underflow from

catchment in the weathered basement aquifer or deep percolation through

fractures in the bedrock i.s negligible.

Estimates of actual Et rates for dambo andinterfluve areas* have been

made based on the calculation of Penman potential Et for Lilongwe

(Table 6A - 6C), resulting in an average of 779 mm over the whole

catchment.

In the darnbo areas the actual Et is considered to be 120% of potential

during the wet season (once any moisture deficits have been made up)

because of the inte~c'='!pt:!'("'In ll"\f?'e:~~ from the grasses and direct evapo­

ration from the ,Slow moving surface run off. Field observationS of

dambo vegetation and soil moisut~e conditions from aeriel photos and

satellite images taken during the d~yseason suggest that active

evapotranspiration continues and actual Et does not reduce as much

as observed by Balek and Perry (1973) for wooded dambo catchments in

Zambia. A reduction to 50% of potential Et by the end of th~ dry

season is considered appropriate. The figures .are v,ery generalised

and in practise concEdo"" ·"il1 vary cor.siderably over the catchment

dependin9 on factors such .as antecendent moisture conditions, rainfall

intenSity and ~uratton, relative humidity, temperature and vegetation

type.

Estimates of average actual Et losses·have.also been made for the

interfluves area (Table 6B and 6C). The overall losses are considered

to be much less than those observed by Balek and Perry (1973) for the

wooded catchments (where the deep tree roots and permanent vegetation

mmmi!liilm!1!Hm!H!l!ll!m!mm!mHlmii!HHHj~!!!m!!imH~i!jlm!lmWn!!lmfmmHlmmHm"!!mmHHl!!!fmmm1l!!mm!1!IH!mmlHlmmmmmmmmi!r

* Helpful discussions on eva~otranspiration rates were held with W. Stephens from I<asinthula Agricultural Research Station, Chikwawa

31.

ensure active transpiration throughout the year, possibly directly

tapping groundlrater. The greater p<3rt of the plateau area of the Bua

catchment is cultivated. Average actual Et from the cropped and fallow

aroas n2'!$ ~:::~~ -::.c;t{mtd~c:1 using a root constant of 140 mm and a limiting

soil mixture d,~ficit of '{SO mm (Table 6B)" 'fhe figures show that

actual Et quickly deops belmr potential after rainfall ceases, and the

limiting soil moisture defici t8 result in very low actual Et by the

crop harvesting ti.me in June. Further evaporative losses from the

fallot< land and the bare soil are small and drop to zero by the end

of the dry season.

This overall drop :i.n ?<,tual Et over the catchment is confirmed by

measurements of relative humidity (tal,en at 1400 hours at Lilongwe

Airport d.\lring 1981 and .1982). These ShOlv a mean monthly figure of

73% in January and fall. to mean monthly figure of 39% relative humidity

in September. The mositure in the air in September is probably derived

from evapotranspiration in the dambo areas. The drop in actual Et

rates during the dry season has been observed for various crop types

in Kenya (Edwards, 1979).

The proportion of the catchment of permanent trees is taken as 5%.

There is a greater leaf area for interception than on cultivated '"

vegetation or dambo grasses, so that once moisture deficits are made

up, .which will tc,;:0 :O.1S::".>t' \ .. >:..<',: :L".:- t:.::_ c;t:nG::" v~set~tion types, actual

Et is taken to occur at a rate greater than potential (up to 12!;%)~

Average· actual Et during the dr~.season has been estimatea using a

root constant of 2()O 1,"" ".1<,1 a limiting soil moisture deficit of 230 mm

(Table GC). It can be s(.en that Et. rates do not fall off as rapidly

as for the cultivated areas and that some moisutre continues to be

lost to the atmosphere right until the end of the dry season:

There have been [j,V ih::lu l1k:a;;::"w.i..:ements vi:: acc.ucll Et: nor soil moisture

deficits and the figures pre'sented in Tables 6A - 6C can only give a

generalised id~a of .possible catchment conditions. It is realised

that there will be conswerab.Le variation on a local scale depending

on the crop type, date of planting, climatic conditions and antecedent

soil moisture status. ~'he meteorological conditions (rainfall inten­

sity and duration) I~ill be particularly important in the early wet

season when moisture deficits are being restored. On the i·nterfluves

the deficits will take longer to satisfy than in the dambo areas so

maximum Et rates will be achieved later •.

Table 6A Estimates of Actual Evapotranspiration, Dambo Area

Month Potential Estimated Actual Estimated Actively Estimated Actual Et Estimated l-,ctual Et Et (mm) Et as % of Potential Actual Et Transpiring from surface RO from baseflow

(mm) dambo areas as (mill over oatchment) (mm over c<,tchment) ,% of catchment

Jan 110 120 132 20 26

Feb 85 120 102 20 21

March 95 120 114 20 23

April 90 110 99 20 20

May 95 100 95 19 18

June 75 90 ! 68 18 12

July- 75 80 60 17 10

Aug 110 70 77 16· 12

Sept 135 60 81 15 12

Dct 155 50 78 15 12

Nov 150 80 120 17 20

Dec 110 100 110 20 22 -Total 1285 1136 130 78

Table GB : Estimates of Actual Evapotranspiration, Interfluve Area (Cropped and Fallow Area.

Month

Jan

Feb

March

April

May

June

July

Aug

Sept

OCt

Nov

Dec

Total

Rainfall (mu)

224

212

120

63

14

0

0

0

0

9

58

206

904

Potential Et Estimated Actual (mm) Et as % of Potential

110 100

85 120

95 120

90 , 100

95 100

75 82

75 13

110 6

135 2

155 0

150 0

110 100

-. -1285

* Limiting SMO 2 190 mm

RC = Root constant

Estimated Actual Et*· RC = 140 mm

(mm)

110

102

114

90

95

62

10

7

3

0

0

110

703

Interfluve Area as% of catcliinent

75

75

75

75

76 -

77

78

79

80

80

78

75

Estimated Actual Et (nml over

catchment

83

77

86

68

72

48

8

6

2

0

0

83

533

... w •

Table 6e Estimates of Actual EvaE2transpiration, Interfluve Trees

~lonth

Jan

Feb

March

April

May

June

July

Aug

Sept

Oct

Nov

Dec

. Total

Rainfall (mm)

'224

212

120

63

14

0

0

0

0

9

58

206

904

Potential Et (mml

110

85

95

90

95

75

75

110

135

155

150

110

1285 ..

* Limiting SMD = 230 mm

RC = root constant

t

Estimated Actual Et as % of Potential·

105

125

125

105

100

100

43

8

3

1

1

100

Estimated Actual Et* RC = 200 mm

(mm>

115

106

114

95

95

75

32

9

4

2

1

110

763

Interfluve Area as %of catchment

5

5

5

5

5

5

5

5

5

5

5

5

Estimated Actual Et (mm over catchment)

5.7

5 .. 3

5.9

4.8

4.8

3.7

1.6

0.5

0.2

0.1

0.1

5.5

38.2

w .... •

3.3. SURFACE HYDROLOGY

There are nine streamflow stations in the Bua Catchment where the

river and its tributaries are currently gauged (Figure 1).

