GROUNDWATER RECHARGE AND FATE OF GROUNDWATER

STORAGE OF THE WEYBO RIVER

CATCHMENT

WELAYITA-HADIYA ZONES SOUTHERN ETHIOPIA

BACKGROUND

  Most activities come to rely on groundwater resources than on surface water resources mainly due to their sustainability and quality 

The Potentialities of Groundwater resource is mainly factored by the Rate of Recharge

Changes in the Geo-Environment of the catchment and the entire Omo-Gibe Basin is manifested by changes in Recharge Rates

OBJECTIVES Estimating the Optimum Groundwater

Recharge  Showing the Temporal-Trend of Recharge

w.r.t. the Temporal Changes of the Hydro- Meteorological Parameters

Assessing the Fate of Groundwater Storage

To Show the Major Controlling Parameter of Groundwater Recharge in the Catchment

 

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Study area

O m o-G ibe Basin

Location of the study area in the O m o-G ibe Basin

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DISTANCE FROM ADDIS ABABA AND ALTERNATIVE ROADS   Along Addis Ababa – Butajira – Hosaina Road = 300Km Along Addis Ababa – Shashemene – Welaita Road = 410 Along Addis Ababa – Durame – Areka = 440Km

AREAL EXTENT = 574Km2

PERIMETER = 124Km

60 55’ 42 - 7010’28N370 31’ 39 - 37046’40E

PHYSIOGRAPHY  50% of the area has a slope 2-6%, 25% has 0-2%, 15% has 6-12%,

6% has 12-24%     Elevated areas are found in the

boundaries of the area Topographic relief ranges from 1800-1900m from the highest peak Damota to the lowest Weybo valley

GEOLOGY

The main lithologic units that are outcropped in the study area and near to its adjacent watersheds are the teritiary volcanics

  The Nazareth Groups (of Miocene to Pliocene age) and the

flood basalts (Eocene to early Miocene age) are the dominant units that widely cover the study area

  About 90% of the area is covered by the Nazareth Group,

which comprises of a series of rhyolite-trachyt flows, ignimbrites, pumice and ash falls

The Nazareth Group unconformably overlies the early flood basalts

 

 

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Flood basalt

Rhyolite

Trachyte

Ignim brite

A lluvium

Inferred faults

Known fault

E levation contoursm eter1800

HYDROGEOLOGY

HYDROGEOLOGIC UNITS AND AQUIFER SYSTEMS

AQUIFER FORMATION

-Weathered and fractured ignimbrite/welded tuff

-Sediments associated with weathered pumice

-Weathered and fractured rhyolites and Trachytes

TYPES OF AQUIFERS IN THE CATCHMENT  

-Dominantly leaky aquifers

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LegendH ydraulic conductiv ity zones

Low H ydraulic conductiv ity

M oderate hydraulic conductiv ity

H igh hydraulic conductiv ity

(K<=0.3)

(0.3<K<0.5)

(0.5<K>0.8)

Know n fault

In ferred fault

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R egional groundwater flow d irection

Local groundw ater flow direction

Inferred region of groundwater outflow

Inferred region of groundwater inflow

D rainage netw orks

Sub-catchm ent boundary

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Know n region of groundw ater outflow

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Boreholes

Springs

W ater type and TD S rangesNa-C a-HCO 3-Type(70-100m g/l)Ca-M g-HC O 3-Type(25-80m g/l)C a-Na-HC O 3-Type(200-300m g/l)Na-Ca-HCO 3-Type(100-190m g/l)Na-Ca-HCO 3-Type(80-185m g/l)

HYDROGEOLOGICAL MAP OF THE STUDY AREA

    

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Boreholes and their d istribution used to m ap hydraulic conductivity o f the area

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  Correlation has been made between Geological structures and hydraulic conductivity values

A greater extent of the study area possesses a low permeability zone.

HYDRAULIC CHARACTERISTICS

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R egional groundw ater flow d irections

Local groundwater flow directions

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Fig. R egional and local ground w ater flow system s as in ferred from ground w ater e levation contours

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GROUND WATER FLOW SYSTEMS AND POTENTIOMETRIC CONTOURS Regional, intermediate and local flow systems

RECHARGE AND DISCHARGE ZONES  

Zonation based on topography 

 

    

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RECHARGE & DISCHARGE ZONES AS INFERRED FROM ELEVATION CONTOURS

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Zonation based on peizometric patterns

Flow lines tend to diverge from recharge areas and converge toward discharge zones

    

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Fig. C onvergence and d ivergence zones of flow lines show ing recharge and d ischarge zones

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Zones of converging flow s

C atchm ent boundary

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Same clusters showing aquifer interactions

Recharging zones, Na-Ca-HCO3-Type waters(cluster-2)

