Analysis of floods recession constant of several karst springs in the north side of Karangbolong (Gombong) karst area, Central Java

This research was conducted in three karst springs (Kalisirah, Jumbleng, and Kalikarak) located on the north side of the Karangbolong karst area (Gombong), with objectives to analyze the differences of karst aquifers in releasing flow components from the small void (diffuse), moderate void (fissure), and large void (conduit). Three sets of HOBO U20L-02 water level data loggers were installed over 11-16 months to obtain a time-series discharge hydrograph. Then, the recession period is selected for several flood events that are eligible to calculate the recession constant of diffuse (Kb), fissure (Ki), and conduit (Kc). The results showed that there were differences in the three springs in releasing diffuse, fissure, and conduit storage. Jumbleng spring has the slowest release of diffuse storage (Kb), although the difference is not too significant when compared to the other two springs. Meanwhile, Kalisirah spring is indicated to have the most developed conduit with the smallest Kc value. Besides, this karst region has the shortest time to peak value (Tp) when compared to values in other karst regions on Java, which may be caused by the morphological control of the steep karst cone (runoff is quickly concentrated to the recharge point/ponor).