35.

Resource Units 5D, SE and 5F ,3re drained principally by dambos developed

on the shield surface. The dambo land form is very delicately balanced

and there is a complex interrelationship of climate, soils, vegetation

and base level of erosion. The dambo character varies with small changes

in any of these controlling factors. For example where dykes of more

resistant rock type crop out the valley slopes are steeper, the dambo

cross-sectional area is reduced and as a result the flow velocity is

higher and a more defincd or evcn incisecl channel may occur. Balek and

Perry (1973) give a generalised consideration of dambo form and evolution.

The genesis of dambo landforms is complex and further study of the effect

of climatic conditions, landform evolution and anthropogenic influence

is required. Agnew (1971) argues that the recent stripping of vegetation

for cultivation which had been present on the plateau for most of the

Quaternary is likely to result in major changes to the hydrological cycle.

It is possible that the transpiration and surface protection will have

been reduced significantly and as a result runoff and erosion increased •.

This could lead to incision of well defined channels, faster flow and

possibly drainage of some dambos. Incision of some dambos is also

occurring towards the rift valley escarpment where rejuvenation is taking

place due to tilting or upwarp (Agnew, 1971), and the dambo form appears

to be very unstable. The complexity of the nature and ~haviour of . ~.

dambos is not yet well understood and a detailed study of their hydrology

and interrelationships with groundwater is required.

The dambo hydrographs often have a very distinctive "square.iI shape

(Figure 11) especially where the catchments are large. The rising

limb is very· st""p., "ossiblv because early rai)')fall over the large

area of the dambo (20 % of the land area for much of the Bua catchment)'

and limited idfilt~ation into the dambo clays (due to their low

permeability and negligible storage) resu]c':s in a 'steady and fairly

rapid flow build up at the gauge., The size of the catchment acts .to

"buffer" the peaks of irregular rainfall and isolated storms and this

together with the dampening effect of dambo vegetation and gentle

slopes retarding overland flow results in a flattened hydrpgraph

with none of the peakiness that o0ulcl be expected, and is obtained

in catchments with few dambos. It is possible that there is temporary

Figure 11:

125

..,. v

'" £ 75 S '-' 0)

e' So

~ 2.S ~

0

\00

RIVER HYOROGRAPH

... ,. ".~;

STATION 501 1964/65

A Separotions on normal scale'·

- i.cttAI cli~5e n~~ ____ 9(()IA.VI(:\WG.\:e;r cl.i~e I-\~d->'O~ ~ 1:>000~\oW s~()'"'

.. ...... h~0!3~ of .f,ve ~ Mi"i""",

-.-.-~ ¥~og"',¥h

~ ----1---."' ---- ----." ----- -----

I I \

B Semi-log base flow separation

i \0 __________________ L ________________ -=-_

- ---~

t "6 0·\ r-----.J

No" .la" Feb

1%5

36.

bank storage of water in the dambo clays during the rains. Following

the end of the wet seaSon the temperature is high, relative humidity

falls and the high evapotranspiration over the large dambe area results

in a sudden decrease in flow rate, as the dambe drains. Low flows

are maintained in the dambo for a period of several months with

seepage occurring around all dambo headwaters, probably sustained by

groundwater discharge, although it is possible that some of the base flow

is from inter flow and temporally perched water tables. Subsu'rface' flow

towards the valleys is maintained throughout the year which ensures that

large moisture deficits do not build up in the dambe area. However the',

contribution to surface runoff is considerably reduced by high evapotrans­

piration and eventually river flow ceaseS. This is only a tentative

explanation of processes controlling dambe hydrograph form, and their

behaviour is not fully understood (Grey, 1980).

Discrete groundwater discharge points in the dambe areas may be marked

by 'salt licks' where animals are attracted by the salts which have

precipitated on evaporation of groundwater. Seepage lines are commonly

marked by a line of termitaria parallel to the edge of the dambes, as

termites like to be able to reach water but avoid waterlogged conditions. '

As groundwater levels rise, seepage is thought to occur through fossil

termitaria, which,may have punctured confining clay layers. Discharge

may occur along the top of lated te ZO,','::8 where these have temporarily

perched groundwater 1 these ar,e generally associated with fluct'uaUons of

water level but some fossillate!:ites maybe found higher on the interfluves.

Stream hydrographs from smaller catchments tend to show more flashy

responses to rainfall because there is less dambe area for temporary

ret.ention of the water. Hydrographs are also flashier near the

uplands, where the slopes are steeper and dambes comprise a smaller

proportion of' the catchment area. This, occurs in the Mtiti catchment

(gauge 50 3) which drains part of

waters of the Bua (~E 6 and SE 2)

the Oowa Hills, and also the head­

close to the Mchinji Ridge, and the

upper reaches of the Namitete (SE 1) near the Ozalanyama Range. At

all the gauging points there is little or no flow towards the end of

the dry season even near the higher land where rainfall is more

dependable.

37.

The gauge at Chizuma (SC 1) on the lakeshore plain shows a runoff per·

unit area which tends to be proportionately greater than at the plateau

stations. This is due to higher rainfall and possibly lower evapo­

transpiration on the escarpment section. The flow can be rather flashy

caused by rapid runoff from the many tributaries in the steep escarpment,

substantial floods occur seasonally but the river may cease to flow in

the dry season so the flow duration curves are very steep.

3.4. SURFACE WATER HYDROGRAPH ANALYSIS

The gauge at Chizuma on the lakeshore plain gives a record of the total

runoff from the catchment (Table 7A). There is a wide stable channel

with good natural rock bar control, and a single well-defined rating

curve has been used to estimate discharge from stage measurements.

Since there will be little groundwater contribution from the steep

slopes of the escarpment detailed analysis of hydrographs has been

restricted to those on the plateau.

Data from the gauging station &t Bua Drift (5D 1) have been analysed

since this monitors dambe discharge from the whole of the plateau area.

The station has a control rock bar and the record from 1959 is considered

to be good except for 1970/71 and 1974/75, where total discharge and

low-flow gauge readings are unreliable. The inconsistent rainfall­

runoff relationship meantioned by Chilton (1979) is to be expected

,since eimple rainfall-t:unoff m~ls can rarely reflect the complexity

of conditions within the catchment.·

The hydrographs have the distinctive ·square" form "tyPical of large dambo

catchments and the flow-duration curves are very steep as a .consequence

of the very large annual range in discharge rate (Orayton et al, 1980).

Data from the gauging station on the Bua at Kasese (SO 2) have also , been analysed as there is a good rock control for high flows and a

sandy bed for Iml flOl'5, and the whole record' is considered to be

reliable.

38.

