An Unexpected Factor Affecting Recharge from Ephemeral River Flows in SWA/Namibia

Abstract

The processes controlling recharge to alluvial groundwater have presented complex and even intractable hurdles to accurate estimates of this parameter. Casual observations in ephemeral rivers in SWA/Namibia have indicated that variable recharge quantities result from similar flood events. For this reason the Department of Water Affairs initiated a research project to identify and investigate relevant parameters. To achieve these objectives three carefully chosen natural channel sections were identified for instrumentation, a simulated channel was constructed under laboratory conditions and the basis of a mathematical model laid down. This paper comments on the field and laboratory instrumentation applied and the results derived from the first fully instrumented natural river channel for which two seasons of flood flow and resultant recharge data are available. These results are compared with laboratory trials for a range of flow regimes. Whilst this work is at an early stage it has already become clear that silt carried by flood waters can effectively seal the alluvial surface even during the flood event at unexpectedly high flow velocities. Thus the other processes controlling recharge in the unsaturated zone may become relatively unimportant for much of the duration of any given flood event.