Hydrological Sciences -Journal- des Sciences Hydrologiques,40,1, February 1995 97

Hydrological modelling and the sustainable development of the Hadejia-Nguru Wetlands, Nigeria

J. R. THOMPSON & G. E. HOLLIS Wetland Research Unit, Department of Geography, University College London, London WC1H OAP, UK

Abstract The Hadejia-Nguru Wetlands produce agricultural, fishing and fuelwood benefits of upto 1277 Nairahr1 (Nl = US$22, October 1994), over five times the productivity of formal irrigation schemes. The wetlands play a vital role in aquifer recharge. The key is the annual wet season flooding of over 2000 km2 in the 1960s and around 1500 km2 in the 1970s. A water balance model, utilizing monthly hydrological and meteorological data simulates flood extent and groundwater storage within the wetlands. The model was operated between 1964 and 1987 and was calibrated using observed flood extents ranging from 50 to 3265 km2. Subsequently elements were added for dams and irrigation schemes. Results indicate that full implementation of all the schemes constructed or planned would cause flooding to be less than 375 km2 for 60% of the time and groundwater storage to fall by over 5500 106 m3. It is possible to define an operating regime for the basin's hydraulic structures which could provide artificial floods and enable a distribution of water between formal irrigation, small scale irrigators, the wetlands and downstream users. This regime would provide assured flooding, of around 1000 km2 each year, and a reduced loss of groundwater storage. Such a sustainable development scheme could offset decades of piecemeal development and bring a philosophy which emphasizes water use throughout the basin not just in irrigation in the upper reaches.

Un modèle hydrologique pour le développement durable des zones humides de Hadejia-Nguru, Nigeria Résumé Les zones humides de Hadejia-Nguru fournissent des produits agricoles et halieutiques ainsi que du bois de chauffage pour une valeur jusqu'aux 1277 Nairas par hectare (NI = US$ 22 en Octobre 1994), ce qui est plus de cinq fois supérieur à la valeur du rendement des projets d'irrigation conventionnels. Les zones humides jouent un rôle essentiel pour la recharge des eaux souterraines pendant la saison des pluies, dont la clé fut l'inondation de plus de 2000 km2 au cours des armées soixante et d'environ 1500 km2 au cours des années soixante-dix. Un modèle de bilan d'eau utilisant des données hydrologiques et météorologiques mensuelles permet de simuler la surface des zones inondées et le stockage souterrain de l'eau dans les zones humides. Ce modèle a été appliqué à la période allant de 1964 à 1987 et a été calé à partir des surfaces inondées observées, dont l'étendue s'échelonnait entre 50 et 3265 km2. Des éléments concernant des projets d'endiguement et d'irri­gation ont ensuite été ajoutés au modèle. Les résultats indiquent que la mise en oeuvre de tous les projets existants ou proposés pourrait réduire la surface inondée à moins de 375 km2 pendant 60 % du temps et amener le stockage souterrain à moins de 5500 millions de m3. Il est possible de

Open for discussion until I August 1995

98 J. R. Thompson & G. E. Hollis

définir pour les aménagements hydrauliques du bassin des règles de gestion qui maintiendraient artificiellement des inondations et permett­raient une répartition de l'eau entre l'irrigation conventionnelle, les petits arrosages, les zones humides et les usagers de l'aval. Ceci assurerait l'inondation d'environ 1000 km2 chaque année et limiterait les pertes de la nappe phréatique. Un tel plan de développement durable pourrait compenser des décades d'aménagement au coup par coup et promouvoir une politique de valorisation de l'eau dans l'ensemble du bassin, qui ne se limiterait pas à l'irrigation des régions amont.

INTRODUCTION

Wetlands are now recognized as one of the World's most productive ecosystems which through their functions provide goods and services for the health, safety and welfare of human populations. Adamus & Stockwell (1983) documented the functions which furnish wetlands with their important status. These included groundwater discharge and recharge, sediment trapping, flood storage, water quality improvement and fishery and wildlife habitat. It is uncommon for a single wetland to possess all of these values, while different wetlands perform the same functions to different degrees. However wetlands are often the only places where such values are to be found and, once lost following the destruction of a wetland, they are frequently lost forever.

Unfortunately, as well as being some of the most productive ecosystems, wetlands are also among the most threatened. Threats facing the World's wetlands can be broadly categorized into those related to agricultural intensification, pollution, major engineering schemes or urban developments. The most significant threats to wetlands in dry regions originate from the reduction of water supplies by the construction of water resource schemes coupled with the expansion of intensive agriculture. In West Africa alone, Ketel et al. (1987) identified 114 dam projects, 51 then operational, capable of having significant effects upon wetlands. A further 78 "wetland intervention" projects were also recognized.

