A theoretical approach to the remediation of saline streamflow conditions downstream of the Nwanedi catchment reservoir

Abstract

The Nwanedi River Catchment in Limpopo Province, South Africa, experiences seasonal salinity accumulation that constrains downstream water use and undermines agricultural productivity. This study develops a hydrology-based framework to define environmental flow requirements (EFRs) to mitigate salinity through flow-driven dilution and flushing. A desktop methodology integrating the Revised Desktop Reserve Model (RDRM), GIS-based catchment characterisation, stakeholder insights, and empirical water quality analysis was implemented. Results reveal a distinct downstream increase in salinity, with electrical conductivity (EC) reaching approximately 146.8 mS/m during the dry season. Modelled EFRs indicate that, under Environmental Management Class D, minimum low-flow allocations correspond to 18.7% and 21.0% of mean annual runoff for sub-catchments A80H and A80J, respectively. Monthly dry-season flow releases range between 0.35 and 1.05 Mm3, while maintaining an average residual volume of approximately 1 Mm3 to support agricultural demand. These environmental flows are derived from naturalised flow regimes and are assumed to re-establish the critical hydrological conditions necessary to dilute and mobilise saline baseflows. The findings, therefore, do not quantify direct reductions in salinity but demonstrate that maintaining these flow thresholds can recreate the hydraulic conditions required for salt flushing, particularly in downstream reaches where salinity accumulation is most pronounced. This study provides a preliminary, process-based foundation for developing adaptive water allocation protocols. It establishes a testable framework for future cause-and-effect analyses, in which controlled dam releases and systematic water-quality monitoring can be used to quantify the extent to which EFRs reduce salinity in semi-arid, data-scarce catchments.