Integrating hydrology, hydraulics and ecological response into a flexible approach to the determination of environmental water requirements for rivers

There are many different approaches to environmental water requirement (EWR) determinations that have been reported in the international literature. Many of these focus on different aspects of the problem, while few of them consider all of the issues associated with the eventual implementation of EWR as part of integrated water management. It is also necessary to recognize that there is a wide range of different types and resolution of information available in different parts of the world. This paper presents an integrated framework that has been in use in South Africa for several years and which is based on integrating the impacts of changing flow regimes on different ecosystem response components through indices of ‘stress’ measured on a common scale (0–10). Software to support its implementation has been included as part of an existing hydrological modelling framework package that includes a GIS interface and database management procedures. The framework is flexible enough to be used with different approaches to analyzing ecosystem responses, ranging from complex hydraulic habitat assessments to the interpretation of expert opinion and therefore should be widely applicable. The framework can also be used to design a modified flow regime for a given set of ecological objectives, or it can be applied to assess scenarios of flow regimes based on a range of possible future water management options. The paper explains the approach, provides some illustrations of its application and discusses some of the issues associated with its more widespread use.