Groundwater availability and suitability evaluation for irrigated agriculture in Mbale district

Abstract

In the last years, farmers have been affected by drought in Mbale district and this has been attributed to climate change which causes variations in temperature and rainfall. This study aimed at evaluating groundwater availability and suitability for irrigated agriculture to mitigate drought. The study considered the potential, quality and vulnerability of groundwater. Groundwater potential was assessed using geospatial technique through Analytical Hierarchy Process (AHP) model. Thematic layers were prepared namely: land use and land cover, slope, soil, rainfall, lineament density, lithology, geomorphology and drainage density. All layers were integrated using the Mulvariate Clustering (MLC) technique. Ranking of each parameter was performed using Weighted Overlay Index Analysis (WOIA). Weights were assigned to each subject class basing on AHP results. The consistency of the outputs was tested by computation of Consistency Ratio (CR) and was at a reasonable acceptable level (0.029 < 0.1). Groundwater potential was delineated basing on the values of Groundwater Potential Index. Groundwater quality was determined using Irrigation Water Quality Index (IWQI) method. Thirty-three water samples were collected. Five chemical parameters were tested in the laboratory: chloride, electrical conductivity, bicarbonate, sodium and Sodium Adsorption Ratio (SAR) to generate the water quality database using Kriging interpolation technique. Computation of IWQI values for each source was made and was used to generate the IWQI map using the weighted summation. DRATIC method was used to delineate vulnerability of groundwater. Layers were generated using the following parameters: depth to groundwater, net recharge, aquifer media, topography, vadose zone impacts, and hydraulic conductivity. The aquifer vulnerability map was prepared by overlaying layers. Three different vulnerability zones were determined according to DRASTIC scores low (<100), medium (100-140) and high (>140). Integration of maps of quality, vulnerability and potential of groundwater was made using an unsupervised MLC classification method. The resulting clustered map was classified into five categories with their respective regions: 17.58% very poor (36.56km2), 13.84% poor (28.77km2), 12.69% good (26.39km2), 31.46% very good (65.39km2) and 24.43 % excellent (50.78km2). The sub counties in Mbale district that have inadequate and unsuitable groundwater where irrigated agriculture cannot be practiced are Budwale, Wanale, Busano, Bubyangu, Bufumbo and Nyondo since they belong to the zones of very poor and poor. The remaining sub counties belong to the zones of good, very good and excellent which implies that irrigated agriculture can be successfully practiced there.