Hydrology and land use in a sahelian floodplain wetland

Abstract

Africa’s numerous floodplain wetlands provide a host of hydrologic, ecological, economic, and social benefits. They play a fundamental role in supporting significant human populations. In common with all wetlands, the hydrology of Africa’s floodplains is central to their functioning and in turn plays a key role in determining the benefits that they provide. The timing and extent of wet-season inundation are major influences upon the temporal and spatial distribution of floodplain resource use. Using results from intensive hydrologic surveys, supplemented by analyses of river flow data and flood extent maps, this paper delineates areas within the Hadejia-Nguru Wetlands of northeastern Nigeria according to their hydrologic characteristics. This delineation is used in combination with a land-use map derived from aerial and field surveys to examine the impacts of hydrology upon human activities and land use. The timing of different cultivation practices in the wetlands is shown to be in tune with the annual flood cycle. Rice is cultivated in inundated areas that are then planted with other crops after the floods recede. The intensity of fishing and cattle grazing also varies with the pattern of rising and falling water levels. The distribution of major land uses is strongly influenced by spatial variations in hydrologic characteristics. The location of rice cultivation and small-scale irrigation is determined by water availability in the wet and dry seasons, respectively. Extensive flood rice farms are found along the Jama’are River, which contributes over 70% of the river inflow to the wetlands. Within the hadejia River system, important flood rice areas are the Madachi Swamp and the Marma Channel. The primary area of small-scale irrigation is along the Burum Gana River in which small, but year-round, flows are maintained by return flows from upstream irrigation schemes. This area, which covers only 13% of the land-use map, contains 48% of the area of small-scale irrigation. Extensive inundation seems to act as a barrier to human activities in some areas and is responsible for the maintenance of natural vegetation within central parts of the wetlands. Modifications to the hydrologic characteristics of some parts of the wetlands resulting from drought and upstream irrigation schemes have required local communities to adapt, with varying degrees of success, their modes of floodplain resource use. Farmers along the Burum Gana have embraced small-scale irrigation, while those along the Keffin Hausa River, which has all but dried up, have had no option but to concentrate on rain-fed agriculture. The strong inter-connections between hydrology, land use, and human activites dictate that any further hydrologic modifications should not be undertaken without a full appreciation of their potential impacts.