EVALUATION OF GROUNDWATER POTENTIAL USING INTEGRATED GIS-BASED-MCDA AND WETSPASS MODELS: A CASE STUDY OF GELANA WATERSHED, ETHIOPIA

Abstract

Unlike Surface water, groundwater is a precious fresh natural resource in sub-surface formations that is very important for the water supply utilization of human beings. However, assessment and estimation of the groundwater potential in a certain area are still challenging in terms of time, and cost for resolving water scarcity problem and the management of groundwater systems, especially over such a highly heterogenous geophysical environment with limited Spatio-temporal data like the study area Gelana watershed in Abaya Chamo lakes basin. This study aims to identify and delineate the groundwater potential zones using MCDA and WetSpass model. The available two river gauging stations in the watershed are used to validate the WetSpass model for its applicability in simulating the long-term averaged annual water budget. The simulated annual water budget reveals about 77.22 % of total precipitation in the watershed is lost through evapotranspiration, 16.54% through surface runoff and only 6.24% recharges the groundwater system. Multi-thematic layers such as lithology, geomorphology lineament density, and drainage density, including recharge from the WetSpass model are considered as potential factors, which influence groundwater occurrence and formation-based-MCDA then employs analytical hierarchy process (AHP) techniques to identify the relative determinant contribution among selected five thematic layers in groundwater potential zonation. The weighted factors in AHP are procedurally aggregated using a weighted linear combination to provide the groundwater potential index. The results are then classified into low, moderate, and high zones based on groundwater potential index values. The identified groundwater potential zones are also validated using the wells and springs discharge data available in the region and the validation revealed that 70.67% agree. The result from weight comparison shows that lithology, geomorphology, and recharge were the dominant factors among others. About 45.88% of the area is identified as a high groundwater potential zone situated mostly at the central part corresponding to alluvial plains, high water bearing formation with low slope, whereas 39.38% are moderate groundwater potential found in the central axis from the southeast toward the north part where there is tertiary volcanic lower basalt or thick basalt regolith with gentle to moderately rolling topography features and moderate diffuse recharge available which enhance the productivity of thick basalt regolith, and 14.73% are low XII groundwater potential found along the western portion and scattered on the central southeastern portion of the study area due to high distribution of mountainous geomorphology and relatively less water bearing formation like quartzo feldspathic schist, fels, and gneiss. The result provides valuable insight into the groundwater potential in the study area for policymakers and the community toward efficient groundwater resource management practice.