Effects of reservoir operation on the hydrological ...

Man's Influence on Freshwater Ecosystems and Water Use (Proceedings of a Boulder Symposium, July 1995). 1AHS Publ. no. 230, 1995.

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Effects of reservoir operation on the hydrological regime and water availability in northern Nigeria

LEKAN OYEBANDE Hydrology Laboratory, Faculty of Environmental Sciences, University of Lagos, Nigeria

Abstract Much of the northernmost part of Nigeria has experienced typical Sahelian climate during the last 20 years, and the cumulative effects of the prolonged desiccation has produced Sahelian ecological conditions even in areas that were more humid. The major sources of surface water in the Sudano-Sahelian zone are the Lake Chad, K. YobeHadejia, Sokoto-Rima and Gongola river systems. The seasonal and long-term variations in the flow of the rivers underline the importance of over-year storage as a means of making optimum use of the water resources. For example, most of the Sahelian rivers dried up during the 1973 and 1984 droughts, making the existing manmade reservoirs look like oases in the desert. The high rates of evaporation from the reservoirs, persistent droughts, faulty designs and operation of the multipurpose dams and reservoirs have often led to drastic reduction in the areas of wetlands and available water in downstream areas.

INTRODUCTION Nigeria can be divided into four broad ecological zones: equatorial/tropical forest, Guinea savanna, Sudan savanna and the Sahel (Fig. 1). The Sahel with the northern Sudan zone is a sub-humid to semiarid region and is currently studied intensively under the appellation "Sudano-Sahelian zone" (SSZ). The rate of decrease in the rainfall of the SSZ since the mid-1960s has been interpreted to represent a southward progression of desert conditions at a rate of 33 km per decade (Jensen, 1990). The SSZ of northern Nigeria is approximately bounded by latitudes 11 and 13°N and by longitudes 4 and 15 °E. The land area covered is about 26 160 km2 or 28% of Nigeria's total land area. The major sources of surface water in the SSZ are the Lake Chad, the K. YobeHadejia, Sokoto-Rima and the Gongola river systems. The seasonal and long-term variations in the flow of these rivers as shown by the range of the seven-year means and the ratio of the 90% probable flow to the mean annual runoff (1:65 for the Sudan and 1:250 for the Sahel) underline the importance of over-year storage as a means of making optimum use of the water resources. Eighty to ninety percent of the flood flow is concentrated in three months from July to September. Most of the Sahelian rivers dried up during the 1972/1973 and 1984 droughts and the existing manmade reservoirs were like oases in the desert.

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Fig. 1 Annual rainfall and hydroecological zones of Nigeria (isohyets in mm).

Present approach to water management The Federal Ministry of Water Resources and Rural Development (FMWRRD) is charged with the overall responsibility of policy objective formulation, data collection, monitoring and coordination of water resources management at the national level. It supervises the twelve River Basin Development Authorities (RBDAs) whose functions in their areas of jurisdiction include development of water resources infrastructures. These functions are carried out through construction of dams and reservoirs, boreholes and irrigation systems to catalyse surface and groundwater development. They are also mandated to provide raw water from their multi-purpose reservoirs for urban water supply systems. All RBDAs use their dam projects mainly for irrigation. The National Electric Power Authority (NEPA) dedicates its own dam projects solely to hydroelectric power generation. The 30 states and Federal Capital of Abuja have primary responsibility for water supply in urban, semi-urban and jointly with other local agencies in the rural areas. These functions are carried out through the State Water Supply Agencies. DAMS, RESERVOIRS AND THEIR OPERATION Altogether 162 large, medium and small dams have been constructed and are being operated in Nigeria. They have a total storage capacity of more than 30 109 m3, i.e. less than 10% of the country's total potential surface water resources. Eighty five percent of the larger dams are located in the Sudano-Sahelian zone of the country. A sample of 52 dams indicates that 79% have domestic industrial water supply components, while

Effects of reservoir operation on the hydrological regime in Nigeria

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33% have irrigation as a major use to which the stored water is put; 4% are also for hydro-electric power generation (HEP); 29% for fisheries and 16% for recreation. All the dams contribute to flood mitigation and affect the area of wetlands (floodplain) in their downstream areas. The three largest hydropower dams are under operation and control the flow of the Niger and Kaduna rivers (Fig. 2). These are Kainji, Jebba and Shiroro with total active capacity of 18.6 x 109 m3 and total power capacity of 1920 MW. In terms of storage usage however, irrigation accounts for 36%, water supply 3% and hydropower 61%. Two samples of 132 and 94 dams give the distribution of reservoirs by capacity and area for the SSZ and the country. 64 dams have reservoir capacity greater than 10 x 106 m3 while 20 dams have capacity exceeding 100 x 106 m3. In terms of reservoir area however, 87% of the sample has reservoirs occupying large areas extending for more than 5 km2. The slight topographic differences result in large reservoir areas which in turn contribute to large evaporation losses.

