Groundwater Resource Potential Assessment in the Wama Catchment East Wollega Zone Western Ethiopia
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Date
2021-09-06
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Addis Ababa University
Abstract
This study aims to evaluate the groundwater resource potential of Wama catchment based on hydrometeorological, hydrogeological, and hydrochemical data analysis. Wama Catchement is the major tributary of Didessa Sub-basin and is situated between 36⁰ 24′ 0′′E to 37⁰ 24′ 0′′E longitudes and 8⁰24′0′′N to 090 0′ 0′′N latitudes. The total area of the watershed is about 3385.5km2. It is characterized by highly rugged topography, dendritic drainage pattern, and tropical to sub-tropical climate condition. Elevation of the area varies from 1300 to 3100 m.a.s.l.
From the long term mean monthly rainfall data, annual precipitation of the catchment is calculated as 1715 mm/ year. The PET value obtained by Penman method is estimated to be 1059.1 mm/yr. AET obtained from Turc and soil-water balance methods are 717mm/yr. and 860mm/yr., respectively.
The annual groundwater recharge estimated from water balance and chloride mass balance methods were 481mm and 204mm respectively. The wetSpass model have simulated the average annual long-term groundwater recharge, actual evapotranspiration and surface runoff in the catchment. The results are found to be 333.5 mm/ year, 908.2mm/ year and 473.3 mm/year respectively.
Lithologic units in the catchment include, granite, Quaternary sediment (Alluvial and Eluvium), Tertiary volcanics (Lower, Middle and Upper basalt, Lower and Upper Silicics, Trachyte flows and plugs) and Mesozoic Sedimentary rocks (Getema and Wama sandstone). Tertiary volcanic rocks are the most extensive lithology in the study area.
Based on qualitative and quantitative analysis of aquifer/aquitard characteristics, the basin is categorized in to moderately productive porous aquifers, moderately productive fissured aquifers, moderately productive mixed porous and fissured permeability aquifers, low productive fissured crystalline rock and aquitards. The fractured and weathered volcanic rocks are the main water bearing unit in the catchment. From the existing borehole data, the higher values of discharge, hydraulic conductivity and transmissivity zones are mapped at or near fractured regions.
Water flows from surrounding highland to the discharge areas following low hydraulic gradient and regional groundwater flow is generally toward the southwestern direction.
The hydrochemical analysis result shows that Ca and HCO3 are dominant cation and anion, respectively. Based on graphical plots, the major water type found in the area is Ca-HCO3 and Ca-Na/Mg-HCO3 with low TDS and EC value in highland areas (recharge zone) and Na-Ca/Mg-HCO3 with relative high TDS and EC value compared with the recharge area. Most of the water samples quality result shows the area is suitable for domestic and irrigation purpose.
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Keywords
Groundwater Potential, Hydrochemistry, Wama Catchment, Water Quality, Wetspass