The water use sector in South Africa is currently implementing the National Water Act (Act 36 of 1998), which requires, amongst other things, improved planning of expected water demands. A need was identified for the development of a computer model that can assist Water User Associations (WUAs) to easily and effectively capture data for management and planning purposes. The existing Water Administration System (WAS) computer program was used and a new module developed to capture data and

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... calculations so that irrigation demand can be determined for different time periods (daily, seasonally, annually, etc.) and management levels (field, farm, and scheme). The crop yield (ton/ha) can also be captured at the end of a growing season and used to calculate the total yield (ton) and the yield per unit of water (g/m 3 ). A summary of water demand for a specified period can easily be generated per crop type, and all the crop water demand information can easily be linked to a geographic information system (GIS). The module was implemented at the Orange-Riet WUA where it is used to model irrigation demand for 16700 ha of mixed crops irrigated from river and canal distribution systems.