HYDROLOGICAL SYSTEM ANALYSIS AND GROUNDWATER RECHARGE ESTIMATION USING SEMI-DISTRIBUTED MODELS AND RIVER DISCHARGE IN THE MEKI RIVER BASIN

Abstract

Grounqwater recharge estimated from a soil water balance model- and from river
discharge records were compared in the Meki River basin located in a clo~ central Main Ethiopian
rift. The first method is based on soil-water balance model (WATBAL) whIch uses long~term average
monthly hydrometeol'Ological data to quantify lumped soil water balance in three regions (rift,
escarpment and highland). The second method involves estimation of recharge using discharge records
with a computer code called BFI which uses a digital recursive filter to separate base flow from total
daily discharge. The base flow was considered to be lumped catchment groundwater recharge with
some adjustments for upstream channel losses (14.4 mm annually) estimated from field systematic river
discharge measurements. The results from BFI and WATBAL show general agreement on annual basis.
However, the base flow seems to indicate realistic temporal variations, especially in dry seasons. The
annual weighed basin reclw'ge excluding channel loss from BFI model and WATBAL is SO.1 mm and 79.2
mm respectively. The BFI model seems to provide realistic estimates of recharge for input into regional
transient hydrological models and it also appears to be a viable method of defining the base flow from
the river discharges against the classical manual base flow separation using regression curves. Both
long-term and field discharge records clearly revealed that the total. flow and recharge shows
substantial temporal and spatial variations. The model results and discharge records demonstrate that
the Meki River basin has relatively lower storilge capacity and fast response to rainfall