Requirements of efficient reservoir operation and uncompleted irrigation systems Accurate demand survey and assessment is a pre-requisite for efficient reservoir operation. In Nigeria, the range of purposes served by storage reservoirs includes water

Fig. 2 Major dams and reservoirs in northern Nigeria.

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supply for irrigation, domestic and industrial uses, hydroelectric power (HEP), increasing water depth for navigation, flood control, reclamation of low-lying lands and recreation. Some of these uses conflict (e.g. flood control and HEP and other uses) and priorities and proper balancing need to be carefully considered. In order to carry out proper reservoir operation, certain manuals, basic data and drawings are required at the operation and maintenance office. Unfortunately, most of the above drawings and data are not available in many operation and maintenance (O&M) offices at dam sites. As a result most of the monitoring works needed for efficient and proper reservoir operation are not carried out. In reservoir management, the reservoir operation rule should take into account reservoir inflow, evaporation losses, and irrigation demand both in wet and dry seasons, among other demands. Observed hydrological data at dam sites are indispensable for such considerations. Table 1 shows the status of existing irrigation schemes in the country. About 50% are designated to have standstill status. In addition to these dormant projects are those in which the irrigation systems are only partially completed or are still under construction because of shortage of funds. In all less than some 15% of the reservoir water is currently released for downstream irrigation. Many of such reservoirs have been exposed to the atmosphere for years and the stored water is being lost annually by evaporation. In such cases insignificant quantities are released to feed water supply systems of nearby major urban centres.

EFFECTS OFRESERVOIR OPERATION ONTHEHYDROLOGICALREGEVIE Table 2 depicts the natural fluctuation of the flow of rivers in northern Nigeria's SSZ. Five of the eight rivers are not perennial. The regulation of such rivers could obviously enhance their utilisation to meet certain needs. The effects of such regulation on the hydrological regime has been investigated in detail for the Kainji hydropower Dam on River Niger (Oyebande et al., 1980) and Tiga Dam on River Kano (Oyebande & Nwa,

Table 1 Status of existing irrigation projects in Nigeria (X 103 ha). Region

Fully developed

Partially developed Standstill

Total

Northwest

7.7

30.7

79.5

117.9

Northeast

27.1

49.1

108.5

184.7

Central-west

11.1

45.7

-

57.5

Central-east

11.7

28.7

37.9

78.3

Southwest

1.4

21.0

24.0

46.4

Southeast

9.2

21.1

10.0

40.3

Total (per cent of total)

68.9(13.1)

196.3(37.4)

259.9(49.5)

525.1(100)

* Construction of irrigation system is under design and may take some 7-8 years to complete.


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Table 2 Variation of monthly discharge at selected stations during 1970-1989. River

Station

Monthly discharge (m3 s"1) maximum

Niger

Kainji Dam

2430

400

900

Sokoto

Bakolori

170

0

23

Rima

Wamako

310

0

39

Kaduna

Wuya

4970

5

470

Gongola

Dadinkowa

410

1

64

Kano

Tiga dam

220

0

26

Challawa

Challawa rai Iway

290

0

26

Hadejia

Hadejia

140

0

24

1980). The first study shows that the Kainji Dam is filled mainly by storage of the "white flood". The post-Kainji flow duration curve shows that the post-impoundment minimum discharge was at least 500 m3 s"1 while maximum discharge was reduced to less than 3500 m3 s"1 from the previous level of 6000 m3 s"1. The Kainji Lake also trapped more than 70% of the river's sediment transport. This significantly affects the water quality regime, especially turbidity. The regulation of River Kano has had similar effects in the river segment between the dam and Hadejia. Some 80% of the runoff from the upper parts of the basin occurs during August and September. The dam controls 92% of the total flow and took nearly four years to fill. During the filling stage, the small release of 8 m3 s"1 into River Kano for the Greater Kano water supply simulated the effect of the use of the entire storage for irrigation and urban supplies. The Hadejia became a perennial river up to Wudil more than 200 km downstream and the base flow continued to rise in 1976. In 1981 the baseflow equalled the flood discharge of 1968 and 1973. The huge storage (2 x 109 m3) released into this area of low relief floodplains and lakes which attenuates the flows into a single seasonal peak which moves steadily downstream (Hollis et al., 1993). The Tiga dam was meant to serve the Sudano-Sahel zone of north-eastern Nigeria. One of the impacts of the lowering of flood levels on the downstream areas is the drastic reduction in the area of the floodplain wetlands or fadamas which serve as lifelines for farmers in this semiarid area. The average annual extent of the wetlands progressively decreased from 3250 km2 in 1950 to 2350 km2 in 1969 and further to 750 km2 in 1987. The extent was less than 300 km2 during the drought of 1984. A model simulation suggests that the droughts lowered the flow of the Hadejia at Gashua by 23 % while Tiga dam further reduced it by a least 25 %. It is predicted that the combined effect of the Tiga Dam and other dams such as Challawa Gorge and Kafin Zaki in the Yobe Basin will be a further desiccation of the wetlands below the permissible range of 6251500 km2 and lowering of the water table in the downstream areas, particularly in Borno state.

