Understanding groundwater/surface-water interactions through hydropedological interpretation of soil distribution patterns

Abstract. Understanding and quantifying groundwater/surface water interactions is important for effective water resource management. Characterisation of these interactions are however difficult due to heterogeneities in landscapes on difficulties in measuring hydrological processes at different scales. Although soils play an integral role in the hydrological functioning of landscape, very few groundwater/surface water interaction studies consider soils as key components of hydrologic variation. In this study, 21 catchments in South Africa with available stream attributes such as baseflow index (BFI) and an index of streamflow variability (CVB) were identified. The soils of the catchments were interpreted and grouped into four classes based on their dominant hydrological response namely: Recharge, Interflow, Responsive (shallow) and Responsive (wet). The dominant soil distribution patterns in the catchments were then determined. Significant positive correlation coefficients (r) exists between BFI and soil attributes such as depth (r = 0.72), clay content (r = 0.50) and percentage coverage by ‘Recharge’ soils (r = 0.78). The occurrence of "Interflow" and shallow soils decreased BFI significantly (r = −0.79 and −0.66 respectively). CVB are however positively correlated to the area of "Interflow" soils in the catchment (r = 0.73) and negatively to the area under "Recharge" soils (r = −0.65). Soils dominant in the valley bottom suggest that there are considerable differences in groundwater/surface water interaction mechanisms. Based on the results three perceptual models were constructed: (1) vertical drainage through soils and recharge of groundwater in the upper slopes of the catchment are dominant, with return flow to the soil in lower lying positions, both the soil and groundwater contribute the stream) (2) vertical drainage through soils and recharge of groundwater dominant in upper and lower lying positions, no return flow to soils, only groundwater contribute to stream and (3) lateral flow at soil bedrock is dominant throughout catchment, limited recharge, stream fed through lateral flow from soils with limited groundwater contribution.