THE HADEJIA-NGURU WETLANDS, NORTHERN NIGERIA

The Hadejia-Nguru Wetlands are located in semiarid northeastern Nigeria, around and upstream of the confluence of the region's two principal rivers, the Hadejia and the Jama'are (Fig. 1). Nguru's mean annual rainfall for the period 1942-1990 was 487 mm. The climate of the region is dominated by the annual migration of the Inter-Tropical Convergence Zone which reaches its most northerly position above Nigeria in July or August and whose influence produces the distinct wet and dry seasons characteristic of Sub-Saharan West Africa (NEAZDP, 1990). Consequently, the region's rivers exhibit ephemeral flow patterns (Umar, 1985) with periods of no flow in the dry season (October to April) and a marked concentration of runoff in the wet season with almost 80% of the total annual runoff in August and September. It is during these

Hydrological modelling of the Hadejia-Nguru Wetlands 99

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100 J. R. Thompson & G. E. Hollis

periods of high river flow that the fadamas (a Hausa word for land which is seasonally waterlogged or flooded) of the lower part of the Hadejia-Jama'are basin flood. It is this flooding which imparts the many valuable functions and values to the wetlands.

According to Adams (1994), the Hadejia-Nguru wetlands have for centuries played a vital role in the regional economy, being one of the most productive areas of northeastern Nigeria. They can be likened to the Sokoto flood plain of northwest Nigeria of which Adams (1986) stated "the flood plain and the floodwater ... represent important ecological and economic resources for the development of the region".

Analysis of the wetland economy undertaken by Barbier et al. (1991) reveals that the present value of the agricultural, fishing and fuelwood benefits provided by the wetlands is between 849 and 1277 Naira per hectare. Table 1 provides a "breakdown" of these benefits and shows that they are substantially greater per hectare than those deriving from the Kano River Irrigation Project, a formal irrigation scheme on the Kano River, a tributary of the Hadejia.

Besides the quantifiable agricultural, fishing and fuelwood benefits, the wetlands possess values less readily evaluated on an economic basis: dry season grazing for livestock from the surrounding Sahelian pastures; non-timber forest products; groundwater recharge; refuge for human populations during times of drought; and habitat for both Afro-tropical and Palaearctic birds. Arguably the most important of these functions is the recharge of the shallow and deeper aquifers of the Chad Formation which is facilitated by the wet season flooding of the fadamas (Schultz, 1976; Diyam, 1987). These aquifers are extensively utilized for domestic and agricultural purposes by large human populations both within the flood plain and beyond.

However, the maintenance of the many values possessed by the flood plain wetlands of the Hadejia-Jama'are basin is threatened by the combined influence of drought and the uncoordinated development of the basin's scarce water resources. Grove & Adams (1988) stated that the succession of drought years experienced throughout Sub-Saharan West Africa in the last two decades

Table 1 Present value of benefits from the wetland and Kano River Irrigation Project (after Barbier et al, 1991) (NI = US$22, October 1994)

Wetland value

Agriculture Fishing Fuelwood Total

Kano River Irrigation Project2

Present value of wetland benefit (Naira ha"1)1

Lowest estimate Highest estimate

558 197 91

849

153

838 300 139

1277

233

1 Averaged over die total production area of 730 000 ha (230 000 ha for cropland, 100 000 ha for fishing and 400 000 ha for fuelwood). 2 Based on the 1985/1986 cultivated area of 19 107 ha and allowing for project operating costs which have ranged from 7.7% to 37% of the total value of crop production.

Hydrological modelling of the Hadejia-Nguru Wetlands 101

has led to a marked reduction in the discharge of the region's rivers. Reduced flow within the Hadejia and Jama'are has diminished the area of wet season inundation within the wetlands (Hollis et al., 1994; Hollis & Thompson, 1994a). In the wet seasons of 1969 and 1974, 2350 km2 and 2004 km2 respec­tively of the flood plain were inundated (Schultz, 1976). More recently, aerial surveys undertaken by the Hadejia-Nguru Wetland Conservation Project have revealed that in the 1991 wet season, flood extent was 962 km2 (Morgan, 1994). In the next two years 525 and 413 km2 respectively were inundated (Table 2).

Great emphasis within Nigeria has in the past been placed on the development of large scale formal irrigation schemes as a means of achieving national self-sufficiency in food (Lele & Subramanian, 1989). Within the Hadejia-Jama'are basin a number of irrigation schemes and their associated dams have been constructed or are currently under construction (Fig. 1). Early in 1993, the contract to complete the construction of the Kafin Zaki Dam, the

Table 2 Records of flood extent in the lowland catchment above Gashua

Date Flood extent (knr) Source

1950

October 1969

22 August 1974 27 September 1974 13 October 1974 6 November 1974 4 December 1974 10 February 1975 6 March 1975

3265

2350

1763 2004 1846 1527 1085 345 50

This is the area liable to flood on DOS 1:50 000 maps derived from 1950 air photography. This has been grossed up for the whole of the catchment using areas and proportions derived from Schultz (1976).

