Hydro-ecology: Linking Hydrology- and Aquatic Ecology (Proceedings of Workshop i ÎW2 held at Birmingham. UK, July 1999). IAHS Publ. no. 266. 2001. 101 Streamflow regime change and ecological response in the Lake Chad basin in Nigeria LEKAN OYEBANDE Department of Geography, University of Lagos, PO Box 160, Akoka, Lagos, Nigeria e-mail: [email protected] Abstract Three major factors account for the streamflow regime change in the Komadugu-Yobe river system. The combined effects of the Sahelian drought and the partial conversion of flood flows to dry-season releases by Tiga Dam reservoir has decreased the coirtribution from the Hadejia to the Komadugu- Yobe River since 1974. Blockages by weed growth and siltation in the Hadejia have also jointly contributed to the decline in river flow. The change in the flood regime has directly impacted certain components of the Yobe basin ecosystems including plant and fish species diversity and quality. A certain minimum flood extent is required in the Jama'are and Hadejia rivers to sustain their rich wetland ecosystems at reasonable level, particularly areas of high biodiversity such as the Dagona water fowl sanctuary, the Gorgoram forest reserve and the fish resources. Key words Remote sensing and G1S; flooding; drought; ecology; wetland; birds; dams; Sahel; siltation; degradation CLIMATE AND HYDROLOGY The part of the Lake Chad basin within Nigeria is drained by five river systems. The largest of them, the Komadugu-Yobe river system is the focus of this paper. The Komadugu-Yobe river system drains an area of 147 840 km2 (85 470 km2 within Nigeria), before reaching Lake Chad. The Komadugu-Yobe River in turn comprises three main rivers: the Hadejia, Jama'are, and the Misau. The Hadejia is formed by the confluence of the Challawa and Kano rivers about 20 km south of Kano (Fig. 1). For the first 48 km of its course the Hadejia River maintains a gradient of approximately 1 m km"1. As it descends from the crystalline basement complex and enters the ancient lacustrine basin of Lake Chad however, this gradient reduces abruptly and the channel becomes poorly defined and characterized by numerous small oxbow lakes. The Jama'are River, like the Kano, has its headwaters in the Jos Plateau where it begins with a relatively high gradient, cutting through volcanic and metamorphic rock terrains before entering the lacustrine basin of Lake Chad northeast of Foggo. The Misau River rises north of Bauchi and flows northeast to join the Komadugu-Yobe River about 128 km from Lake Chad near the town of Damasak. Most of the headwaters have high sediment carrying capacities, hence the deposition of their load of silt and fine sand ftirther downstream, and the resulting aggraded valleys of poorly-defined channels with numerous small oxbow lakes. Annual rainfall varies from less than 400 mm in the north to over 1200 mm in the south, and the length of rainy season from 3'/2 to 6 months. Four rainfall zones have been identified (Northeast Arid Zone Development Programme, 1995). Zones (3) and Fig. 1 The Yobe basin. Streamflow regime change and ecological response in the Lake Chad basin in Nigeria 103 (4), the two northernmost zones do not experience overall water surplus anytime during the year. These two zones receive 400-600 mm and less than 400 mm of rainfall annually respectively whereas potential evaporation can range from 3000 to more than 4000 mm annually. The result is that high evaporation and low rainfall preclude surface streams except flashy flows generated during intense falls of rain. The northernmost zone (Zone 4) is the driest, real Sahelian zone typified by conditions in Damasak, Gashua, Geidam and Kukawa. The water balance is such that soil moisture deficits occur from September to July and soil moisture recharge only occurs during August and September. Surface streams are rare while those that flow from humid zones have their water depleted by evaporation and infiltration. The coefficient of runoff ranges from 13 to 22% and approximately 80%o of the runoff occurs from July to September with peaks occurring in late August. About 70%o of the total annual flow is lost downstream as far as Gashua, and another 10% between Gashua and Geidam. Water losses among the streams of the Yobe system are due very largely to évapotranspiration in poorly drained flood plains and also to a small but significant portion of surface runoff, which infiltrates to groundwater storage. Such infiltration, which serves to recharge the upper zone aquifer of Lake Chad (the Lake Chad basin has three regional aquifers: the upper, middle, and lower zone aquifers) is possible in the area west of Gashua (Table 1). Alkali & Carter (1996) estimated the