DEVELOPMENT OF AN OPERATING RULE MODEL TO SIMULATE TIME-SERIES OF RESERVOIR RELEASES FOR INSTREAM FLOW REQUIREMENTS

The output from many of the instream flow assessment workshops, curren tly being held in South Africa whenever a major water resource develop ment is proposed, is a matrix of monthly flow rates that describe the recommended nature of a river's modified regime (IFR) that will mainta in the river in a pre-determined ecological condition after the develo pment has been implemented. This paper describes a technique that may be used to translate the IFR information into reservoir release operat ing rules for both low-flow and flood event releases and that can gene rate a time series of releases. The technique is based upon the use of a reference time series of daily flow data to represent the prevailin g climate and trigger the various releases. To avoid the necessity of having a reference-flow time series available for the specific IFR sit e, the release trigger is based on duration curve percentage point dat a. These values are believed to be more closely equivalent across adja cent catchments than are weighted flows. Ultimately, the technique is expected to be used for planning purposes, to illustrate the effect of a give IFR on the likely day-to-day pattern of releases, as well as o perationally, to control releases. This paper explains the technique a nd how it has been incorporated into a preliminary version of a model to simulate the pattern of releases that is expected to occur. The def inition of the operating rules and the application of the model are il lustrated using an example from the Luvuvhu River, Northern Province.