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EFFECTS ON DEVELOPMENT DOWNSTREAM OF EXISTING RESERVOIRS Water availability for hydropower generation Dams in Nigeria are generally associated with large reservoir areas, because of slight topographical differences that prevail so that available storage at the full water level (FWL) at the end of the wet season is often drastically reduced by large evaporation losses during the dry season (November-May in the northern Nigeria). The losses reach 10-30% of the active reservoir capacity and even when the reservoir is filled to the FWL at the end of the wet season only 70-90% of it is available for the dry season. Unfortunately too, inflows into the reservoirs have been decreasing since the late 1960s. The annual inflows into the country's first large dam - Kainji Hydropower Dam - decreased from 51.5 x 109 m3 in 1969 to 17.9 x 109 m3 in 1987/1988 as a result of the severe Sahelian drought. The hydropower dams at Kainji and Jebba have faced serious problems because the Kainji reservoir has not reached the full water level since the 1980s due to decrease of the imported "black flood" from 27 x 109 m3 in the 1970s to 18 x 109 m3 in the 1980s because of prolonged droughts and upstream development in the riparian states such as Mali and Guinea. The Kainji dam reservoir has been operated so as to store part of the inflow and control outflow. For this reason, the outflow in the 1980s decreased considerably. By June 1993, the hydrological stress had worsened, and the inflow was very low at the critical period of the annual cycle, so that the reservoir was being drawn down toward the design operating minimum level. It is thus necessary to review water use and demand based on the reduced water availability in the reservoirs until firm agreement is reached on fair share of the Niger water with the upstream states. Part of the decrease in the inflow is due to reduced "white flood", particularly the contribution originating from the Sokoto-Rima basin. The planned operation of the four irrigation dams in the basin (the first four dams in Table 3) indicates that only 38% of the total inflow to the dams would be released down stream for irrigation, water supply and river flow. Fifteen percent of the quantity would be released during the wet season

Table 3 Evaporation losses from dam reservoirs in Nigeria. Dam (1)

Zone (2)

Active capacity (106 m3) Evaporation losses (10* m3) (3)

Loss capacity ratio (%) (4) = (3)/(2)

Jibiya

Sahel

121

36.4

30

Zobe

Sahel

170

54.0

31

Bakolori

Sahel

403

96.0

24

Goronyo

Sahel

933

280.0

30

Kiri

Sudan

325

132.0

41

Challawa Gorge Sahel

904

120.0

25

Tiga

Sudan/Sahel 1845

214.0

12

Kafin Zaki

Sudan

2500

300.0

12


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and 23 % during the dry season. Evaporation losses amount to 29 %. The actual releases seem to be lower than the planned quantity because the irrigation projects have not fully taken off in three of these dams. The Kainji and Jebba dams are thus deprived of more than 60% of their former inflow from the basin before the rivers were dammed between 1982 and 1990.