Table 7A

Rydrograph Analysis SC 1 (catchment area 10600 km2j

Water Year Average Catchment Total Total Rainfall Ra infal! * (mm) Runoff Runoff minus runoff

(mm) (10"m') (mm)

1959/60 970 35 373 935

1960/61 1,340 68 721 1,272

1961/62 1,310 133 1,412 1,177

1962/63 1,250 143 1,510 1,107

1963/64 1 ,0110 114 1,204 966

1964/65 1,250 82 870 1,168

1965/66 840 48 509 792

1966/67 1,100 46 489 1,054

1967/68 890 18 185 872

1968/69 1,360 B3 882 1,277

1969/70 980 33 349 947

1970/71 1,460 192 2,035 1,268

1971/72 1,150 59 624 1,091

1972/73 1,080 64 675 1,016

1973/74 1,250 158 1,674 1.092

1974(75 1,010 80 849 930 ".

16 Year Mean 1,145 84.8 897.6" 1,060.2

* Estimates as used by Dray ton et al (1980)

Computer-generated hydrographs are available on both arithmetric and semi­

logarithmic scales; tnese were used to obtain an estimate of groundwater

discharge. On a semi-l09 plot, the straight line portion of the falling , limb represents the g.round"Jater recession. From a backward extrapolation

of this line to a point just after the total hydrograph peak a continuation

of the falling limb of the groundwater hydrograph can be produced. This

can be replotted on the arithmetic scale and the rising limb drawn in by

eye (Figure 11).

Annual groundwater discharge can be estimated by measuring the area under

the groundwater hydrograph. Rainfall, total runoff (derived from monthly

flow summaries)and groundwater discharge for the catchments to 5D 1 and

39.

5D 2 are shown for the period 1959/60 - 1974/75 in Tables 7B and 7C. The

data is all. expressed in millimetres per unit area of the catchment to

allow a water balance to be made, but it should be recognised that surface

runoff, groundwater discharge, and evapotranspiration will all vary from

area to area within the catchment.

Table 7B

Hydrograph Analysis 5D1 (catchment area 9410 km 2

Water Average Year Catchment

Rainfall (mm)

1959/60 805

1960/61 986

1961/62 1,095

1962/63 1,059

1963/64 805

1964/65 933

1965/66 707

1966/67 850

1967/68 698

1968/69 1,058

1969/70 680

1970/71

1971/72

1972/73

1973/74

1974/75

16 Year

1,100

763

788

1,095

1,039

Mean 904

14 Year Mean (omitting 1970/71 and 1974/75) 880

Total Runoff

(mm)

42

69

153

138

99

71

38

28

8

80

30

171*

67

24

140

52*

75.6

70.5

Total Runoff (10"m')

397

653

1,438

1,299

933

668

353

260

91

757

277

1,608*

631

225

1,325

490*

711

665

Estimated Groundwater

Discharge (mm)

16

11

28

38

12

15

6

2

27

41

'. 23

·6

37

10

17.8

16.8

Groundwater as % -rotal

Runoff (nun)

38

16

18

27

12

21

22

22

24

33

22

24

35

25

26

19

24.0

24.4

<Or iver gauge readings suspected to be unreliable

Rainfall Minus Runoff

(mm)

763

917

942

921

706

862

669

822

690

978

650

929

·696

764

955

987

828

810

Recession Constant

!(

0.911

0.981

0.984

0.984

0.993

0.989

0.982

0.984

0.911

0.972

0.979

0.992

0.975

0.981

0.985

0.984

0.9824

0.9816

40.

Table 7C

Hydrograph 'Analysis 50 2 (catchment area 6790 km')

water Average Total Total Estimated Groundwater Rainfall Recession Year Catchment Runoff Runoff Groundwater as ,% Total minus Constant

Rainfall (mm) (lO'm') Discharge Runoff (mm) Runoff K (mm) (mm) (mm)

1959/60 765 24 164 8 31 741 0.971

1960/61 1,089 61 417 10 16 1,028 0.982

1961/62 1,144 155 1,053 31 20 989 0.984

1962/63 1,149 123 833 32 26 1,026 0.984

1963/64 828 99 669 7 7 729 0.994

1964/65 997 81 549 12 15 9Hi 0.986

1965/66 722 42 288 9 21 680 0.980

1966/67 841 27 184 6 21 814 0.985

1967/68 744 7 48 2 29 737' 0.977

1968/69 1,222 110 750 34 31 1,112 0.976

1969/70 678 36 247 7 20 642 0.974

1970/71 1,023 164 1,113 21 13 859 0.986

1971/72 805 55 375 13 23 750 0.978

1972/73 829 17 114 3 18 812 0.987

1973/74 1,185 158 1,072 34 22 1,027 0.980

1974/75 917 72 492 12 16 845 0.982

16 Year Mean 934 77 523 -.15 20.5 857 0.982

The semi-log hydrographs have been used to estimate the recession constant

(R) for the groundwater component using Barnes equation:-

Log K = Log Qt - Log Qo

t

, Wher'e Qt = discharge at time t

Qo = Ini tial discharge

The recession constants obtained are similar from year to year but relati­

vely low for groundwater contributions. This rapid recession of water

levels may indicate that the discharge contribution is from interflow or

perched groundwater. The effect of high evapotranspiration in the dambo

will also have the effect of increasing the rate of recession.

41, ..

Usually about 4 - 6 weeks elapse after rainfall commences before signifi­

cant flow occurs in··the dambo. The period of surface flow after the end

of the rains appears to be related to the length of the wet season rather

than the amount of rainfall although t.hIs effect ",ay be masked by other

factors. Usually about one month elapses after rains have ceased before

the drainage from storage on the inundated dambo surfaces is complete,

after which flow is likely to be entirely groundwater.

The total runoff can be broadly related to the annual rainfall over the

catchment, although the latter can only be estimates at best.

Hill and Kidd (1980) derived linear regression equations relating these

variables but using only two or three rainfall stations:-

Annual yield for 50 1 (mm) = 0.27 [Annual Rainfsll (mm) - 584)

Annual yield for 5D 2 (mm) = 0.24 [Annual Rainfall (mm) - 5981

with correlation coefficients of 0.89 and 0.77 respectively. They suggest

that a refined equation taking into account the" increased evaporation

associated with dambos might be more appropriate.

