Flood hydrograph parameters in the Pindul underground river, Gunungkidul, to characterise the release of stored water from karst aquifers

Abstract

This research was conducted in the Pindul underground river, a cave tubing tourism site in Gunungsewu Karst, Indonesia. The purpose of this study is to identify flood hydrograph parameters that can be used to characterise how karst aquifer release groundwater storage. The recording of water level fluctuations was carried out with the installation of HOBO U20L-02 (November 2020-August 2021). The discharge measurement was carried out 20 times to create a stage-discharge rating curve. Furthermore, eleven flood events were selected to calculate the recession constant of diffuse (Kb), fissure (Ki), conduit (Kc), time to peak (Tp), and time to baseflow (Tb). The results of the calculation of the hydrograph parameters show the number Kb=0.996, which indicates the slow release of groundwater storage supported by the Tb value = 20.72 hours (long enough). Compared with Kb and Tb values in other karst springs in Java, the Pindul underground river has a slightly slower than average releasing of groundwater storage, although its Tb value is shorter than the average for other karst springs. Meanwhile, based on the Kc value (0.429), the Pindul underground river is faster in releasing its conduit flow which is strengthened by a reasonably short Tp data (3.1 hours).