Modelling Surface and Ground Water Interactions of Borkena River Catchment, Ethiopia

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Addis Ababa University


Inadequate water resources supply among different water users can be viewed both from social and economic dimension. The Borkena Catchment is affected by recurrent draught due to climate change in every eight to ten years. To sustain the water security of the catchment understanding the groundwater surface water interaction and management is necessary. The general objective of this study is to analyze the surface and groundwater interactions; specifically to estimate the recharge, to identify the regional and local ground water in the system and to asses if the borkena river is gaining or loosing river. ArcSWAT was applied to understand the water balance components of the study. The SWAT model result was calibrated by using SWAT CUP SUFI2 algorithm with daily stream flow data ranging from 2000-2007 and validated with stream flow data ranging from 2008-2014. The groundwater and management parameters are more sensitive (groundwater revap coefficient, groundwater delay, threshold depth of water in the shallow aquifer vital for return flow to occur (GWQMN), deep aquifer percolation fraction (RCHRG_DP)). The model goodness of fit was evaluated with coefficient of determination and the Nash-Sutcliffe Coefficient on a monthly basis for both calibration and validation periods and resulted 0.68 and 0.66 for calibration period and 0.64 and o.63 for validation periods respectively. After calibration the ArcSWAT model was conducted and the result was good. From the SWAT model the annual rainfall of the whole catchment is 1017.5mm. The recharge calculated is found 12% of annual rainfall, surface runoff is 62% of the annual rainfall and evapotranspiration is 58% of the annual rainfall. Based on the produced hydrogeological cross sections, hydrogeological map, and groundwater level map from deep boreholes water level and physico-chemical analysis water types data, there is groundwater transfer from Abay basin (Gerado) to Awash basin (CombolchaKemisse). The water types at Gerado/Abay basin (KCTVW-01-19) are Na-HCO3-CO3), water type at Combolcha/ Awash basin (KCVTW-02-19) is Ca-Na-Ca-Mg-HCO3 and water type at Kemisse/Awsh basin (KCVTW-03-19) is Na-Ca-HCO3-SO4 which are similar but due to geo-chemical processes evolution of compounds is identified. This indicated that there is regional groundwater flow (transfer) from Abay (Gerado) to Awash (Combolcha-Kemisse) and from the NW-N-S or Kutaber-Desse to Combolcha and Kemisse locally. The surface water and groundwater exchange is analyzed based on the boreholes groundwater elevation and borkena river elevation data the positive values in each river and borehole data indicated that there is river water seepage to aquifers and negative values indicated groundwater flows from aquifer to river. The analysis result indicated that 41.66 % is effluent or gaining river, 36.11 % is influent or losing and 22.22% is effluent or Influent River. This also confirmed with the SWAT-MODFLOW model result for 7448 river cells. A further study to identify Regional groundwater flow from Gerado/Abay to Combolcha/Awash basin and Surface water and groundwater interaction using Isotope analysis method is recommended.



Groundwater-Surface water Interactions, ArcSWAT, SWAT-MODFLOW, Borkena Catchment.