The groundwater component 0;: ~",. :cc,,,," ",,,,,off estimated using the .conven­

tional method described, appears to be mainly in the range t5- 30' of

total. flow. The groundwater discharge is variable with a mean oflS mm

and fa mm over the catchments to 50 2 and 501 respectively assuming no

significant Qhahges in gt"ouncl't'7ater ~to!,,~cre~ This ::"e eqUivalent .to ,

approximately 1.5 - 2% of the mean annual rainfall. . ~

These estimates of gr.ound,,'Ja( er. discharge and; ts proportion of surfaoe

runoff are low, and the method of analysis used for theil:' derivatlon

is considered to be too sill':olistic. The very high evapotranspiration

at nearly, or even above, p,)tential rate which OCCUI:'S in the dambo

areas during much of the ye~r is not tak~n into account in the conven­

tional ~nal.ysis ">">;0:: :Ch'" . 'y:'ve. It!.s pnsgib!.e :';-.)'0 " significant

proportion of tlhe gr?undwater dis.oharged upstream of the river gauging

station is evaporated 01:' transpired to the atmosphere before :it r.eaches

the gauge because of th" \1( .:y slcvl,river flo,,' ,:atc"s and the large

surface area of the dambo. This effect will be more marked in that

period after the rainy season when runoff is almost entirely ground­

water. The groundwater contribution to stream flow observed at the

gauge could thus represent only a residual proportion of the total

groundwater discharge and could give cc" erroneously emall idea of

,

42.

reobarge over the catchment, It should however give some idea of the

millimum recharge, An attempt to obtain an order of magnitude estimate

cd! grounc'{!mt€'t dlsch:-:=90 (and by implication recharge) is made in the

gr.oundwater. ba.lanc;e calculations (Section 3.5).

1

1 :::i;:::~:u::c o:r:::::~::~ ~:::::e~:s (::::1:~:: ::d::~i:~C:~e m:::.::ver

/

1 flow using the mi.nima of successive non-overlapping five day periods.

Where the minima are less than 0.9 times the adjacent points they are

used to construct a hydrograph which for UK catchments seems to provide

a valid estimate of groundwater discharge. However for the catchments to

Sf) 1 2inc1 5D 2 the gi:ounowc:.":t:r discharge est"J .. Hlated by t:his method is con­

siderably greater than th'at derived by subjective baseflow separation and

it is likel.y to include much of the water which is temporally stored on

the dambo surface ana released slowly (Figure 11). The average Base Flow

I IndIces of 5D 1 and 5D 2 are 85% and 86% respectively which are considered i I' to be much greater than the groundwater contribution to total flowl this

lY;0thod of analysis is not recommended for dambo catchments in tropical

climates ..

3.5.1. Int.E.9.duc~

Usi.ng hydrological and meterological data an attempt to pt;oduce a water '.

balance can be maae using the general. equation:.

p :;; R j. R + E ± S -± Sg ± SMD s g 1£ s'

l<1here P ~ precipitation

R -s surface runoff .. R ~ grounawater aischarg~ g ,.

r. ,;C ;,J('J; 1 €\?apot:r aris~)l r ,~I !~~ i·:)I'l

S = cniJnge in surface storage s S = change in groundwater storage

9 Sbl:J 'Cl-.", ... ';I';';'· . in soil moistUi.e defioits

Assuming a sufficiently long period of analysis the changes in surface

and qrotmd"ater stor;1lge and soil moisture deficit can be considered to be

negl.igible ana the equation reduces to:-

p =

43.

However it is clear from the preceeding sections that determinations

of ground>later dJ.scharge from hydrograph separation could be, under­

estimates of l:'GC!~'{E.o::'0e p.r:.1 that tht:: Sp&t:t:;,.1 ~.r~.t'iation of actual evapo~

transpiration could be considerable. A simplistic consideration of

the water balance would thus be misleading; An attempt to produce

a possible model of dambo hyClt·ology processes (shovm in Figure 12)

and order of magnitude esti.matGs for the components within the

hydrolog~cal cycl.e helve been made (Section 3.5.3).

3.5.2. ~he Oambo M~~

Dur ing the wet S0i:.iSOrl.. t'2cba;:ge from prec,ipi tation occurs to the

weathered basement aquifer OVer parts of the interfluve area of the

catchment, once the soil moisutre deficits have been satisfied.

There may also be scme temporary perching of water, for example on

laterite layers, and interflow towards the valley bottoms.. Evapo~

transpiration occurs at or above the potential rate all over the

catchment becuase of interception on vegetation surfaces. The inter~

ception effect will be greater on trees and shrubs than grass because

the leaf surface area presented is greater" Rapid surface runoff occurs

over the interfluve area because of the high intensity of tropical

storms and the low infiltration capacities of the soil. In the

valley bottoms. the groundwater level is at or near the surface, and

overland flow occurs due to saturated soil conditions (although infil­

tration capacities are also low in the damboareas). Baseflow contri­

butes' to the total runoff; it..is possible that this is entirely

groundwater discharge but there could also be some component of inter­

flow. Evapotranspiration from the dambo area during the wet season

is considered to occur from the surface runoff alone, a~d losses from

baseflo~l are tai<en to be negligible.

After the wet season surface runoff ceases, the storage in the dambo

grasses drains, the discharge rate falls rapidly and dry season flows

are maintain4d by.baseflml. These are affected by evapotranspiration

which has a very significant effect over the 'large dambo area.' Any

observed runoff is therefore only residual baseflow. Eventually the

evaporative'losses keep pace ~/ith the baBeflow discharge so there is

no runoff from the catchment until the following wet season. Actual

evapotranspiration rates are conI) i.d~red to reduce during the dry season

because of the build up of soil moisture deficits. However, they

remain relatively high in the dambo areas where moister conditions

Wet Season Figure 12 DAMBO MODEL

Evapotranspiration from surface runoff at or above potE,nloial rate

Precipitation Evapotranspiration at less than potential rate unti I soil moi s tUre deficits restored

Runoff from catchment .~-I

Dambo Surface runoff Interfluve

isurface runoff plus baseflow) Interflow

ground water discharge

deep groundwater percolation

Dry Season

Evapotranspiration. of baseflow at less than potential rate with build up of soil moisture deficits \ . in clays

Runoff from __ -I catchment

Dambo

. residual baseflm1 ) Interflow

recharge

Evapotranspiration at potentio rate reducing to zero as largl soil moisture deficits build UI

Interfluve

groundwater discharge

deep groundwater percolation ~--------~~--------~----------------~

44.

are maintained by groundwater discharge, and the dambe grasses .emain

green. In the interfluve areas actual evapotranspiration rates reduce

rapidly as, moisture availability falls and the crops ripen. After crop

harvesting, evapor,~ tion fro!:'! the bar~ 805.1 and transpiration from the

remaining scrub vegetation is considered to be much reduced because of

the large soil moisture deficits and negligible active plant growth.

It is poss ibIe that there may be a component of groundwater uridt,r'flow

through' the weathered basement aquifer and/or deep percolation via

fractures in the bedrock.

A water balancH hc{i::) been attempted using the c1ambo model as described

abeve using meteorological and hydrological data typical for the

catchment to the gauging station at 5D 1. This area forms a large

natural geographical unit typified by dambo drainage on the plateau

(see Section 3.5.3).

Aerial photographs and satellite images show the dambe are to be 20%

of the catchment. Ground observations and satellite images taken at

different times of the year show that the area of actively transpiring

green dambo grasses reduces slightly, to perhaps 15% on average by the

end of the dry season. Conditions vary from dambe to dambe depending

on many interrelated factors including the bedrock type, depth of

weathering, soil character, slope vegetation types, climate and lOcal , moistur~ conditions within the dambo.