1:40 000 air photographs taken by die Directorate of Overseas Surveys by Burnett Resource Surveys Ltd. for the Schultz (1976) study.

Seven flights between 22 August 1974 and 6 April 1975 were made by the Schultz (1976) team who marked the flooded area on 1:100 000 DOS map sheets.

November 1978

November 1986

Mid September 1987

23 September 1991 8 October 1992 5 October 1993

1825

1186

700

962 525 413

Landsat TM image (Since the image was in November it was assumed that all of the upstream areas of flooding had drained by the time of the image.)

Landsat TM image (Since the image of the eastern part of the flood plain near Gashua was not available for analysis, the measured figure was grossed up using the Schultz (1976) assessment of area liable to flood in the area not analyzed and the proportion of the area liable to flood (Schultz, 1976) that was analyzed and was actually flooded in 1986. Since the image was in November, it was assumed that the upstream area of flooding had drained by the time of the image.)

Field and air-borne mapping by W. Bentham.

Air-borne mapping by the Hadejia-Nguru Wetland Conservation Project who marked the flooded area on 1:50 000 DOS map sheets (Morgan, 1994).

102 J. R. Thompson & G. E. Mollis

largest dam planned in the basin, was awarded and work at the dam site begun. A year later construction ceased following the revoking of the contract by the Federal Government due to lack of funds (Daily Champion, 1994).

The locations of these schemes, all upstream of the wetlands, will result in consequences for the flood plain similar to those associated with the drought, principally a reduction in water supply. The viability of the development projects downstream of the wetlands, which have been developed by the European Community funded North East Arid Zone Development Programme (NEAZDP), will also be eroded due to the reduction in water availability.

Since the many values provided by the flood plain wetlands are dependent upon water, serious economic and environmental consequences will result if the operation of these upstream schemes does not take into account the requirements of downstream users. Declines in the flood plain's fisheries, the productivity of which is directly related to flood extent, have already been reported. Thomas et al. (1994) blamed such declines on reduced flooding resulting from the recent drought conditions and the operation of the Tiga reservoir.

In addition to these "downstream" problems, the development of large scale formal irrigation schemes in Nigeria has been fraught with a host of economic, social and environmental problems (Mafara, 1992; Adams et al., 1994). Similar difficulties are repeated on large irrigation schemes throughout Sub-Saharan Africa (SSA) to such an extent that Le Moigne & Barghouti (1989) stated that "irrigated agriculture has had limited success in SSA". This view is echoed by Adams & Carter (1987): "the manifold problems associated with making these schemes effective in either economic or human terms has increasingly been realized".

Given the poor performance of formal irrigation in Nigeria and the contrasting highly productive economy of the flood plain wetlands, the water resources of the basin clearly have to be managed so as to sustain the multiple productivity of the wetlands for use by the local, regional and national popu­lation and to guarantee some water for the people, including those in Niger, downstream of the wetlands. It would of course be unrealistic to ignore the dams and irrigation schemes already constructed within the basin. Instead these structures must be operated in order to supply the water requirements of the current areas of irrigation while maintaining adequate flows downstream to facilitate the inundation of the flood plain and provide water to those down­stream of the wetlands. To date no such holistic vision of the basin has existed in Nigeria. A partial explanation is the division of responsibility for the water resources of the basin between two bodies: the Hadejia-Jama'are River Basin Development Authority (HJRBDA) in the upper part of the basin and the Chad Basin Rural Development Authority (CBRDA) in the lower parts of the catch­ment. In short, operating regimes allowing an equitable distribution of water amongst all the users within the basin are required. The formulation of such regimes will require a mechanism which will allow their potential effects to be evaluated.

Hydrological modelling of the Hadejia-Nguru Wetlands 103

THE MODEL

A water balance model, initially developed by Adams & Hollis (1988), was refined using a monthly hydrological and meteorological data matrix for the period January 1964 to November 1987. The most extensively used sources of data were Diyam (1986) and Umar (1985). Additional data were obtained from Yearbooks and manuscript records of stage and rating curves obtained from the Water Resources, Engineering and Construction Agency (WRECA). The model was programmed using Quattro Pro spreadsheet software and was set up to run as a menu driven system (Thompson, 1993).

