Quantifying hydrological fluxes of contributing hillslopes in the Weatherley catchment, N. E. Cape, South Africa.

Abstract

Hillslope mechanisms and processes are a complex and dynamic set of interactions, but are nevertheless vital components of hydrology due to their critical interactions with surface and groundwater (Lorentz, 2001a). In order to observe and quantify these flow generating mechanisms, the Weatherley subcatchment was selected where the components of streamflow generation have been studied and can be quantified separately. Surface, shallow subsurface and the deeper groundwater interactions are particularly important when quantifying runoff generation from within hillslope, riparian and wetland zones as they are the dominant runoff generating zones within the Weatherley catchment. These components of flow are important to quantify for the further study of flow generation mechanisms, their dynamics and fluxes at the hillslope and small catchment scale, low flow contributions, climate change as well as the consequences of land use change (Lorentz, 2001b). Transfer functions were found to be the best adaptation of hydrograph separation for distributed hydrological modelling purposes when attempting to quantify the various streamflow hydrograph components. In this study, the runoff components were simulated along transects using the HYDRUS-2D model, where the simulated soil water dynamics are compared with the observed tensions and water contents at different depths within the soil profile in order to quantify the contributing hillslope fluxes to streamflow generation. The 2001 data set was used with the rainfall and potential evapotranspiration data being converted into rates according to the breakpoint rainfall data. The HYDRUS-2D modelling exercise is performed to calculate the variety of flux rates (timing and quantities) within the subcatchment, so that the overall stream hydrograph can be properly deduced when modelling this catchment with transfer functions in the future. An understanding of the driving forces as well as the behaviour of sources and flow paths was extracted from this thesis, along with gaining some knowledge about the mechanisms and behaviour of streamflow generating mechanisms at the hillslope and small catchment scale.

Troch et al (2003) clearly encapsulates the essence of modern day catchment hydrology in stating that hillslope response to rainfall remains one of the most central problems of catchment hydrology in order to quantify catchment responses. The processes whereby rainfall becomes runoff continue to be difficult to quantify and conceptualise (Uhlenbrook et al., 2003) and this is because the characterisation of subsurface water flow components is one of the most complex and challenging tasks in the study of the hydrologic cycle (Achet et al., 2002). Since trying to understand the temporal and spatial variability of moisture content and the subsurface flow mechanisms is a complicated problem (Achet et al., 2002), an attempt is made in this thesis to gain insights into the temporal and spatial variability of soil tensions and soil moisture content at various depths on hillslope transects by combining modelling exercises with field observations. From this modelling, the hillslope water balance and contributing fluxes are derived in effort to augment, at a later stage, the hillslope response functions.