3.5.3. Catchment Water Balance and Groundwater Resource Evaluation

A water balance has been attempted using hydrological data presented

in Sections 3.1 - 3.4 and a summary is given in Table 8. It'must be

noted that there are errors in estimating each hydrological component

of a water balance I the groundwater discharge estimates should, thus,

be treated with caution as they are a relatively small component which

can easily be iost vithin the margin of error of larger components

(e.g. rainfall, evapot.ronspiration). The i.mbillance of the equation

is therefore acceptable, although it could reflect a component of

groundwater under flow through the wea'thered zone or deep percolation

through fractured bedrock.

The estimates of actual evapotr?M,c.piration are slightly less (779 mm)

than rainfall minus runoff for the catchment (828 mm). This could be due

to an underestimate of actual Et or groundwater underflow as mentioned above 0

Table 8

§.~§!E,Y_C:f. AV$E.0..~..J~X5~L.(),L~\))·aal, f.omR2n~

~!i~:.!-!:_:,,""Ti~I,~'J;:~£.'!~'lfQ~. to 5D 1

Anmlal rainfall (P)

Total runoff gauged at 50 1

Rainfall minus total runoff

Residual ground>later discharge at 50 1 (Rg) (from hydrograph separation)

Surface runoff at 50 1 (R ) s

Estimated actual E (averaged over t '

\1hole catchment)

Estimated actaul Et from dambo bet>leen June and November

'"

'"

'"

-

'"

'"

'"

904 mm

76 mm

904 - 76

18 mm

76 - 18 '"

208 + 53J

78 mm

'" 828 mm

58 mm

+ 33 '" 779

Possible maximum ground;'1ater dischD.rge '" t'E:'sidual qroundwater +

mm

Et from dambo 1n dry season

Water Balance equation

'" 18 t 78 '" 96

RS + Rg + Et ± imbalance

904 '" <;A ... 1 R + 779:1: imbalance

p =

mm

Therefore imbalance '" 49 mm

The groundwater recharge is difficult to determine with any accuracy

witho~~~: fu:rt::i2f ,:'2;-,':,;.1E:c monit,oring of dambo catchments~ At minimum

it "ill be represented

from simple hydrcgraph

by the residual ground"ater discharge deriVed

separation (18 mm), but i,t could be .signifi->,

cantly greater due to the evapotranspiration of baseflo~ in dambos

upstream of the gauging station. Total ground"ater discharge could

45.

in theory be' as much as 96 m;n if interno" is negligible and the evapo­

transpiration from dambos is not derived from storage in the .clays.

Since base flow extends for a period of several months into the dry

season it i-s suspecte(~ ~':h:?}: 9l:"o:.lnd~rater is sllstnining t.he bulk of the

flow and that interflo" is not a major component, other"ise flow would

cease soon after the wet season. It should be noted that the aquifer

properties (e.g. transmis'l;.v;ty), hydraulic g::',vl:lel'f. and surface water

drainage pattern are such that the annual groundwater discharge to

surface "ater (and hence recharge) is unlikely to exceed 40 mm (see

Section 2.4). The average annual recharge is therefore more likely

to fall somewhere between this limit and the minimum estima~e from

simple hydrcgraph separation (i.e. 'J j ." i,O nun).

It should be noted that conventional hydrogra.ph analysis of the

River Livulezi (Y7&ter Resource Unit 3) and Ri.ver Lm.elezi (Water

Resource Unit 6) suggests a groundwat0t' component comm.only of 60 -

46.

80 mm per unit ci:cea. :(8p.ceH~~'r{i.:ir:'J br.uund 30~t of the total discharge.

1'hese catchments are similar to the Bua Plateau in terms of under­

lying geology (l<1eathered ba.sement) and climate, but the principal

difference is the presence of ~lell defined river channels and few

damboa. The larger groundwater cmnponent obtained from simple

Hydrograph analysis for these catchments corroborates the suggestion

that a significant amount of groundwater discharge from catchments

largely drained by dambos could be evapora~ed upstream of the gauging

station. There ~lill hOI,'ever be a component of recharge from infil­

tration of ~Iater from river beds into the outwash material at the

base of the escarpment in the Livulezi Catchment.

The alternative methods of estimating temporary replenishment resources

in the plateau area discussed in previous sections are insufficiently

preci.se or are not cons idered to be very reliable. For example the

analysis of ground~later ::'c,ve: [:," :,.,:,'cicms suggests that recharge ia

likely to be 13 - 35 mm but a lack of knowledge of specific yield and

sufficient hydrograph data mean that the estimates cannot be better

defined. The flOw net analysis is hampered by evaporation of ground-

geological complelCi ty; this method is thought to be inaccurate' and

a larg~ underestimate. Infiltrometer tests and' poorly draining soak­

away pits suggest that ),e' :1arge is slow but the groundwater quality

confirms that it is lil<el,/ to b2 recent.

The permanent resources oan be evaluated in terms of aquifer.' geometry

and physical properties. Bm1ever, the lack of detailed knowledge of

speci fie yield ard v~,,:;: :L. l,(j?l :t n saturat~~t:~ ~-l.,: 0\' ~':'::: :.! pr<?clude an~l

accurate estimate of permonent resources.

In the escaprment ;':;2:)tic:'~ C;-.2 ·Bua catch. ... .:. ...... "L.:.2 ~_·oundwater contri­

bution to the' river dischnrge is expected to be negligible because the

weathered zone is very thin or non existant. There could be a small

contribution from fractured bedrocl< but this will be minor. On entering

the lakeshore plain, the rivers may lose some or all of their flow thus

recharging the alluvium where P':"""",,;Jilities are sufficiently high.

The reverse could occur nearer Lake Malawi with groundwater discharge

47.

from the alluvium to the river although there is inusuffioient data

to show this. There is likely to be some groundwater underflow through

the alluvium discharging into the lake (Mauluka, 1983) and possibly

some deep percolation ill fissures in the underlying bedrock; these

are difficult to quantify but are .likely to be small components of

the hydrological balanoe considering the narrow portion of the lake­

shore in question.

"

48.

4. GROUNDWATER DEVELOPMENT

4.1. EXISTING WATER DEMANDS AND SU~~

The present dem2ua fo;~ <;.<,"ater j.8 1ars,:.::.y for aCl'l10stic supplies.

Agricultural consumption is small, though this has increased with the

rapid development of commercial tobacco estates which require water

for nurseries and planting. This is usually provided from dams and

rivers, though some is obtained from boreholes. Water is abstracted

from the Bua River for· the irrigated rice scheme on the lakeshore

plain.

The rural population of the oatohment (estimated at 524,000 in 1977)

present a total water supply demand of 5.2·x 10· mS/year at a design

consumption of 27 l/head/day (although the existing actual consumption

per capita is estimated to be only 10 - 15 l/head/day). The projected

1990 rural population of .786,000 will require an estimated 1.8 x 10·m'/

year, again based on a design consumption of 21 l/head/day.

