Decision support for open-air irrigation reservoir control

Abstract

The availability of irrigation water greatly impacts on the profitability of the agricultural sector in South Africa and is largely determined by prudent decisions related to water release strategies at open-air irrigation reservoirs. The selection of such release strategies is difficult, since the objectives that should be pursued are not generally agreed upon and unpredictable weather patterns cause reservoir inflows to vary substantially between hydrological years. In this paper, a decision support system is proposed for the selection of suitable water release strategies. The system is based on a mathematical model which generates a probability distribution of the reservoir volume at the end of a hydrological year based on historical reservoir inflows. A release strategy is then computed which centres the expected hydrological year-end reservoir volume on some user-specified target value subject to user-specified weight factors representing demand satisfaction importance during the various decision periods of the hydrological year. The probability of water shortage for a given year-end transition volume may be determined by the decision support system, which allows for the computation of acceptable trade-off decisions between the fulfilment of current demand and the future repeatability of a release strategy. The system is implemented as a computerised concept demonstrator which is validated in a special case study involving Keerom Dam, an open-air reservoir in the Nuy agricultural district near Worcester in the South African Western Cape. The system's strategy suggestions are compared to historically employed strategies and the suggested strategies are found to fare better in maintaining reservoir storage levels whilst still fulfilling irrigation demands.