SIMULASI OPTIMASI (SIMOP) WADUK PENGGA DAN WADUK BATUJAI PADA DAS DODOKAN UNTUK PEMENUHAN KEBUTUHAN AIR BAKU DI KAWASAN EKONOMI KHUSUS (KEK) MANDALIKA

Abstract

The Mandalika Special Economic Zone (SEZ) in the southern part of Lombok's river basin requires a large supply of raw water.  The Mandalika Special Economic Zone (SEZ) is located in an area with less supportive water potential, so it requires water supply from other sources, namely the Pengga reservoir in the Dodokan river basin (578.92 km2). In the Dodokan watershed there are 93 headworks (headwork / HW) in the form of 65 weirs, 26 reservoirs, and 2 dams downstream of the main river (Batujai and Pengga). The Pengga reservoir (Veff = 21000000 m3) is located downstream of the Batujai reservoir (Veff = 15429300 m3). The Pengga Reservoir is currently operated for irrigation (including a micro hydro power plant).  In addition, the reservoir is planned to serve 200 l/sec raw water for The Mandalika Special Economic Zone (SEZ). The reservoir is in the HLD (High Level Diversion) interconnection system so that it is interdependent with the allocation of water in other HWs. To obtain optimal water allocation, a water balance behavior study was conducted during the 25 year timeseries (1994-2019). Optimization simulations (simop) in each cascade reservoir use a mass balance equation with sustainability-efficiency-equity (SEE) criteria, constraints and main inputs in the form of local inflow and water demand. In the optimization simulation, pay attention to: i) overflow from HW in the upstream Batujai dam, ii) existing water needs in Batujai (2860 ha) and DI Pengga (3189 ha), iii) existing raw water needs in the Batujai reservoir 130 l / s, and iv ) as a result of the supply from DI Gebong to DI Pengga so that it reduced the water demand load to the Pengga reservoir by about 35%. From the hydrological analysis, the mean: i) rain in the entire catchment area is 1308 mm, ii) rain in DI 1366 mm, iii) local inflow in the Batujai reservoir 28.68 million m3 and the Pengga reservoir 26.42 million m3, iv) irrigation water needs (existing cropping pattern rice-paddy / palawija-palawija, planting intensity 300%, the beginning of the planting season in Batujai, Des1 and DI Pengga, Nov3) 99.84 million m3, and v) raw water needs of 10.41 million m3. It is recommended to take 200 l/s of raw water in the Pengga reservoir. Based on optimization simulations, the allocation of irrigation water can be proportional-evenly with the Kmin achievement of 41% -42% (K3 class), there is no K4 class (20% -40%) even in the year of the minimum local inflow. Reservoir performance with the coefficient of Reliability (R) = 100%, Resiliency (R) = 100%, and Vulnerability (V) = 0% which indicates that operations can be sustainable. The target of raw water supply to the Mandalika SEZ can be achieved provided that real-time water allocation must be tight, with centralized control by the WS management.