A small proportion of these demands are met by a gravity fed piped

water scheme taking ''later from springs on' a tributary to the River Rusa

on the North East side of the Mchinji Hills. The network of reticulation

pipes with taps serve an area of 400 km' for a design population Qf

20,000. The scheme has a design discharge of 200,000 m'/l~ar

(21 l/head/day) and the estimated present consumption is 125,000 rn~/year.

The scheme was completed in 197fi and was largely construc~ed by self­

help labour from the villages. It.has operated successfully apart

from some water shortages felt at the far ends of the pipe network in

the dry season, and some pipe replacements have 'been neoessary because

of poor installation. Tt.ere are virtually no other areas w.tth protected

upland sources and perennial river flO\qs whioo. would be suitable for

further piped water !Joh",nes apart from some springs in the Pewa Hills

\,hich could possibly supply small areas.

In 1981 there were about 700 bor.eholes and 350 prctected shalloW wells

over the plateau providing clean, ·safe but untreated water. The

protected shallow I.ells, equipped with shallow lift hand pumps have

all been constructed in the past decade and are concentrated mainly

in Pewa and Kasungu districts. Boreholes are found all over the plateau.

They are mostly equipped with hand pumps although about 100 have

motorised pumps, these large'lY being found on estates or at institutions

49.

such as schools or health centres. On average there is one protected

water point in every 10km'. There are only about 25 boreholes in

Resource Unit se because of the sparce population and unreliable

groundwater yields.

Each borehole produces on average perhaps 700 l/hour and 5,0.0.0. l/daYI

a protected dlJg well ",ould produce about half this quantity. The total

groundwater abstraction for rural 11ater supplies is thus in the order

of 1.6 x 10.' m' /year, the abstraction from the escarpment and lakeshore

areas being v"ry minor. The annual abstraction from the plateau area

is equivalent to 0.2 mm over the whole area which is considerably less

than the available replenishment resources by whatever· method reoharge

is estimated. There is therefore considerable scope for further

development of groundwater for rural domestic demands without a danger

of depletion of permanent stock resources.

Each bore hole is estimated to serve perhaps 350. people at present

(each abstracting about 15· l/head/day) and there are often very long

walking distances of several IdJ.W'~~~e.s i.,wolved. It is clear that

the existing \later points are insuffioient and too widely spread to

serve the dem,md for rural domestic supplies, and as a result many

unprotected sc>urces are used in addi Hon. The water taken from

unprotected wells i ~pr~],gz ~ d2.nIs and r:l.vers is a health hazard since

it can transmi.t waterborne diseases such as cholera, typhoid and

dysentary. "

It is estimate'd that a total of only 2.2 x 10.' m'/year of clean, safe

water is consL'med by villagers at present. ..: Less· than half of.

the el!isting population have access to these protected suppl:!:es and

the long walking distances may result in inadequate quantities of

water being c~'llecte(L. Th5.,·. '.ea"es a t"equirem(mt of '" f"rther

5.6 x 10.'m' /year which ,/ill be necessar.y to serve the 1990 population

at the design cbncumption of 27 l/head/day.

The district centres and some sub-c~ntres are supplied with water by

the Urban Water Supply Branch of the Department of Lands Valuation

and Water. These are served by ground\~ater from boreholes equipped

with motor pumps or by abstraction from rivers. The storage reservoirs

are linked to reticulation system~ 1:;".C;1 c;llpply to standpipes serving

several houses or to individual houses if the owners can afford the

50.

connections, A supply of 70 to 100 l/head/day is made in urban areas

but the service does not always extend to the entire population of

the centre. An estimate of the average annual consumption is given

in Table 9, Hchinj i 'co,m used to be served by direct abstraction frem

the Bua River but since the 10~1 f10~1 was unreliable the town now

depends on supplies piped from springs to the north-east in the

Mchinji Hills where flow is perennial. The Water Supplies Branch are

also responsible for some institutional supplies 'and their maintenance

(for example, secondary schools and health centres) and they supply

water kiosks in Mchinji as well.

Location

Mponela

Kabudula Health Centre

Kochilila Health Centre

.Mchinji

Ntchisi

Type of Source

Boreholes

Borehole

Large Diameter Wells

Springs

Ri,ver

Total

Table 9

Urban Water Supplies

'.' Borehole Numbers

5044 (A41)

Estimated Annual

Consumption 19B1 ' (m'/yr)

54,000 SDttl8(SM1S6i 5D224(W11~)

5D212(W32~) , " 5D223 (Rl'!'2g),'

SE 49 (W155) 5,000

-, 12,000

50,000

33,000

154,000

4.2. GROUNOWATER ABSTRACTION METHODS

Estimated 1982

Population served

4,200

150

300 -

3,500

2,000

Estimated Average

Consumption (l/head/day)

70

100

100

70

70

Where the depth to groundwater is 'shallow (less than 3 m) 1n the dambo

margins the most appropriate method for abstraotion for rural suppli~s

is hand dug wells. Where grou .. ,,::',,':ctc>: levels are deeper, drilled bore­

holes 'offer the only solution for seasonably reliable water supplies.

51.

4.2.1. Boreholes

Boreholes have been drilled from t.he 1930's onwards using percussion

methods. ~Jany have been drilled since 1970 under a dispersed proqramme

at villages selected throuqh District Development Committees and funded

by the Christian Services Committee of the Churches of Malawi.

Resistivity surveys have been routinely used to locate all the sites.

However, the reliability of these surveys in predioting depth to

bedrock and groundwater levels is oomplicated by the presence of

laterite which gives anomolously high resistivity and/or graphitic

bedrock causing 1011 resistivity anomolies which may be oonfused with

a thiok weathered zqne. Other geophysioal methods for groundwater

exploration in Malawi have been investigated (Carruthers, 1981) but

no completely reliable method has been found. He found that maqneto~

meter surveys are complicated by the presenoe of magnetite and ferro~

megnesian minerals; the changes in depth of weathering tend to be

masked by variation in bedrock mineralogy. Seism~c refraction aurveya

were hampered by heterogenous conditions, the laok of well defined

layering and no simple bedrock refractor", Electromagnetic (EM) surveya

may be able to pick up fraoture traces, but the presence of graphite

and surface conductors will tend to have overriding effects. EM

surveys might be a cheap and rapid method of determining areas of

shallow bedrock and help to avoid abortiveboreholes in marginal areas.