Following the approach of Schultz (1976) and Sutcliffe & Parks (1987), the model calculates, on a monthly basis, the volume of water entering the wetlands via Wudil, Chai Chai, Bunga Bridge and some smaller tributaries but not leaving via Gashua (Fig. 1). The water retained within the wetlands is supplemented by local rainfall and runoff in the wet season and depleted by evaporation, soil moisture recharge and infiltration to groundwater. The resulting volume is converted into an area of inundation by means of a synthetic volume/area relationship, a methodology also adopted by Sutcliffe & Parks (1987, 1989) in a number of other African wetlands. Such a relationship was required due to the absence of any elevation data on the maps of the area.

The following equations are used for balance calculations for each month of the simulation period:

*t ~~ t-\ + W- *if wudil-' + *^chai chai + ^rbunga + » f jam.trib ~~ ^fgashua

+ (Rt - E) x At_x + (Rt - ETt when > 0.0) x (Aiot?i - At.x) (1)

-RCxAt_x = -SCx{At-At_x when >0.0)-HVPt

where t indicates the current month; t—\ indicates the previous month; V = volume of water in the flood plain (106 m3); Q — discharge at the named gauge (106 m3). The discharge at Wudil is reduced by 25% to allow for infiltration between Wudil and Hadejia (Diyam, 1986); Qjmn trib = flow in the Iggi, Dogwala and inflow to the Jama'are between Bunga and Foggo as estimated from the ratio of the 1965 km2 of extra catchment area to the 3080 km2 above Bunga multiplied by the flow at Bunga; R = weighted mean rainfall for Kano and Nguru; E = open pan evaporation from Class A Pan at Nguru multiplied by a coefficient of 0.7; A = inundated area (km2); ET = weighted mean potential evaporation by the Penman method at Kano and Nguru; At0t3l = maximum area of the flood plain (set at 5780 km2 after Schultz (1976)); RC = monthly rate of infiltration beneath the inundated area (mm) and optimized to a value of 0.2 m per month. Hollis et al. (1994) presented preliminary results from an on-going field programme which confirmed infiltration rates of this order of magnitude; SC = recharge of soil moisture when floods advance over an area of flood plain and optimized to 0.3 m; and

104 J. R. Thompson & G, E. Hollis

HVP — estimated diversion to the Hadejia Valley-Project Trial Farm.

A t = aVbt (2)

where b = 1.0 throughout; and a was varied to fit the model but was optimized to 1.0.

These equations provide an initial estimate of Vt and At. Since the inundated area which loses water through evaporation and infiltration is strictly the mean of the initial and final values for the month these estimates were used to adjust the estimates for evaporation and infiltration to the mean flooded area to give in a second iteration:

(A+A, ,) (A,-At ,) l i tlL(R(-Et) and l . M RC 2

HVPt = IAXIRXIP

(3)

(4)

where IA is the area (ha) irrigated in any one year (1978: 145, 1979: 290, 1980: 435, 1981: 580, 1982/84: 727, 1985: 145, 1986/87: 201); IR is the assumed irrigation rate of 15 000 m3 ha4 year4; IP is the proportion of irrigation demand each month after Haskoning (1977) (J: 0.13, F: 0.15, M: 0.14, A: 0.11, M: 0.06, J: 0.06, J: 0.06, A: 0.01, S: 0.07, O: 0.07, N: 0.07, D: 0.09).

4000-

3500-

3000-1

E 250O1

S 2000-

o o 1500-

1000-

500-

1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986

Date Predicted « Observed

Fig. 2 Flooding under natural conditions.

Hydrological modelling of the Hadejia-Nguru Wetlands 105

GWSt = GWSt_x+RCt- •RF-ET. phreatophytes

,6 „ , 3 \

(5)

where GWS is the groundwater storage (10° nr); RF is the regional flow in the aquifer estimated by Schultz (1976) at 41.7 106 m3 per month. The accuracy of this estimate was plus or minus an order of magnitude and consequently it provides only a very crude estimate of the regional flow. Further hydro-geological analyses are planned for its refinement; and ETr

évapotranspiration from phreatophytic vegetation. phreatophytes IS

ET phreatophytes = A phreatophytes 5X£rfx(GWSr/GWSstart) (6)

where A phreatophytes is the area of phreatophytes estimated by Schultz (1976) at 614.4 km2; and GWSstM is the starting groundwater storage.

The model was optimized manually by adjusting: (a) the pan coefficient for open water evaporation; (b) the infiltration rate (RQ beneath the flooded soil; (c) the soil moisture recharge (SQ for newly flooded areas; (d) a in the volume/area relationship; and (e) b in the volume/area relationship in order to bring the predictions of the model into as close accord as possible with the observed flood extents in Table 2 (Fig. 2) and to maintain a fairly stable level of groundwater storage from 1964 to 1975 as in the Schultz (1976) simulation (Fig. 3).

