A simple flow-concentration modelling method for integrating water quality and water quantity in rivers

As part of the determination of the ecological Reserve for rivers in South Africa (National Water Act, 1998), flow requirements are assessed for maintenance low flow, drought low flow and flood conditions. Since water quantity and water quality are often closely linked, it is necessary to ensure that in setting the recommended flow regime, the appropriate water quality will be attained. This paper presents a simple method (based on solute rating curves) for predicting the instream concentrations of chemical constituents that will arise from different flow regimes. The method uses monthly mean flow (discharge) at a given site plotted against monthly median concentration of each chemical constituent. This is carried out for both the reference condition (the natural, or least-impacted state) and for the present state. The flow-concentration relationships obtained are used to predict the expected monthly concentrations under the recommended flow regime. The computed concentrations can be compared with the reference condition and present state values to assess the degree of river modification. This paper outlines the modelling protocol to be followed, considers the limitations and assumptions inherent in the approach, and the application of the resultant information. It is concluded that the modelling method is a useful screening tool for identifying sites where, without reduction of pollution, the water quality component of the Reserve is unlikely to be attained under the recommended flow regime.