recharge of the upper zone aquifer in three sections Gashua-Geidam-Damasak-Yau (a distance of 286 km) as 16.98 106 m3 year"1. Inflow from Lake Chad adds 0.73 106 m3 year"1. In analyses of water losses from the Komadugu-Yobe River between Gashua and Geidam, the possibility exists that floodwaters from the Komadugu-Yobe may cross over the flood plain to drain through the Misau River or vice versa (Diyam, 1996; Oyebande, 1997). Similar losses occur along the Misau River. The Hadejia splits into three channels between Hadejia and Likori (downstream of the Madachi Swamps and near the main road). The three channels are the old Hadejia Table 1 Annual river flow and groundwater recharge. (a) River flow balance in the Hadejia-Nguru Wetlands Year 1975 1977 1979 1981 1983 1984 1985 1987 1989 1991 1993 1995 Outflow(106 1139 907 1016 993 699 381 931 682 1208 1425 1004 <115? m3) Outflow as % of 37 35 34 39 52 48 53 44 49 52 52 <5? inflow (b) Groundwater recharge between Hadejia and Nguru, 1991-1995 along the Hadejia River Changes in groundwater level 1991 1992 1993 1994 1995 Observed rise in groundwater level (average, cm) - 198 200 255 166 Groundwater level rise due to rain (cm) 21 71 123 39 Groundwater level rise due to river bed recharge 129 132 127 (cm) (c) Percentage of flow contributed by the Jama'are, Hadejia and Kafin Hausa rivers River 1991 1992 1993 1994 1995 1991- 1995 Jama'are 48 48 45 50 39 Averaj je = 46 Hadejia 47 48 51 45 53 Averaj ?e = 49 Kafin Hausa 5 4 4 5 8 Averaj ze = 5 Source: Hadejia-Nguru Wetlands Conservation Project (1996), Goes & Zabadum (1996). 104 Lekan Oyebande which leads to Gashua to form together with the Jama'are the Yobe River; the Marma channel which flows into the non-returning Nguru Lake; and the relatively small Burum Gana River (Fig. 1). An analysis of four years of pre-Tiga records (1964-1967) in the Yobe river system indicates that an average of more than 68% of measured runoff was lost from the Yobe system upstream of Gashua. Only 18% of total runoff reached Geidam (Oyebande & Nwa, 1980). Along the Misau River the average flow lost between Kari and Dapchi was 68% of the flow at Kari. Based on this analysis it is estimated that less than 10% of the total surface runoff from the Yobe river system reached Lake Chad even in the pre-Tiga era. STREAMFLOW REGIME CHANGES AND THEIR CAUSES Figure 2 shows the annual runoff of Hadejia at Hadejia and Komadugu-Yobe at Gashua between 1963 and 1996. The level of runoff dropped suddenly and significantly in mean (b) ! .J July - Nov [__"] Dry season Fig. 2 Annual runoff at Hadejia (1964-1995) and (b) at Gashua (1963-1996). Streamflow regime change and ecological response in the Lake Chad basin in Nigeria 105 1972/73 and remained low until 1975/76. In the case of the runoff at Hadejia, low flow or dry season flow increased at the expense of flood flows after 1976. These changes in streamflow regime coincided with the onset of the sahelian drought and the closure of Tiga Dam, which was filling during 1974-1976. (Oyebande & Nwa, 1980). A basin hydrological model that used monthly water balance data for the period 1964 to 1987 was used to simulate flood extent and groundwater storage in the Yobe Basin (Hollis & Thompson, 1993; Thompson & Hollis, 1995). The model indicated that the recharge of groundwater was lower than évapotranspiration from flooded soils, but higher than the discharge of Gashua (24% of river inflow). Groundwater storage was largely stable during 1964-1971 and 1975-1983, but fell in the early 1970s and especially in the late 1980s by an estimated aggregate of 5000 x 109 m3 as a result of drought and reduced flooding. The situation in the 1990s is indicated in Table 1, which shows that secondary recharge, that is, riverfed recharge, is consistently higher than direct or rainfed recharge. Causes of change in flow regime Two major and primary factors account for the streamflow regime change. The first is the prolonged Sahelian drought, which began in the mid-sixties. The droughts of 1971/72 and especially that of 1983 and 1984 devastated many parts of northern Nigeria, in particular the eastern half of the Komadugu-Yobe basin. Annual rainfall data for Kano in 1983 and 1984 amounted to only 60%> and 57% of the long-term average for the respective years (1905-1982). The corresponding values for Nguru station are 44 and 65% of the 1942-1982 average (Oyebande, 1997). The Ngadda, Yedseram and Komadugu Gana rivers did not flow at all, and the Lake Chad level dropped to an all time low, below the minimum of 2.0 m recorded in 1907. The Misau River, which derives from the Komadugu Gana and usually carries flash floods during the months of July-September was completely dry at Kari.