Modeling of ground-water flow for the Amran Valley basin

Abstract

The 800 km2 Amran Valley is a basin lying in an intermountain graben produced by tectonic movement associated with the Red Sea rifting. The basin is filled by quaternary alluvial deposits to depths of300 m. The Amran Valley quaternary section constitutes the region's only known aquifer, the Valley aquifer. Groundwater occurs under water table conditions (unconfined aquifer) in the uppermost layer and under artesian conditions (semi-confined aquifer) for the lower layer. The aquifers are replenished underflow from wadis and surface inflow. The Valley aquifer suffers from severe overdrafts, which caused depressed water levels in the whole aquifer. Constructing a numerical flow model for (simulating) steady-state and transient-state conditions to better understand the hydro logic system of the basin is the main objective of this study. The steady-state condition was run for 1973 to produce the annual initial heads for that year, which was used as initial configuration heads for the transient-state conditions for the period 197 4 to 1977. Annual outflows as underflow from the basin to Wadi Attaf were estimated at 5.5 Mm3/year (NORDEP). The ground-water losses were estimated as 42% (NORDEP) from the total pumped irrigation, which is considered a low estimation, and it is increased to 60%. The surface recharge to the aquifer was estimated as 11 Mm3/year (NORDEP); it is considered high estimation and is reduced to 0.5 Mm3/year. Further investigations are needed to improve the steady-state model and recalibrate transient simulation for predicting responses of the hydro logic system to climate and anthropogenic stresses for the last time. Improved information on hydraulic parameters like hydraulic conductivity and transmissivity are needed to improve the model.