ASSESSMENT OF GROUNDWATER POTENTIAL AND RECHARGE ZONES IN ERER SUB-BASIN, WABESHEBELLE RIVER BASIN, ETHIOPIA

Abstract

Groundwater is a valuable and important natural resource for humankind for drinking, industrialization and irrigation in present day, but the availability is reduced gradually due to over exploitation and lack of management. However, identifying the groundwater potential zones in certain areas of Ethiopia is still challenging in terms of time and cost for resolving water scarcity problems and the management system of groundwater. The broad aim of this study was to assess of groundwater potential and recharge zones using integrated approach of Geographical Information System (GIS) and Remote Sensing (RS) Technologies with Analytic Hierarchy Process in Erer Sub-Basin, Wabeshebelle River Basin. Nine parameters (lithology, geomorphology, slope, lineament density, topographic wetness index, land use/cover, rainfall, drainage density, and soil) that affect groundwater potential and recharge were identified based on theoretical selection criteria setting (frequency of usage, criticalness of the factor and ease/difficult of data access) according to their significant role in groundwater occurrence. The thematic layers of lithology, geomorphology, slope, lineament density, land use/cover, rainfall, topographic wetness index, drainage density, and soil were prepared, with the weight assigned to each thematic layer of feature for developing the groundwater potential and recharge zone map. The weights of each thematic layer were determined by AHP method through Pairwise Comparison Matrix (PCM) and the reclassified thematic maps were integrated by a weighted sum overlay analysis (WOA) tool in ArcGIS 10.4.1 to develop a groundwater potential and recharge zones map. Accordingly, the result groundwater potential zone of the study is highly influenced by lithology (30.7%) and lineament density (21.8%), whereas rainfall (30.7%) and lineament density (21.8%) highly influence the groundwater recharge zone of the study area when compared with other factor. The result of generated groundwater potential map has five reclassified classes namely: very high, high, moderate, low and very low. The areas of very high groundwater potential are estimated to cover 391.19 km2 (9.84% of the study area), high groundwater potential 1672.59 km2 (42.06% of the study area), moderate potential 1216.65 km2 (30.6%), low potential 600.71 km2 (15.11%), and very low potential 95.43 km2 (2.4%). It was also found that the groundwater recharge zones has five reclassified classes namely; very high, high, moderate, low and very low. The areas of very high groundwater recharge are estimated to cover 463.53 km2 (11.66% of the study area), high recharge 1199.96 km2 (30.18%), moderate recharge 1661.07 km2 (41.77%), low recharge 546.72 km2 (13.75%), and very low recharge 105.30 km2 (2.65%). Finally, the groundwater potential zone map was validated using the existing water source inventory point (pumping well yield), and so most well yields are observed in the high and moderate GW potential zones in the west and from the central to the south and southeast parts of the study area, respectively. This indicated a good prediction accuracy of 89%. Thus, potential zones identified and delineated in the Erer Sub-Basin, Wabeshebelle River Basin by the GIS and RS based multi-criteria decision analysis technique are reliable.