Quantifying the impact of integrated watershed management on groundwater availability in Gerduba watershed, Yabello district, Ethiopia

Abstract

Integrated watershed management has a positive impact on groundwater balance. However, most parts of the study area particularly, Borana is the one which is suffering from severe rangeland degradation. Among others, heavy grazing, bush encroachment, gully expansion, topsoil fertility loss, sedimentation and less adequate water availability account for the greatest noticeable rangeland degradation phenomena. Due to these, water remains the most limiting resource for the pastoral and agro-pastoral communities of Borana (Coppock et al., 2006). To overcome these problems, efforts have been made to launch integrated watershed management programs; however, knowledge to quantify the impact of integrated watershed management on groundwater availability has been limited to date. The hydrology of the area was characterized based on its land use, land cover, soil type, slope position, rainfall, humidity, wind speed, temperature, evapotranspiration and runoff. Thornthwaite's soil-water balance model was used to determine the potential and actual evapotranspiration and results were 796.27 and 465.89 mm, respectively. The mean annual runoff from the catchment was computed using the runoff coefficient method. The catchment is characterized by two rainy seasons during the year. The mean annual rainfall of the catchment is 585.1 mm. As the result of soil and water conservation measures, the volume of surface runoff was reduced from 45.98 to 33.44% of the mean annual rainfall of the catchment. Inversely, the groundwater recharge increased from 12.8 to 55.14% of the mean annual rainfall of the catchment. Though, the difference in groundwater level in cistern and hand-dug wells after interventions was found to be 1.1 and 1.3 m, respectively. Thus, construction of additional physical conservation structures is suggested to further improve the groundwater availability in the area.