11.a

10.0---

^ 4 ? 8.0

« t . 7.0-

o

4 . 0 \ HI ill l ill til H II! I«I 111) 111 111 I! W III i II! Si i !H il & ill I il> Hi r Hi II H Hi li! IIII H il 111 i! ill h I ill ill ! Ill II il !l i l il ii 11! ;; |„ U \ IS iii H || H! H II! 1II IS li IIII!1! Si à I «, H. t Si !i i ill i Hi l« S III * I !!i Hi I S u b

1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984 1986

Date Fig. 3 Groundwater storage under natural conditions.

106 J. R. Thompson & G. E. Hollis

SIMULATIONS OF DAMS AND FORMAL IRRIGATION SCHEMES

Elements representing four of the engineering schemes extant or under construction (Tiga Dam and the Kano River Irrigation Project (KRIP), Challawa Gorge Dam, the Hadejia Valley Project (HVP) and Kafin Zaki Dam) within the basin were programmed. For each of these schemes it is possible to set the area of irrigation and volume of monthly regulated releases. The inclusion of these elements required the development of a procedure to calculate the flow downstream of the wetlands at Gashua since the operation of dams and irrigation schemes upstream would change the discharge at Gashua from that observed from 1964 to 1987. A relationship was derived by regression between the outflow from the wetlands and the aggregate inflow during the four most recent months:

gGashua, = - 5 . 2 + 0 .34(0 .2 X Inflow, + 0.4 X Inflow,.j

(7) + 0 .3 X Inflowf_2 + 0.1 X Inflow,_3)

where gGashua is the monthly discharge at Gashua; and inflow is the aggregate monthly inflow via Wudil, Chai Chai, Bunga Bridge and Jama'are tributaries.

The reservoirs at Tiga, Challawa Gorge, Kafin Zaki and the Hadejia Valley Project were simulated by interposing them in the river flow and computing a water balance. The area-volume relationships from Diyam (1986) were used in the simulations of Tiga, Challawa Gorge and Kafin Zaki reser­voirs while a similar relationship was determined for the storage pond on the Hadejia Valley Project using Diyam's suggested operational water levels and Haskoning's (1977) plans of the scheme.

Evaporation from the computed areas of each reservoir was simulated using Class A Pan evaporation rates for Kano with a coefficient of 0.7, while rainfall data for Kano and Nguru were employed to simulate rainfall onto the open water surface of the reservoirs. Water was drawn from reservoirs, subject to the current volume in storage, to meet the demands of irrigation and water supply schemes. The monthly demand for irrigation water was simulated as a proportion of the annual total using the proportions of Haskoning (1977). In addition, the simulation of Tiga reservoir supplied 103 106 m3 each year to the Kano City Waterworks. This was distributed according to the proportions provided by Diyam (1987). The river flow downstream of each scheme was taken to be the volume of spill (calculated as the volume of storage in excess of reservoir capacity after evaporative and water resource demands) and any regulated releases. Additionally 10% of the irrigation water supplied to the Hadejia Valley Project was assumed to drain back into the Hadejia River. These downstream flows were supplemented by runoff from the basin below the dams, either through the use of gauged river data where available, or by assuming a direct relationship between catchment area and runoff and in turn evaluating non-gauged flows from adjacent gauging station records. Down-

Hydrological modelling of the Hadejia-Nguru Wetlands 107

stream discharges were also depleted by infiltration through river beds and the flood plain.

EVALUATION OF TWO ALTERNATIVES FOR THE DEVELOPMENT OF THE BASIN'S WATER RESOURCES

The model was used to simulate many alternative scenarios for the development of the basin's water resources (Hollis & Thompson, 1994b). Two are exem­plified (Table 3). The first envisaged the full implementation of all the schemes within the basin while the second was an attempt to permit both the current extent of formal irrigation and the maintenance of a regulated flooding regime for the wetlands. The effects of each of these scenarios upon the area of flooding, groundwater recharge, river flow and the volume of storage behind the dams were evaluated.

Table 3 The water resource development scenarios evaluated

Scenario Dam Releases (106 m3) Irrigation

Full Tiga none KRIP1 at 27 000 ha implementation Challawa Gorge 348 per year1

Kafin Zaki none 84 000 ha Hadejia Valley Project none 12 500 ha

Regulated Tiga 350 in August KRIP1 at 14 000 ha flooding Challawa Gorge 248 per year1 + 100 July regime Kafin Zaki 100 between October and none

March + 550 in August Hadejia Valley Project Dam open in August 8000 ha

KRIP1: Kano River Irrigation Project Phase 1. 1 Distributed according to the monthly irrigation proportions of Haskoning (1977).

