Effects of Reservoir Operation on the Hydrological Regime and Water Availability in Northern Nigeria

Man's Influence on Freshwater Ecosystems and Water Use (Proceedings of a Boulder Symposium, July 1995). 1AHS Publ. no. 230, 1995. 25 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. Yobe- Hadejia, 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 reser voirs, persistent droughts, faulty designs and operation of the multi purpose 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. Yobe- Hadejia, Sokoto-Rima and the Gongola river systems. The seasonal and long-term varia tions 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. 26 Lekan Oyebande 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 responsi bility 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 27 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 how ever, 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 opera tion. In Nigeria, the range of purposes served by storage reservoirs includes water Fig. 2 Major dams and reservoirs in northern Nigeria. 28 Lekan Oyebatide 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 indis pensable 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 construc tion 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. Effects of reservoir operation on the hydrological regime in Nigeria 29 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.

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