Dryland salinity management in the semi-arid Western Cape (South Africa)

Abstract

Dryland salinity is a major factor affecting the water quality of the mid-to-lower-reaches of the Berg River, a pivotal source of water to Cape Town. The implementation of salinity management strategies is therefore essential. Distributed hydrological modelling has been identified as a potential management tool. The success of any mitigation strategy is however dependant on the comprehension and quantification of catchment salinity fluxes. The salt storage (in the regolith and underlying shales), input (rainfall) and output (in runoff) in a 152 km2 test catchment were quantified. The data was also used to facilitate the development and validation of a beta salinity module for the J2000 distributed hydrological model. Salt storage ranged between 19 ¿ 1 740 t ha-1, with the spatial distribution being a function of elevation and groundwater salinity (r2 = 0.75). Rainfall exhibits a salt content in the range of 14 ¿ 37 mg L-1 and salt output from the catchment is on average 6 700 t a-1 (0.5 t ha-1 a-1). The module is based on a mass balance approach and is able to simulate inorganic salt fluxes at a catchment scale, being sensitive to land use change. The model was able to account for all processes of the catchment salt balance. Scenario simulations can be performed to identify suitable land uses and management practices to mitigate the salinity problem. It is envisaged that the model and the methodology applied in this investigation may be extrapolated to other semi-arid catchments where dryland salinity is evident.