The maj6rity of the, boreholes are' poorly designed with very high

construction and"maintenance costs. ' Most of the boreholes are 40 m

or deeper often reaching well into fresh bedrock. They are all

completed with imported steel lining (mainly 150 mm diameter), which . is usually slotted for the lower third or half.. The hacksaw'slots

are widely spaced

very low (0.1%).

so that the open area of the ~creened portions is

In addition the higher yielding levels in the

weathered aquifer are often cased out and groundwater is forced to , pass vertically down'to the fresh bedrock before it,can enter: the

borehole. The head losses are larqe, the entrance velocity of water

is high and the boreholes are qener'ally very inefficient, The low

specific capacities reflect poor borehole design as muoh as a low

yieldinq aquifer. Where there is a gravel pack it,is ,usually very thin

and comprises coarse crushed 9 - 12 mm roadstone, but often there is

no pack. In either case there is no effective filter and sand from

the aquifer is drawn into the boreholes. ' As a result the boreholes

tend to silt up with time. Also there is often exoessive wear on pump oomponents (especially oup leathers in handpumps which need to

be replaced tldcs a year on average), so maintenance is costly and

required frequently.

Most of the e'tist~.ng 1::-ot'2f;;-:.len e.::0 Gcr ... ;ir::rcG Hi tll handpumps J there

are several different types in use but all are imported, expensive

52.

and difficult to maintain. A truck \1ith a winch is required to remove

all the downhole components to enable even the most basic repairs to

be carried out. Haintenance is a great burden on Government and

cpsts have risen to 1(200 per borehol€' pet' year (1982 prices). There

are about 150 private estate, urban and institutional supply bore­

holes which have motor pumps.

4.2.2. Improved Borehole 1)e9,1gns for Rural Wat,er Supplies

Since 1980 a considerable effort by the Groundwater Project and the

Groundwater Section of the Department of Lands Valuation and Water

has been devoted to improving the bore hole designs at the same time

as reducing the costs. This has been achieved by matching an under­

standing of the geology and groundwateroccurrence with the most

economic and appropriate 'methods of abstraction. The improved desiqns

have been successfully implemented in other parts of Malawi with' a

weathered basement aquifer, and are being used in a rural water

supply programme constructing bore holes in Oowa west durinq 1982/83.

Borehole designs have been improved by several changes. The recog­

nition.that the weathered basement is a more important aquifer than

the fractured bedrock below enables. bcreholes to be drilled to much

shallower depths. A minimum saturated aquifer thickness of 10 m is

aimed for, and usually a total depth of only 20 - 25 m would be

necessary. Borehole diameters have been reduced to a maximQm of

200 mm. This makes the use of smaller,:',rigs, v.ehicles and crews

possible, drilling tim~s and costs are both considerably reduced.

Adequate yields for rur!.ll water supplies (0.25 -0,'5 l/second) should

be possible o"er tht;! greater part of the plateau where the weathered

profile is well develop",(J. As a consequen,e 'the need for detai1'ed

geophysical site surveying will be unnecessary except for urban or

institutional supplies requiring higher yields, and possibly for

delineating areas where depth to bedrock is shallow.

Borehole designs ,,"'VB been improved by i.ncreasing the open area of

the slotted screen to 8 % a"cl reducin9 the slot size to 0.75 mm.

53.

The lo\,r0~:" en~:xan('!? velooi ties and correct pl.s:cin::,!, ef '::11'2 scre.';;n results

in increased hydral11l.c effid,ency and improvea yielas. The use of a

locally manufactured ana slotted PVC lining (1 io lMl diameter) is

considerably cheaper than imported steel linIng (150 mm diameter) and

probably results in increased borehole life due to its inert nature.

It may also reduce the problem-of hi9h iron concentrations in ground­

water which could be partly associated 11ith dissolution of steel

lining.

The use of a correctly graded gravel pack has also improved hydraulic

efficiency and reduced the inflUl( of sand into the boreholes. Surveys

have sho,m that Lake t"I,,1,,",1 beach sand at: several. locations has the

ideal grain size distribution (0.7 - 2.5 mm diameter). A thicker

pack is achieved by inserting smaller diameter lining (usually 110 mm rather _than 150 mm) ,rith a drilled diameter of 200 mm.

Borehole surrounds ate «ell ,,~'"'''., .,,,~c,_, with brick based concrete -

aprons ana channels for the drainage «ater. Soakaway pits are dug

at distances of at least 5 m away from the borehole to avoid surface

pollution of the groundwater. It i3 intended that eventually rural'

communities. To enable this a hundapump has been developed in

Malawi with ease of maintenance bPeing a major design feature together

with 10'1 cost and the po' ";nth). for loc-11 rcanufacture •• Repairs or

servicing will be possib,e by hand and it is intended that spare

parts made local1y \\.Ul 'Je available at village stores. The frequency

of handpump repairs requ'red «ill be considerably reduced with the

construction of impl'OV{2d design boreholes."

4.2.3. pug Wells

Shallow hand dug \.,'.:211-8 &J:'f: iilOS'i: c~ppr:Cp'.::i.Ed=:G [er t'ural ~\~ater supplies

in areas \-There tht'? 'v,. Sfc)undwater L~ ~,_;""~"_,,,,, ;,"1> Yi~lds are

lower than for boreholes because of· the low permeability and they

cannot serve as many people. To avoid- pollution of the water supply

the wells are protected by coverin9 the top, lining -the sides and

installing a shallow lift handpump. Some of the ~l<3lls in the plateau

area are also backfilled for ,,,;6,_.~_ "c0tC!ction from pollution, although

54.

this makes access difficult i.f the handpump fails or the well requires

deepening. The protected \~ellG have 1:>2en constructed since 1975 usinq

self-help J.abour from the villages involvedll ThG~ vIGIls ate dug to a

sufficient depth (gen",ral1y 3 - (; co) to maintain a relaible dry season

supply. A minimum depth of 3 In of water in the well is desirable.

Construction ia carried out in the dry season wherever possible so

that groundwater levels are as low as possible and the pumping required

to keep the well drained is' kept to a minimum. Locally made shallow

lift hand pumps are being developed for the wells programmel it is

intended that these could eventually be largely maintained by the

community.

4.2.4. Integrated Projects

The DDC Programme of C;dlling dispersed b'j~eholes has suffered from

a lacl< of planning, and management, inefficient. use of equipment

and from poor supervision" As result the boreholes have very high'

construction costs as well as being poorly designed. The concept of

"Integrated Projects for Rural Groundwater Supplies" has been developed

by the Groundwater Section of U',,, i)"partment of Lands Valuation and

Water to provide more efficient low cost water supplies (Groundwater

Project, 1982). An Integrated Project aims to provide complete

coverage of an area ~lith water points within a maximum walking

distance of 500 11,. "'L~£ iG ""C::.:·:·,: :...". ~'::·,"':"::':'i,;;"i:.ing existing

boreholes, protecting suitable springs and existing dug wells and

construeting new, better designeot boreholes or wells •. The'yields

required for rural dClin(i;s d.o use are ,small so geophysical. surveying

is generally not necess5ry to choose the new sites.

Aer ial photographs are used to locate areas t<hich should be "voided

where BedrOCk outcrops, and to pick out possibl.e fracture traces.

