"Optimal" reservoir operating policies with explicit consideration of economic and environmental requirements

Reservoir operation has traditionally been dominated by the need to 'optimise' the 'economic' returns achievable from the reservoir. With the growing emphasis on environmental considerations, especially in water resource development, reservoir operating policies that enable the best economic performance to be obtained while meeting specified environmental requirements are now being developed. Various methods which recommend general guidelines for the specification of these environmental flow conditions were examined. Detailed biological studies in the region of a specific reservoir, which give the best indication of the river's requirements, and therefore provide the best parameters to incorporate into optimisation models for reservoir operation, are very costly. Even when such studies have been completed, it is still often difficult to specify what levels or type of flows are environmentally acceptable. Recent research in Australia has suggested that in the absence of detailed ecological studies, environmental flow targets should be specified in terms of certain percentiles of historical flow. A stochastic dynamic programming approach to optimising the 'economic' operation of a reservoir while maintaining some of the statistical characteristics (variation) of the natural flow regimes in the system has been developed. The approach is run on a scenario basis in which the environmental release strategies are specified as targets for environmental flows expressed predominantly in terms of percentiles of historical flows. The probability distribution of total releases (for economic and environmental purposes) from the reservoir, as determined from the steady state operating policy, is used to derive the probability distribution of the flows available for environmental purposes. This probability distribution, or more correctly this variation, of flows can then be compared to the variation associated with the natural regime. In this manner, the trade-off between economic and environmental demands on a reservoir can be assessed on the basis of both total flow released and how closely the pattern of releases of that total flow matches the natural regime. This model is demonstrated on the Fairbaim Dam system in Central Queensland.