A desktop model used to provide an initial estimate of the ecological instream flow requirements of rivers in South Africa

Abstract

The South African Water Act of 1998 stipulates that future water resource developments should be environmentally sustainable and that a component of the natural flow of rivers should be reserved to ensure some level of ecological functioning. Detailed methods for quantifying the environmental instream flow requirements of rivers have been available internationally and in South Africa for some time, but the implementation of the new act introduced a degree of urgency and pointed towards the need for rapid, low-confidence assessments that could be used for initial planning. The desktop reserve model was developed to fulfil this requirement, but since its development in 1999 there have been many more detailed IFR determinations. During 2001 the model underwent a complete review based on the results from 97 detailed IFR determinations involving groups of specialists applying the building block methodology (BBM). The paper explains the conceptual links between the BBM and the model, describes the algorithms that constitute the model and discusses some of the problems associated with developing a model of this type from limited data and limited scientific understanding of the underlying principles. Most of the problems are related to the lack of a consistent understanding of the relationships between hydrological variations and ecological functioning of rivers across a range of river types within a large geographic region (South Africa, for example). The paper indicates that despite these problems the model has found a niche in the water resource management industry within South Africa and has the potential to be improved as our scientific understanding of eco-hydrology improves.