Characterizing flow release from the aquifer of Guntur Spring in Gunungsewu Karst Area, Indonesia

Karst aquifers have three flow components, namely conduit, fissure, and diffuse. This research was designed to characterize the flow release from the karst aquifer of Guntur Spring. It is part of the hydrogeological system Panggang Block, part of Gunungsewu Karst Area, Gunungkidul, the Special Region of Yogyakarta. The water level and discharge data from May 2018 to May 2019 were processed to create flow hydrographs. Two different methods were applied to measure discharge variation in one year, namely sudden injection in dry seasons and velocity area in rainy seasons. While baseflow separation analysis was employed to calculate the percentage of baseflow, recession constants computation was used to derive the three flow components of the karst aquifer. The recession constants, Kb (diffuse flow), Ki (fissure flow), and Kc (conduit flow), were calculated from nine selected flood events. From the hydrograph analysis, this research found that the average discharges in rainy and dry seasons were 56.7 l/s and 13.25 l/s, respectively. These relatively small flows are mainly because Guntur Spring is an epikarst spring with layers of limestone or thin aquifer. The recession constants were Kb=0.998, Ki=0.933, and Kc=0.500, meaning that Guntur Spring has three flow components, including diffuse, fissure, and conduit. Although the baseflow separation analysis found that the diffuse flow prevailed (93.07), the influence of conduit and fissure flows was apparent, especially during rainy seasons. The Guntur Spring hydrograph analysis revealed that the average time to baseflow (Tb) and time to peak (Tp) was rather prolonged. Supporting this finding was the mean value of Kc that categorized the recession as medium and signified that the conduit fractures developing in Guntur Spring were not extensive. As a conclusion, diffuse flow mainly characterizes the discharge of Guntur Spring with some indications of growing fissures and conduits.