Hydrodynamic response on recharge events in a sloped aquifer in the May Zeg Zeg basin near Hagere Selam (Northern Ethiopia)

Problem setting : Sloped aquifers on elevated plateaus may respond hydrodynamically different than subhorizontal aquifer systems. Although they are often rather thin, they may be very important for local agriculture development in highlands. Controlled by gravity driven flow, the sloped aquifers become nearly completely dewatered in the dry season by downstream outflow and are refilled during the successive rainy season. This makes them extremely sensitive to weather conditions and climate variations. As the groundwater stored in these shallow, sloped watertable aquifers is the only water resource available in the dry season, their importance for local economic-social development is evident. Therefore a profound understanding of their functioning and responses to climate stresses is essential for a better management or for conceiving development strategies. In this paper, the example of a shallow water table aquifer in the May Zeg Zeg basin near Hagere Selam town is given as a representative case of a local sloped aquifer. Field measurements and observations have helped understanding the functioning of the aquifer and a groundwater model has been implemented that reproduces the monthly variation in water availability during a 12 year period. Methodology : Piezometric measurements in observation wells during the start of a rainy season have allowed to follow the refilling of this shallow watertable aquifer after the dry season. Piezometric measurements and field observations were integrated into a hydrodynamic system analysis to give a synthetic scheme of the functioning of the aquifer and to identify the main mechanisms that control the outflow of the groundwater and lead to the dewatering in the dry season. Runoff after rain events was estimated with the SCS-CN method. Based on meteorological data, groundwater recharge is estimated using a soil moisture water balance. Calculations with monthly and daily meteorological data were done and compared. A transient flow hydrodynamic groundwater flow model of the May Zeg Zeg basin has been made to reconstruct the yearly filling/dewatering cycle during a 12 year period (1995-2007) using the calculated aquifer recharge rates as an input boundary condition. Time evolution of the different discharge components and piezometric levels were obtained. Results and discussion : The results of the soil moisture balance model calculations show that the time resolution of the meteorological input parameters has a large impact on the obtained recharge rates. Using daily values instead of the often applied monthly totals, yields up to 30% higher recharge rates. The difference is larger in dry years. The hydrodynamical system analysis shows than the main discharge mechanisms that lead to dewatering include seepage outflow, spring discharge, discharge through fracture zones and percolation to underlying layers. The groundwater flow model results show the variation over more than a decade of the seasonal recharge/discharge cycle and its water balance components.