Discharging zone, reprasented by Na-Ca-HCO3-W ater Type(Clusters 1,4,5)

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C atchm ent boundary

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Hydrochemical clustrs showing aquifer interconectivity and recharge and discharge zones

Zonation Based on Hydro chemical Trends

HYDROMETEOROLOGY

PRECIPITATION = 1340 mm

TEMPRATURE = 19.210C

RELATIVE HUMIDITY = 63.5%

MEAN WIND SPEED = 2.45m/s

POTENTIAL EVAPOTRANSPIRATION = 1074.4mm

ACTUAL EVAPOTRANSPIRATION (AET) = 960.3mm

 

STREAM DISCHARGE OF WEYBO RIVER

(i) Scaling up the stream flow values

Qmouth = (A2/A1) Qgauged

Q = Qan (Pan/P)

(ii) The analogue method

SUMMARY OF WEYBO RIVER DISCHARGE,m3/s, 1992 - 2005

WAT. YEAR JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC TOTAL

1992 0.873 0.742 1.116 2.475 5.521 3.785 5.754 27.050 35.044 34.221 26.519 10.400

1993 1.235 1.942 0.787 1.907 8.330 7.991 8.415 17.491 11.366 4.984 1.654 0.781

1994 0.622 0.548 1.014 1.751 3.505 3.776 17.453 27.442 12.739 4.436 1.430 0.713

1995 0.548 0.557 0.534 3.310 2.833 2.426 11.675 14.278 21.995 11.761 1.996 0.669

1996 0.569 0.348 1.618 3.855 6.759 30.334 22.546 21.688 28.892 7.230 1.878 0.666

1997 0.424 0.345 0.528 3.113 3.575 2.152 4.180 5.406 3.001 11.454 18.458 7.985

1998 2.662 1.598 1.509 2.043 5.394 6.252 14.847 26.248 14.988 16.848 6.432 2.054

1999 1.318 0.928 1.223 2.302 2.553 2.856 9.367 11.557 10.602 19.428 5.146 1.483

2000 0.525 0.427 0.545 1.760 4.710 1.509 2.564 7.310 8.012 12.695 6.160 1.556

2001 0.704 0.542 0.545 1.350 5.071 6.535 13.131 26.761 19.498 12.598 3.060 0.848

2002 0.878 0.793 1.940 1.350 0.943 0.973 1.512 2.673 2.187 0.843 0.366 0.610

2003 0.539 0.454 1.630 2.635 0.914 2.043 6.841 20.403 5.671 0.843 0.365 0.628

2004 0.873 0.742 1.116 2.453 4.051 1.229 3.260 5.208 4.572 5.777 1.129 0.510

2005 0.448 0.422 1.515 4.032 2.554 1.231 3.259 5.207 4.571 5.778 1.128 0.510

MEAN 0.873 0.742 1.116 2.453 4.051 5.221 8.915 15.623 13.081 10.636 5.408 2.101 70.219

GROUNDWATER RECHARGE (GWR)

CONVENTIONAL WATER BALANCE APPROACH

GWR = 88.19mm/annum

STREAM HYDROGRAPHS ANALYSES

Mean long-term minimum flow = 62.5mm/year

Seasonal recession method = 74.5mm/year

CHLORIDE MASS BALANCE = 123.5mm/year

THE OPTIMUM GROUND WATER RECHARGE

ESTIMATION BASED ON GROUNDWATER BUDGET

ESTIMATED GROUNDWATER OUTFLOW = 78.56

GW INFLOW ESTIMATED FROM WATER BALANCE AND SEASONAL RECESSION METHOD = 81.5mm

THE OPTIMUM GROUND WATER RECHARGE = 81.5

TREND OF GROUNDWATER RECHARGE

Trend of Ground Water Recharge, 1992-2005

y = -5.8718x + 93.078

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FATE OF GROUNDWATER STORAGE

Groundwater storage will be heavily affected and appreciable change in storage will be seen after 30 years now on

Equating the linear equation: y = -587x + 93.07

RECHARGE TREND W.R.T. CONTROLLING HYDRO-METEOROLOGICAL PARAMETERS

Mean precipitation is slightly decreasing, however, its rate of declination is not comparable

mean temperature shows an increasing trend

mean wind speed shows an increasing trend

TREND OF PET, 1988-2005

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1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

PE

T, m

m

potential evapotranspiration is slightly increasing

 

-Groundwater recharge is computed using 4 different Methods and four different values are obtained. The optimum=81.5mm -The decline in groundwater recharge is highly attributed to changes in the environment 

-Measurable changes in groundwater storage will be seen in a 30 years period of time if environmental changes are keeping on the same rate 

CONCLUSION

-The groundwater recharge estimated from measurements of chloride yields an over estimated result.