They are also' U$GO to i.~.l~a,-8 'v..;.11ey OO ...... OiU 8itlit$ where the depth to

groundwa ter is less so tha t wells are dug in these areas and bore holes

drilled on the'inter.fluves. The community involvement is maximised

with the villagers 'choosing sites as far as possible and providing

self-help labour. Because borehole drilling is concentrated in one

area the transport costs can be considerably reduced. A low-cost

borehole with a handpump in an Integrated Projeot costs K1,500 (1982

prices) which is appro~imately one quarter of that for an old design

borehole in a dii,spersed programme. 'rhe design consumption is 27 1/

head/day~ a borehole is oonsidered to s~rve 250 people, and a ,

55.

protected shallow I"ell to s","ve 125 people and constructed for half the

cost (1750 in 1982).

An In.t0~;rcated Pro:Jccl; C~YVei.'lng I,,'d£'C tJ): tl1~::: DOII!a West Agricultural Project

Area is currently unc1en,ay and aims to serve '60,000 peopleoduring 1983

and 1984. All the ne\~ly constructed boreholes will be of low cost

improved design and locally produced handpumps will be installed

throughout the area.

4.3.1. Rural Wate,r SURJ2li.9.':.

It is clear that :Ct.1r.tb0)' r:::~ol1ndwater cl$\1;.:-lc:?r.v3:nt. 1s required to meet

the demands of the projected 1990 rural population of about 786,000

(see Section 4.1). More ,rater points are needed to supply an adequate

amount of protected "Iater within a reasonable walldng distance (1&S8

t.han 500, m I~herever possible). It is estimated that a further 5.6 x

'~""" :;;;tIon will be required"

With the improved 1oo1'l')b010 designs, the yields ",ill generally be suf­

ficient for handpump supplies (0.25 - 0,5 1/8'Oc) over most of the

require aettdlec1 site surveying- ei-!cept perhaps to delineate those areas

\,,'here depth to bedrock 13 limite\.-a.Q Dug \>12118 can be consttucted in all

areas close to dambn nib_"'- .. ; n:~ \:'Jl~'~re the f·'al.~("r ":atJ"lf: is ahallo\".. The

t'ur.al village demands (":r'> gr;'oundlivat·2:,r &r-t~ relatively small and can

€:asily be met ,"'d lhout d~. pletlcn of t'cplenishable groundwat®r resources;

they are well 't,,rithin t.h0. n~charge eDcimates by \<!hatever meth6d they are

calculated. Grouncl"',yt",, yiolds&nd recharge in the alluvial lakeshore

lJo't1"ver, there is 1i ttl.v scope for gr:oundvmter development in the

(~!3catpme-rtt. ared except ~::or very lex::al cu:'eas,. because yields are

unreliable and aer:,::nJ c l".~·;:;...~secting f(:;-.:.~L.:":"k_O

It is possible that grou:1o>later development in the plateau area by

drilling boreholes on th" Interfluves could be the best way of maximising

,mter reSOUrces by tappit",g rese~ves ~lhich ~lould otherwise be lost by

evapotranspiration in the ~'be effect might be to reCluce

the saturated area on the dambo followinQ the rainy season and to decrease

the length of surface "at('r flol1 into t.he dry season. Anthropogenic inf­

luence via cultivation could cause incision of channels and dambo drainage.

It ,1.s rGCOmHK~'n(}ed tbat 8,n "Integratecl Project il approach is taken to

provide complete CCiVGn:'ige of pe.r.t.icu:tar rural areas. Sufficient

water: f't."i~~~,.::; .t-'~iUL, '.~_l, •. ,.<~., c-·)ns"i::rnctec1 by the most effic:~/::nt,~ c~conolnic and

56.

appropr.: iatc· rn.uthods of: 'V;ate.r supply (> Further agricultural develop-

ment of pa~ts of ~lahinji, Kasungu and DOVla Districts is planned under

the National Rural DeveLc>pment Programmes, Tt '>JOuld seem appropriate

to i.mprove rural «ater supplies in parallel Vlith this as there are

insLlfficient protected sources serving the community.. Significant

improvements i.n sanitation and infrastructure oan also be achieved

~1i thin these prcgramrnes, thus maximising the impact of development.

4.3.2. ~1 Su.epl~

Locally for small cml",,1 and instl. tuti.onal supplies, the demands per

capita are larger c.I~0 . ',;L',; i.;L-~.:.:ilDtion is mot';.:; ,,;\',;~.c0r~tl:'ated, so more

detailed information is required on local recharge conditions and

Vlhether the permeability of the aquifer could sustain the reqtidlred

yields ",ithout excessive pumping dr/lI,downs. Geophysical surveys and

interpretation of aerial photographs "ill be necessary to help. avoid

areas vlith shallow bedu,ck and to lJcdcE' the most favourable sites for

drilling boreholes where the per.meability is highest and the depth of

~Ieathering is greatest. It is possible that yields of 1 - 3 l/sec

might be obtained on t!1e plateau p but towards the escarpment where the

w ... ~ • .L Le: Hlvt.-e unreliable.

fractured marbles outcrop ':here may be favourable drilling sites.

Yields of 5 l/sec or more night loe obtained from boreholes' 1n the

alluvial lakeshore at-eas ~

Where

Collector ""lls "ith .later ,b drilled from a central shaft might be

another soluHCl1 to provId, larger more re1i.able Yields for small town

.supplies. High€~ yi.elds ,nuid be achieved by increasing chances of

int.ersection of ml.,):Co pE:rl'ii( aO.le zones e.nd providing larger storsg.e

within the v,ell. Collector ,1611 systems would be relatively expensive , to construct and teahnica 1 1.y mor.e, difficult to maintl'lin. Consideration

of each individual site WOJld be required to determine Vlhether it is an

economically viable propos'. tion and whether local recharge could sustain

the proposed abstraction rptes.

57.

<, ~ 3 ~ 3 ~ .!.EE..j~:;;V'l. t;.~

rrhe basement nquifer is unlikely to have sufficient seasonal recharge

n()r high en()ugh tr.ansml.ssivity for the yields required for large

i1'r JS8. tion sohmn28 ~ HOvlCVGl:' f 51';";'-1::' sc~le agt: icul tural plots (say

0.5 - 2 hectares) could successfully be irrigated (requiring yields

of 0.5 - 2 l/sec) using motor pumps).

Yields from the alluvium of the lakeshore plain should be sufficient

for large irrigation schemes where the succession is relatively sandy

and thick (N.S.I.S., 1980).

58.

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Bale 11 , J. and Perry, J .E. 1973. Hydrology of seasonally inundated

African Headwater Swamps. J. Hydrology, 19, 227 - 250.

Balel<, J. 1977. Hydrology and Water Resources in Tropical Africa,

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Bath, A.H. 1980. Ilydrochemistry in Grollndl1ater Development: Report

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WD!OS/gO/20 (Ul1Clllblished) •.

Bellingham, K.S. and Bromley, J. 1973. The Geology of the Ntchisi -

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Caruthers, R.M. 1981. Report on a visit to Malawi to advise on 'the

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-,