Irrigation development Fadamas (the Hausa word for floodplain wetlands) occupy 7% (66 000 km2) of the land in Nigeria; in the northern Nigeria, they cover 5-7% of the land area. The severe climate and the frequency of drought and desertification in the latter area have underlined the potential of the fadamas located within their borders. Often the greatest agricultural activities on an all-year basis are confined to the fadamas. In Sokoto State in the northwest, for example, though both irrigated and rain fed agriculture are practised, the latter is more developed in the fadamas. The Hadejia-Jama'are-Yobe wetland in the northeast is also important (Table 5). It contains a fadama segment known as the Hadejia-Nguru wetland, one of the few remaining wetlands of appreciable size in the zone which has not been converted to cultivation. It is known as haven for migratory palaearctic birds and has attracted the interests of the World Conservation Union (IUCN) and other international organizations. In general the fadamas play an important hydrological function. By trapping flood water, the fadama mitigates the downstream effect of flooding and helped to maintain a dry season as much of the seasonal storage eventually reaches the river channels. The annual river flooding and fadamas are also vital for recharge of the shallow aquifer. However, with extensive clearing of the fadamas for dam projects, the natural conditions and ecosystems that prevailed in the past have been drastically altered. Social conflicts have also become more intense and the hydrological regime has become largely manmade. If the existing dams store all inflow in the wet season in the reservoirs, projects will face critical water shortage problems. We have noted above that some dams have stored water for years only to let it evaporate because the irrigation systems have not been developed. Existing large irrigation projects cover fadama as well as areas of rain fed and irrigated agriculture which have reasonable agricultural productivity. For example, the planned area to be irrigated by the dams indicated under the "standstill" projects listed in Tables 1 and 4 reaches about 155 000 ha. The fadama area accounts for nearly 80% of the area while half of these fadamas will be served by reservoir water. In order to use the existing reservoir water effectively before the irrigation system development is completed, the reservoir water should be released both during the wet and dry seasons to fadama areas where farmers are engaged in traditional irrigation, requiring small weirs, lifting pumps and small flood protection dykes for lift irrigation, as well as flood farming, flood recession cultivation and fishing. Water demands of existing irrigation projects are about 5.46 x 109 m3, of which the Sahel accounts for 58 %. However it accounts for as much as 87 % of the demand meant for operation of irrigation in standstill status. In terms of the areal extent of the projects the SSZ accounts for 98 %. The Sahel is to be beneficiary to the tune of 62% of all the water required to operate irrigation schemes in Nigeria. Water demand of existing and

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Table 4 Irrigation demand of existing public irrigation projects (10 6 m 3 ). Systems fully developed

Systems partially developed

Standstill areas

Total

Northwest

82

332

859

1273

Northeast & central

276

507

1107

1883

Total (Sahel)

358

833

1966

3156

Total (Northern Nigeria)

690

2008

2262

5461

Sahel as % of total

52

41

87

58

Source: FMAWRRD (1993)

Table 5 Selected standstill projects and the fadama. Area

Project

Dams

Total area (ha)

Fadama area {%) (ha)

Northwest

Zobe & Middle Rima Valley, Birnin Kebbi & Zauro Poulder

Zobe, Jibya Goronyo

79 500

75 000 (94)

North-central

Dadin Kowa

Dadin Kowa

38 000

8000 (21)

Northeast

Kano River Project II, Baga, undeveloped areas of south Chad

Tiga, Baga Challawa

108 500

40 000 (37)

155 000

123 000 (73)

Total

planned formal fadama type projects (to be practised only in the dry season) is 1.9 X 109 m3 ha, of which the Sahel accounts for 72% (Table 5). According to the National Fadama Development Project report the potential fadama area to be developed by shallow wells is about 0.86 x 106 ha, of which 0.03 x 106 ha is already developed. There is no doubt that dam construction has radically altered the hydrological regimes of downstream areas including some negative effects that were not anticipated. The loss of flood water has disrupted the activities of Hadejia-Nguru and Birnin Kebbi floodplain traditional wetland (recession) farmers. The case of Tiga Dam has produced a controversy between several groups and communities. A number of conflicts have always arisen among the traditional floodplain farmers: between farmers on higher land who want more flooding and those lower down who want less; between farmers who want to delay flooding and fishermen who want early flooding; and between farmers who require access to fields over a longer post-flood season and pastoralists who desire early dry season access to grazing. Inefficient reservoir operation did not cause, but has magnified, the conflicts between these groups of wetland users. However efficient and well coordinated reservoir management can optimize water availability and reconcile much of the conflicting demands.

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REFERENCES FMAWRRD (1993) The Study on the National Water Resources Master Plan. Interim Report, JICA and Federal Ministry of Agriculture, Water Resource and Rural Development, Lagos. Hollis, G. E., Adams, W. M. & Aminu-Kano, M. (1993) The Hadejia-Nguru Wetlands. IUCN, Gland, Switzerland. Jensen, J. R. (1990) Decrease in point annual rainfall in Northern Nigeria. In: Hydrology in National UNESCO IHP/Federal Ministry of Water Resources, Abuja.

Development.

Oyebande, L., Sagua, V. O. & Ekpenyong, J. L. O. (1980) The effect of Kainji Dam on the hydrological regime, water balance and water quality of the River Niger. In: The Influence of Man on the Hydrological Regime with Special Reference to Representative and Experimental Basins (Proc. Helsinki Symp.), 221-230. IAHS Publ. no. 130. Oyebande, L. & Nwa, E. U. (1980) Effect of drought and water projects on the water balance and flow of regime in the Upper Hadejia basin, Nigeria. In: The Influence of Man on the Hydrological Regime with Special Reference to Representative and Experimental Basins (Proc. Helsinki Symp.), 215-220. IAHS Publ. no. 130. Oyebande, L. & Balogun, I. (1992) Water resources management in the semiarid areas of Northern Nigeria. Can. J. Development Studies special issue, 209-226.