The full development scenario

The implementation of all the water resource developments planned for the basin would lead to dramatic reductions in downstream discharge. For example, assuming a direct feed of irrigation water from Kafin Zaki reservoir to the 84 000 ha of formal irrigation, the flows within the Jama'are at Bunga Bridge would be reduced to around 20% of those experienced under natural conditions. Such reductions in discharge would in turn reduce the area of wet season inundation in the flood plain. Figure 4 presents the model's predictions of the flooding within the wetlands when the full development scenario is simulated. Flood extent would never exceed 1500 km2 while in 13 years out of 22 it would not exceed 750 km2. Such small areas of inundation are similar to those experienced in 1984 at the height of the 1980s drought. Since the recharge of groundwater is dependent on the annual floods, the volume of groundwater storage would likewise diminish dramatically as shown in

108 J. R. Thompson & G. E. Mollis

4000-

3500-

3000-

2500- •

CO CD

« 20001 T3 CD T3

o _o LL

1500-

1000-

500-

M *

M M

1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1£

Date Full implementation * Historic maxima

Fig. 4 Flooding under the full implementation scenario.

11.0

CD C D CO

13 c o (5

0.0-mii!]iiii!iijyi!^!ii;iaHii!ii!iitiii!i^y^yiiwti!iiiy!!ii;y^fmifpnnniNjiiii;i!i!!]ii!iiin;: 1 9 6 4 1 9 6 6 K

(a) Historic

ïTOiiiii!iiirmrîï.iiiii<.«'»i>5iiiMitiiii] 1970 1972 1974 1976 1978 1980 1982 1984

Date

(b) Development ~~x- (c) Regulated

Fig. 5 (a) Historic groundwater storage; (b) groundwater storage under the full implementation scenario; and (c) the regulated flooding regime.

Hydrological modelling of the Hadejia-Nguru Wetlands 109

Fig. 5(b). The discharge downstream of the wetlands would undergo dramatic reductions, the annual total flow at Gashua being only 27% of historic values. This would have serious implications for the water users along the Yobe River.

Diyam (1986) stated that "the combined resources of Tiga and the future (now completed) Challawa reservoirs appear barely sufficient to support the ultimate water requirements of Kano River Phase 1 and Hadejia Irrigation projects and Kano Water Supply". The model confirms this observation. Although Tiga reservoir is shown to be able to supply water to 27 000 ha on the Kano River Project, its live storage would have reached extreme lows in the drought years of the 1970s and 1980s. Similarly, Challawa Gorge reservoir would scarcely be able to fulfil its designed purpose of releasing water into the Hadejia River for use in the Hadejia Valley Project. In eight years of the simulation period the reservoir behind Challawa Gorge reservoir was prac­tically dry and as a consequence the downstream discharge was inadequate leading to prolonged irrigation deficits on the Hadejia Valley Project. The most severe deficits would have occurred in the drought years of the 1970s and 1980s. The model also shows that although Kafin Zaki could have sustained the envisaged 84 000 ha of formal irrigation in the Jama'are Valley throughout the 1960s, during the 1970s and 1980s the reservoir would have experienced periods of desiccation and would only infrequently have realized 50% of storage capacity (Fig. 6).

3000"

OTtiyiiiiiiiiiiiiinrraniiiiiiinmiHiirRrmitiii^ 1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984

Date

Fig. 6 Kafin Zaki storage under the full implementation scenario.

110 J. R, Thompson & G. E. Hollis

A regulated flooding regime

The problems associated with the full development of the water resources of the basin could be ameliorated if a set of operating regimes was developed for the reservoirs and irrigation schemes that would both permit the supply of water to the existing areas of irrigation and allow releases from the reservoirs to facilitate flooding within the wetlands and flows downstream. Such a regu­lated flooding regime was simulated using the model (Table 3).

The levels of flooding within the wetlands experienced under natural conditions are not repeated. However, Fig. 7 shows that the regulated flooding regime does maintain floods between 625 and 1875 km2. Despite the reductions in flood extent, it is possible that the productivity of the flood plain could be increased as a result of the greater certainty of the timing and extent of the annual floods. This could reduce some of the risks associated with the natural flooding regime. Additionally the inundation of some areas throughout the dry season would benefit both the flood plain's fisheries and groundwater recharge.

4000

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1964 1966 1968 1970 1972 1974 1976 1978 1980 1982 1984

Date Regulated regime * Historic maxima

Fig. 7 Flooding under the regulated flooding regime.

