Hydrosalinity fluxes in a small scale catchment of the Berg river (South Africa)

Abstract

The occurrence of dryland salinity is widespread throughout semi-arid regions of the world. The sources of salts may be either rock weathering or rain deposition. Clearing of natural scrubland to make way for cultivated crops and pastures may also change the water balance, trigger salt mobilization and increase the salinity of water resources. These processes are suspected to be the main cause for salinization of the Berg river catchment (South Africa). The objective of this study was to determine the hydrosalinity fluxes associated with overland and subsurface (vadose zone) flow for different soils and land uses. For this purpose, the following data were collected during 2006 in a typical small scale catchment (∼40 ha) located near the town of Riebeek West: weather data, hydrological and water quality measurements, soil water contents and chemistry. The climate of the area is Mediterranean with winter rainfall of about 350 mm a-1. The chemical speciation of water and soil solution in the catchment is conservative, with Na+ and Cl - being the dominant ions. Soil water and salt contents varied seasonally. Due to the typical low intensity of rainfall, the fluxes of salts during individual runoff events were steady. Fluctuations in salinity due to local processes were buffered at a catchment scale. Uncultivated (bare) soil produced more runoff and higher salinity compared to pasture land. Overland flow varied between 3 and 18% of rainfall, mobilizing up to 24 g m-2 of salts during 2006, depending on soil properties, slopes, rainfall intensity and duration, and antecedent moisture conditions. Subsurface fluxes of water and salts were estimated with the HYDRUS-2D model. Management practices at farm scale are required in order to reduce salt mobilization and salinization at catchment scale.