Analysis of Master Recession Curve (MRC) and flood hydrograph components for karstification degree estimation in Kiskendo Cave, Jonggrangan Karst System, Indonesia

Karst that has triple porosity makes the characterization of the aquifer becomes challenging to conduct. Hydrograph analysis is a standard method that commonly used for the characterization. It is considered capable of reflecting the natural characteristics of karst aquifer. This research was conducted in Kiskendo Cave which is part of Jonggrangan Karst, Kulonprogo Regency, Java, Indonesia. Kiskendo Cave is a tourist area that continues to be developed. On the other hands, this location has underground rivers that are utilized by the community to meet their domestic needs. Therefore, karst aquifer characterization is essential to do to support karst water management efforts in its catchment area. This study aims to apply the Master Recession Curve (MRC) to know the degree of karstification in the catchment area of Kiskendo. MRC was developed to calculate the degree of karstification and the analysis of hydrograph flow components was carried out to support the calculation. The data used in this study is the underground river discharge recorded every 30 minutes at the time interval between 23 October 2017 and 16 May 2018. The results show that the karst aquifer in the catchment area of Kiskendo Cave has a complex discharge regime with the degree of karstification 5.5. The complexity is confirmed by the type of conduit, fissure, and diffuse flows, which visible on the MRC calculations. Meanwhile, the analysis of the hydrograph component of flood strengthens the result of calculation of karstification degree, i.e., the time to peak (T_p) is relatively fast(2.5 hours) compared to Petoyan Spring (6 hours) which is recharged by diffuse type karst aquifer. This value shows the existence of conduit flow that fills the underground river during rainfall. Time to the baseflow calculation (T_b) both manually (flood hydrograph analysis) and automatic (MRC) has 40 hours in average which reflects that karst aquifer. This value shows that the channel is still useful in storing diffuse groundwater. Overall, karst aquifers in the catchment area of Kiskendo Cave are still dominated by diffuse-type voids, although it also has sufficiently developed conduit-type voids.