IWRA Proceedings

AbstractGlobally water is unevenly distributed in time and space. In Swaziland the Highveld region receives high rainfall ranging from 1200 to 1500mm annually while the Lowveld region receives the lowest amount of rainfall ranging from 300 to 500mm annually. The Planning, development and management of water storage and transmission facilities is an attempt to change the spatial and temporal availability of water, to make it available where and when needed. The Government of Swaziland has undertaken the development of water storage and distribution facilities as a process towards overcoming the unevenly distribution of the water resource. However, the current water storage facilities with a total capacity of 568.65x106 m3 can not meet the water demand which stands at 1795x106 m3 annually. The overall objective of this project was to develop an optimum and sustainable utilization decision support tool for water allocation and rationing options for national water reservoirs. A Decision Support System (DSS) is defined as a user-oriented computer system which supports decision-makers in addressing unstructured problems. The rationing mode in the developed DSS was used to distribute the available stored water at the beginning of April in order to arrive at a priori predetermined reservoir storage level at the end of October. The DSS has three major components and that is the model input, modeling options and outputs screens. The model input component comprises of technical data (inflow, reservoir level, intended % volume at end of October and allowed lowest rationing %), and policy data (uniform rationing or differential rationing). The modeling options comprise of commands to determine optimum rationing %, provide % rationing and observed dam response and imposed rationing %. The output of the DSS comprises of the optimal rationing (%), monthly reservoir volume from April to end of October as well as a graphical representation of the reservoir response and the final recommendation. The DSS also provides the total crop hectarage and thus the net benefits for the different initial and end of season (October) for the corresponding optimal irrigation water rationing (%). The results of the DSS with a starting reservoir level at 15m at the beginning of April and a 10% reservoir level at the end of October, is that there is no need for rationing (that is the water demand for all uses are met) and the reservoir level stands at around 29% at the end of October. The results of the DSS with a starting reservoir at 6m at the beginning of April is that the rationing is 46% for all uses in order to arrive at a 10% reservoir storage level at the end of October is not recommended. This is because, with 54% deficit, there is a danger of crops to wilt beyond recovery. The final DSS recommendation is irrigation rationing at 60% (i.e. 40% water deficit) combined with a reduction of 14% of the hectares of irrigated fields. The Nyetane dam was used as a pilot reservoir in the development of the DSS. The developed DSS with all the relevant input data can be used to allocate water efficiently in other reservoirs in the country.