The delineation of alluvial aquifers towards a better understanding of channel transmission losses in the Limpopo River Basin

Abstract

Understanding the impact of key hydrological processes on the availability of water resources is an integral component of equitable and sustainable integrated water resource management. Previous hydrological studies conducted in the Limpopo River Basin have revealed a gap in the understanding of surface water-groundwater interactions, particularly channel transmission loss processes. These studies, focused largely on the Limpopo River’s main stem, have attributed the existence of these streamflow losses to the presence of significant alluvial aquifers and indicated that the losses account for about 30 percent (or 1000 Mm3 a-1) of the basin’s water balance. This paper reports on the delineation of alluvial aquifers across the Luvuvhu sub-basin using Landsat-8 imagery and the estimation of potential transmission losses based on the aquifer properties. To delineate alluvial aquifers, general land cover classes including alluvial aquifers were produced from Landsat-8 imagery through image classification. The areal extent of the delineated alluvial aquifers was calculated using ArcMap 10.3 Results indicate that the alluvial aquifers occur as relatively narrow channel alluvial deposits (32-124 m in width) and extensive vegetated floodplain deposits. In the Luvuvhu sub-basin, these are mostly located along the lower reach of the 200 km long meandering Luvuvhu River. The outcome of the delineation of the alluvial aquifer is seen to be consistent with existing regional hydrogeological maps. Based on the characteristics and size of the aquifer it is estimated that the capacity of the aquifer is approximately 9.34 Mm3, which could be ‘lost’ from the Luvuvhu River system at any given point in time. The actual transmission losses however depend on a number of factors including the level of flow, the size of the aquifer in contact with the riverbed, regional slope for water loss into the adjacent areas, antecedent moisture content of the aquifer and riparian evapotranspiration.