Development of an operating rule model to simulate time series of reservoir releases for instream flow requirements

Abstract

The output from many of the instream flow assessment workshops, currently being held in South Africa whenever a major water resource development is proposed, is a matrix of monthly flow rates that describe the recommended nature of a river's modified regime (IFR) that will maintain the river in a pre-determined ecological condition after the development has been implemented. This paper describes a technique that may be used to translate the IFR information into reservoir release operating rules for both low-flow and flood event releases and that can generate a time series of releases. The technique is based upon the use of a reference time series of daily flow data to represent the prevailing climate and trigger the various releases. To avoid the necessity of having a reference-flow time series available for the specific IFR site, the release trigger is based on duration curve percentage point data. These values are believed to be more closely equivalent across adjacent catchments than are weighted flows. Ultimately, the technique is expected to be used for planning purposes, to illustrate the effect of a given IFR on the likely day-to-day pattern of releases, as well as operationally, to control releases. This paper explains the technique and how it has been incorporated into a preliminary version of a model to simulate the pattern of releases that is expected to occur. The definition of the operating rules and the application of the model are illustrated using an example from the Luvuvhu River, Northern Province.