Although peak discharges of the basin's rivers are reduced, the addition of a baseflow component would permit small scale irrigation by petrol-driven pumps. This form of irrigation has undergone rapid expansion in recent years through its promotion by World Bank aided Agricultural Development Projects (ADPs) (Kimmage & Adams, 1990). The discharge of the Yobe at Gashua is

Hydrological modelling of the Hadejia-Nguru Wetlands 111

nearly 220% of that predicted under the full development scenario but still below (54% of historic flows) the politically desirable flows experienced before upstream hydraulic works were begun.

Groundwater storage under this scenario falls further than under natural conditions. However the magnitude of this decline is smaller than that resulting from the simulation of the full implementation scenario (Fig. 5(c)).

Under the regulated flooding regime detailed in Table 3, Tiga reservoir is able, throughout the simulation period, to supply water to the existing 14 000 ha of the Kano River Irrigation Project and make a release of 350 106 m3 in August without desiccation. Only in the most severe drought years does the storage within Challawa Gorge reservoir become seriously depleted while the duration and extent of irrigation deficits on the Hadejia Valley Project are reduced markedly. Kafin Zaki reservoir is capable of main­taining both a flood release in August and smaller releases through October to March without the threat of its desiccation.

The operation of a regulated flooding regime would therefore enable a distribution of water between the existing formal irrigation schemes within the basin, the flood plain wetlands and their many productive values and the river downstream of the wetlands. The scenario examined is the first step in the formulation of such a regime and further research will be required before operating regimes for reservoirs and irrigation schemes can be finalized. It has recently been announced (NIPSS, 1993) that Tiga reservoir's spillway has been lowered by 3.5 m to increase reservoir safety. The reduction in active storage from 1.9 109 m3 to 1.5 109 m3 will not affect the ability of Tiga to supply the 14 000 ha of irrigation but will increase the volume and frequency of wet season spills for the benefit of downstream users (Hollis & Thompson, 1994b).

Flood hydrograpfas: a preliminary analysis

A preliminary analysis of a flood release hydrograph for Kafin Zaki reservoir has been undertaken. The aim of this analysis was to obtain an initial evalua­tion of the size of the outlet structures required for the release of flood flows from the reservoir. This analysis was carried out when Kafin Zaki was under­going the final design stages and although the construction of the reservoir has been stopped, the history of water resource development within the Hadejia-Jama'are basin would suggest that the reservoir may still be built if funds become available. The construction of both the Hadejia Valley Project and Challawa Gorge reservoir was suspended for several years due to similar financial constraints.

The flood release in the regulated flooding regime described above was 550 106 m3 in August, representing a discharge of 205 m3 s"1 if spread uni­formly throughout the month. Such a regime would be unlikely to overtop the river channel and cause significant flooding. Instead an artificial flood hydrograph was determined.

112 J. R. Thompson & G. E. Hollis

Daily flow data were available for Bunga Bridge (downstream of the proposed site for Kafin Zaki) for the years 1964-1972 and 1985-1986. The average date of the peak flow was determined to be 25 August and a strong relationship identified between the model's predictions of flood extent under natural conditions and both peak daily and peak 5-day flow (Fig. 8). Data for 1971 were ignored in the derivation of these relationships since the peak flows were abnormally large. Using these relationships, estimates of the peak flows at Bunga Bridge needed to flood 1250 km2 within the wetlands were determined (Table 4).

Table 4 Estimated discharges at Bunga Bridge required to inundate 1250 km2

Flood extent (km2)

Date of peak flow Peak daily discharge (m3 s'1)

Peak 5-day discharge (m3 s"1)

1250 25 August 650 352

Oj 3000-E — 1

-c 25CO

* 20OO-

1503-

Flood extern = -547+2.92(Peak daily d ischarge)

R sq : 6 5 . 3 %

SCO I COO I D O O 2000 2500

Peak daily discharge at Bunga (mâ'sec)

Flood extent = 8S+3.30(Peak 5<tay discharge) R sq: 837%

Fig. 8 Relationship between flood extent in the wetlands and peak daily discharge and peak 5-day discharge at Bunga on the Jama'are (data for 1971 have been ignored).

2 0 0 4 0 0 600 800 1000 1200 1400 1600 Peak 5-day discharge at Bunga (m3/sec)

An artificial flood hydrograph was determined for the release from Kafin Zaki required to flood 1250 km2 in the wetlands. This hydrograph was designed around the statistics in Table 4 and tuned to give a total volume of 550 106 m3 as set out in the regulated flooding regime. Positioning the date of the peak discharge on 25 August necessitated that the hydrograph be spread over the four weeks from 15 August to 15 September. A second smaller peak was included in the hydrograph since the late reinforcement of the flooding is of great importance to the farmers of the flood plain (ICRA, 1992).

