The water balance of a seasonal stream in the semi-arid Western Cape (South Africa)

Abstract

A detailed water balance and conceptual flow model was calculated and developed for the Sandspruit catchment for the period 1990 to 2010 on a winter rainfall water-year (1 April -31 March) basis. The Sandspruit catchment (quaternary catchment G10J) is located in the Western Cape Province of South Africa and is a tributary of the Berg River. It contributes significantly to the salinisation of the mid-to lower-reaches of the Berg River and thus the hydrological drivers need to be quantified and conceptualised in order to develop salinity management strategies. Various components of the water balance, i.e. precipitation, evaporation, streamflow, recharge, etc., were monitored and quantified. In addition, stable environmental isotopes and water balance modelling were used to perform hydrograph separation as well as to quantify components of the water balance. Annual streamflow in the catchment during the period of observation was variable, ranging between 0.026 mm•a . On average, 6.5% of rainfall was converted to streamflow during the period of observation. Evapotranspiration was found to be the dominant component of the water balance, as it comprises, on average, 94% of precipitation in the catchment. Groundwater recharge was calculated to average at 29 mm•a -1. The water balance model (J2000) performed well during the simulation period with all measures of performance exhibiting acceptable values. Simulation results indicate that streamflow is driven by interflow from the soil horizon (94.68% of streamflow), followed by overland flow (4.92% of streamflow). These results, together with the physiographic conditions evident in the catchment, were used to develop a conceptual flow model. Streamflow is interpreted to be driven by quickflow, i.e. overland flow and interflow, with minimal contribution from groundwater, and is also more dependent on the rainfall distribution in time rather than on the annual volume. The correlation between average annual streamflow and average rainfall was observed to be poor, suggesting that alternative factors, e.g. the spatial distribution of winter wheat, the temporal distribution of rainfall, climatic variables (temperature), etc., exert a greater influence on streamflow. The water balance and conceptual flow model will form the basis for the application of distributed hydrological modelling in the Sandspruit catchment and the development of salinity management strategies. Results from this investigation, e.g. ET estimates, methods to quantify groundwater recharge, hydrograph separation, etc., could potentially be extrapolated to other semi-arid areas.

Quantifying the catchment salt balance: An important component of salinity assessments

“…Salt output is interpreted to be well in excess of salt input and considered to be mainly mobilised by interflow, as interflow from the soil horizon is the dominant contributor to streamflow (94.68% of streamflow). 17 As evapotranspiration exceeds precipitation throughout most of the year, leaching of salts is likely to be limited and soluble salts accumulate in the soil horizon. During rainfall events, the resultant run-off and infiltration periodically flush these salts throughout the landscape and into rivers.…”

“…The contribution from this aquifer to the Sandspruit River is considered to be minimal 17 and thus will also have minimal effect on the catchment salt output via streamflow.…”

“…The TSI for the 2007-2010 rainfall seasons was quantified using catchment average rainfall values calculated by Bugan et al 17 , and the average rainfall salt concentration derived by Flügel 9 . The limited data of rainfall chemistry (temporally and spatially) is a limitation to this study.…”

Validation of remotely-sensed evapotranspiration and NDWI using ground measurements at Riverlands, South Africa

“…The assumptions were closure of the surface energy balance and that the energy used for processes like photosynthesis was negligible. A detailed description of the scintillometer measurements and the set-up used at Riverlands is available in Jovanovic et al (2012).…”

“…Evapotranspiration (ET) and soil water content are the most critical variables in arid areas, where ET may reach nearly 100% of rainfall (Bugan et al, 2012). In addition, soil moisture and ET have a significant impact on the energy, carbon and water cycles.…”