Water availability identification of underground river in the Gunung Sewu karst area using inverse model

Abstract

The karst area is identified in the Gunung Kidul Regency, the so-called Gunung Sewu karst area, as one of the Geopark areas in Indonesia. Gunungkidul Regency has had moderate to high yearly rainfall for the last ten years. However, this region has a problem with a surface water shortage. It is due to the characteristics of the karst, which allows rainwater to penetrate the geological layers accumulating in the certain noose yielding abundant underground water reserves. The purpose of this study is to assess the underground river water availability and the hydrological characteristics in the karst region using Inverse Modeling. Inverse Modeling is a method to identify the features of the underground river by adjusting the hydrological variables as an input and output discharge as a control. The FJ Mock Model as a surface hydrological model is used to characterize the Karst region’s underground river. There were five models in total to be simulated. The conditions under the Mock model show consistent results compared to the stepwise model in terms of low discharge conditions; Coefficient Correlation, RMSE, and R2 values obtain 0.637, 17.29, and 0.407, respectively. In this case, estimating the low flow conditions is more accurate than the high-rate flow conditions. The results are closer to the stepwise model without the infiltration variable involved.