| Summary: | Water supply from South Lombok Interconnection Irrigation Network has not able to meet water demand of irrigation areas in South Lombok optimally, including Pandanduri and Swangi irrigation areas. To overcome this problem the goverment has been building a construction of Pandanduri reservoir, as the regulator of water resource management in Pandanduri and surrounding areas. The plan needs accurate information about water supply capacity of the reservoir according to the cultivation pattern of planting and characteristics of water availability. From previous studies, there was potential of return flow that occurs on land within the irrigation system. Therefore, it needs more detailed study to determine the optimum capacity of Pandanduri reservoir in water utilization by considering all its potentiality. This research is designed to optimize water utilization of Pandanduri Reservoir to supply water irrigation of Pandanduri-Swangi Irrigation System by taking into account the potential of return flow in order to obtain the pattern of optimum reservoir water release.
In order to obtain the optimum water resource utilization of Pandanduri reservoir, an optimization model based on Linear Programming method was formulated and then a water balance of Standard Operating Rule reservoir simulation model was applied in combination with the optimization model. Performance of water allocation is measured by the achievement of annual cropping intensity by considering the ratio of actual release and target release (k factor) and the reliability of reservoir operations. The amount of outflow was calculated based on target release that consisted of water requirements for flushing and irrigation water needs of both Pandanduri-Swangi and Gambir Irrigation System with some alternative cropping patterns and beginning of planting periods. Return flow from drainage water of Pandanduri irrigation area was taken into account to determine the amount of reservoir water release. Modification of Sugawara Tank model was applied in order to determine the amount of return flow.
The result shows that the application of the Tank model to determine the amount of return flow can increase the water utilization of Pandanduri reservoir. Previous studies showed that the available water in Pandanduri reservoir was able to meet the irrigation water demand with maximum annual cropping intensity 258,4% for 4.477 ha of command area. The output of simulation model concludes that supply capacity under constraint of k factor was 0,854 and the reliability of reservoir operation was 82,40%. Optimization results in this study showed an increase in cropping intensity to 269.5% and the reliability of reservoir operations is 83,7% for the same k factor is 0,854. The optimum water utilization of Pandanduri reservoir based on annual cropping intensity is 237,20%, it can be obtained by planting three classes of rotation water supply and cropping pattern paddy-paddy/palawija-palawija that the cultivation is begin in early October. The output of simulation model concludes that supply capacity under constraint of k factor is 0,920 and the reliability of reservoir operation is 90,0%.
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