Towards the incorporation of magnitude-frequency concepts into the building block methodology used for quantifying ecological flow requirements of South African rivers

Abstract

The current approach to setting the water quantity component of the ecological reserve for rivers in South Africa is based on the building block methodology (BBM) and carried out by a group of specialists in an IFR workshop. The BBM concentrates on setting flow magnitudes and there has been no formally defined approach to specify the frequency of exceedance relationships for these flows. This is a problem when attempts are made to implement the recommendations within the context of a water resource plan or management scheme for the river. In such situations information on how often flows of different magnitudes should occur is required. This paper offers a suggestion for improving the BBM by using a model to simulate a representative time series of modified flows and extract from the results assurance levels, or frequency of exceedance, for the different building block components. The implications of adopting such an approach, from both ecological and water resource development perspectives, are discussed.

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