Figure 9 shows the artificial flood hydrograph superimposed upon the recorded discharge in 1972. In this year (as in others) it is at least as good as

Hydrological modelling of the Hadejia-Nguru "Wetlands 113

700

Aug1 Sep1 Date

Artificial flood 1972 flood Fig. 9 The artificial flood hydrograph at Bunga on the Jama'are required to achieve 1250 km2 of wetland flooding and the actual flow in 1972.

the natural flows. It is however suggested that it would not be appropriate to fix the maximum capacity of the outflow structures to 650 m3 s"1 since the artificial hydrograph falls considerably short of the measured peak discharges during wetter years. Instead a maximum outflow capacity of 1000 m3 s"1 would allow almost all the largest peaks to pass through the reservoir without the spillway coming into operation.

It should be emphasized that this analysis is preliminary and is based on flow records for a limited number of years. Further analysis is under way including the collection of daily data from other gauging stations within the basin. In an operational system some flexibility over the timing of the date of peak flow would be desirable to enable the releases from Kafin Zaki to coin­cide with flood flows in the other regulated and uncontrolled rivers of the basin. Operational regimes for all the reservoirs within the basin will have to be coordinated in order to maximize the area of flood plain inundation.

CONCLUSIONS

The hydrological simulation of the Hadejia-Jama'are basin indicates that the full implementation of all the planned water resource developments will have severe consequences for the basin's flood plain wetlands and areas further down­stream. Formal irrigation will consume large quantities of water originally

114 J. R. Thompson & G. E. Hollis

destined for these wetlands. Reduced river flow will lead to a decline in the area of wet season inundation and will dramatically reduce the productivity of an area which is currently one of the most productive in northern Nigeria. The reduced productivity from the flood plain will not be replaced by yields from formal irrigation since the water resources of the basin will be unable to support the envisaged extent of these schemes.

It is argued that the most appropriate use of the basin's water resources is the sustainable utilization of the flood plain wetlands rather than the expansion of formal irrigation. This will require the operation of existing irrigation schemes and their associated reservoirs in a way that will facilitate the distribution of water between formal irrigation and the water users within the flood plain and downstream.

The hydrological model shows that such a distribution of water resources is technically possible. The current extent of formal irrigation can be sustained whilst flood releases from reservoirs permit the operation of a regulated flooding regime within the flood plain. Such a regime would preserve the many functions provided by the wetlands. Greater certainty about the timing and extent of flooding could increase agricultural productivity by removing some of the risks associated with natural flooding regimes. A programme of test releases over a number of years will be required to refine the hydrometric and operational aspects of the scheme. A number of important issues must however be addressed before such a scheme is implemented (Hollis, 1989). These include the social effects of a regulated flood regime. For example, what will be the reaction of farmers whose land will no longer flood? What will be the view of the farmers who do not have access to open water and therefore cannot participate in the expanding small pump irrigation? How will pastoralists react to increasingly restricted access to dry season grazing as the extent of this irrigation increases?

An artificial flood hydrograph to facilitate flooding within the wetlands could be released from Kafin Zaki reservoir should it eventually be con­structed. The hydrograph also determines the required capacity of the outlet structure of the reservoir. At present this hydrograph is only a preliminary estimate of that eventually required and it could be refined substantially through the inclusion of the additional daily data currently being analysed.

The introduction of a regulated flooding regime will require an improved hydrometric monitoring network within the basin. Many hydrometric stations are no longer operational while the quality of data obtained from others is suspected to have been compromised by declining financial resources allocated to data collection. Other prerequisites for the success of such a regime include substantially improved training amongst staff responsible for the operation of the reservoirs and irrigation schemes and close liaison between water managers and users. One obstacle is that the Hadejia-Jama'are basin is managed by two river basin development authorities (one upstream and the other downstream) and five state water boards. There have, however, been concerted calls for a unitary body to oversee the basin's water resources (NIPSS, 1993).

Hydrological modelling of the Hadejia-Nguru Wetlands 115

Many of the arguments presented in this paper were endorsed by the recommendations from a recent workshop on the water resources of the basin (NIPSS, 1993). It was agreed that Tiga, Challawa Gorge and Kafin Zaki (if built) reservoirs and the Hadejia Valley Project barrage should be operated to satisfy downstream water requirements. To this end, the outlet structures of the existing reservoirs are to be tested whilst another recommendation called for an environmental impact assessment of Kafin Zaki reservoir. The requirement for the incorporation of outlet structures for adequate flood releases was also endorsed.

Acknowledgements Financial support for this research was provided by the International Union for the Conservation of Nature and Natural Resources, the Royal Society for the Protection of Birds, the Finnish Association for Nature Conservation, FINNIDA and the British Council. The authors are indebted to the staff of the Hadejia-Nguru Wetland Conservation Project for their continued assistance, support and encouragement.

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Received 22 June 1994; accepted 12 September 1994