Groundwater Recharge under Changing Landuses and Climate Variability: The Case of Baro-Akobo River Basin, Ethiopia

Abstract

Impacts of landuse/landcover changes coupled with climate variability are well felt in areas of pristine environments like the dominantly high rainforest covered Baro-Akobo river basin of Ethiopia that form the Sobbat River system. Recharge is one of the hydrologic components to be influenced by induced anthropogenic changes. Quantifying and understanding changes in recharge and related hydrologic parameters help to properly manage the water resources and protect such vulnerable hydro-ecosystems. Among the various recharge estimation methods, WetSpass model is used for its capability to estimate recharge by coupling surface-subsurface water balances. It is also used to simulate yearly or seasonal averages of groundwater recharge, evapotranspiration and runoff that help to understand the basin’s hydro ecosystems dynamics. Modeling is done for the entire river basin of Baro-Akobo, taking into account landuse/landcover changes, varying climate and other physical parameters over the past four decades. Simulation results are validated using previous estimates, empirical methods using data from monitoring wells and isotope measurements. Bias corrected Climate Forecast System Reanalysis (CFSR) data are used to fill gaps in data scarce areas and to generate potential evapotranspiration. Field measurements, secondary data and fifteen months long three hourly well monitoring data are used to determine seasonal depth fluctuations of the groundwater. Landuse/landcover change of three periods, 1973 to 2014, and planned large scale development activities are used as inputs to see induced corresponding changes in the hydro-ecosystem. The simulated result showed small increase in runoff and despite the increase in temperature, there is a decrease in total evapotranspiration and significant increase in recharge in the recent period. The simulated result is also in line with the hydrogeologic characteristics of the basin making it a basin with low recharge as compared to basins in the central and eastern parts of the country. Coupled with other hydrological and hydrogeological characteristics of the basin, the result helps to understand the reason for finding large volume of surface flow entering the Machar wetlands and eventually reaching the White Nile from a relatively small watershed.

Keywords: Land use- Land cover; Recharge; climate variability; WetSpass; Geology