Alternative model of Sidan Reservoir operation for Ayung River Basin using RIBASIM

Abstract: Bali Province in the next ten years is feared to experience a shortage of clean water. One effort that can be done to anticipate a shortage of clean water is by constructing Sidan Reservoir. Sidan Reservoir is located in the upstream of the Ayung River Basin. Sidan Reservoir is prioritized to meet raw water needs and it is also calculated to meet the needs of irrigation water in the downstream of the reservoir. The initial stage in this study is to analyze the availability of water as an inflow to Sidan Reservoir using the rainfall-runoff model of the ITB water balance method. Synthetic discharge as the results from the rainfall - runoff model is analyzed as Q90 dependable discharge. Furthermore, it analyzes raw water needs and irrigation water needs as an outflow from Sidan Reservoir. After the inflow and outflow are known, Sidan Reservoir operation simulation is carried out in four scenarios with RIBASIM software. Based on the results of the rainfall-runoff model from 2008 to 2017 and the verification with the observational discharge data from 2011 to 2017 showed good model accuracy and the error rate was relatively small with the smallest correlation coefficient of 0.50, as a result the Q90 dependable discharge inflow for Sidan Reservoir is obtained of 3.45 m3 sec-1. The raw water needs up to 2033 is 2.092 m3 sec-1, besides that it can be used to meet irrigation water needs of 0.35 m3 sec-1. The simulation results of Sidan Reservoir operation model based on the second scenario obtained a dependability level of 91.7%, the third scenario obtained a dependability level of 100%, and the fourth scenario obtained a dependability level of 87.5%. The third scenario with 100% dependability is the optimum result. Keyword: reservoir operation, dependable discharge, raw water needs, irrigation water needs, RIBASIM