Optimum water utilization of a cascade reservoirs case study of Batujai and Pengga reservoirs in Central Lombok district, West Nusa Tenggara Province.

Batujai and Pengga Reservoirs are water infrastructure with a cascade operation system. The water source comes from the Penujak River of the Dodokan River Basin in Central Lombok District, West Nusa Tenggara Province. Batujai Reservoir began its operation since 1982 with a storage capacity of 15.59 million m³, and a storage area of 5.89 km2. This reservoir was built to meet the water demands of the Batujai Irrigation Area, which covers 3,390 ha, and domestic water of 130 l/sec. Pengga Reservoir is located about 13 km at the downstream of Batujai Reservoir with a storage capacity of 27.00 million m³, and its operation started in 1995. Pengga Reservoir serves the irrigation water in West Lombok District with an area of 3,189 ha. The two reservoirs have been operating for quite a long time, therefore, there is a decrease in effective storage capacity due to sedimentation. This problem has impact on the operational performance of the two reservoirs in serving the irrigation and the domestic water. Therefore, it is necessary to conduct research to improve the performance of water utilization in the cascade reservoir system by considering the current condition of reservoir and water demands. This study aims to maximize the annual cropping intensity by optimizing release of water in both reservoir simultaneously. The optimization model was formulated using linear programming method by considering water availability, priorities for water supply, storage characteristics, cropping pattern, and cropping schedule. The model formulation used k factor, the ratio between actual release and target release discharges according to the water demand with a minimum value of 0.7 for irrigation water, and 0.85 for domestic water. Evaluation of optimization results uses several indicators of water utilization, namely planted area, crop intensity, and reliability of water supply, which was calculated for a dry year, normal year, and wet year inflow condition. [ABSTRACT FROM AUTHOR]