Assessing the Environmental and Social Impacts of Large-Scale Irrigation Development on Downstream Ecosystems in Kebbi State

This study assesses the cumulative environmental and social impacts of large-scale irrigation scheme development on downstream floodplain ecosystems and dependent communities in Kebbi State, North West Nigeria, contributing empirical evidence to the poorly documented relationship between upstream irrigation abstractions and downstream ecological change in the Sokoto-Rima river basin. The expansion of the Kainji Lake Irrigation Scheme and the Wurno Irrigation Project has substantially reduced wet season flooding in downstream floodplains where communities have historically relied on recession agriculture and fishing from seasonal floodplain inundation. Field measurements of floodplain inundation extent and duration at three downstream monitoring sites were compared against pre-scheme historical records and hydraulic model outputs. Wetland vegetation surveys, fish community sampling, and soil moisture measurements characterised the ecological condition of the downstream floodplain under current hydrological conditions. Social surveys were conducted with 280 households in six downstream communities to document livelihood changes since large-scale irrigation was established. Results show that downstream floodplain inundation extent has declined by 68 percent and duration by 54 percent compared to pre-irrigation conditions. Wetland vegetation cover has contracted, with mesic grassland replacing open water and aquatic macrophyte communities. Fish diversity and catch per unit effort have declined significantly, with net fish yield reduction estimated at 42 percent per fisher household. Recession agriculture area has contracted by 55 percent. Social survey data confirm significant income losses for downstream fishing and farming households. The study recommends a downstream environmental flow allocation under the Sokoto River Basin Authority. Keywords: irrigation impacts, downstream ecology, environmental flow, floodplain, Kebbi State.