Diffuse flow separation in the three largest springs on the north side of the Karangbolong karst area, Central Java, Indonesia

Diffuse flow or slow-flow is the type of storage in karst aquifers that is the longest released during the recession period. For this reason, the length of time of release will affect the availability of reliable discharges in karst springs during the dry period. The higher the percentage of diffuse flow compared to the total discharge, the higher the discharge that can be relied upon to supply irrigation and domestic needs. This study is aimed to determine the spatial and temporal variations in diffuse flow percentages in the three largest springs on the north side of the Karangbolong karst (Kalisirah, Jumbleng, and Kalikarak springs). The primary data used is data from time-series water level recording with the HOBO U-30 logger, which then produces a discharge-hydrograph during the study period (November 2018-March 2020). Furthermore, baseflow separation is carried out automatically using the recursive digital filtering approach, where the BFI_max value and recession constant are carried out manually by interpreting selected floods during the study period. The diffuse separation results show that, spatially, the monthly percentage of diffuse flow in the three springs is in the range of 80%, with the difference between the three springs not too large. The most volatile percentage was found in Kalisirah spring, with the smallest percentage (77.20%) occurring in November 2018 and the highest percentage in March 2020 (88.74%). Meanwhile, the percentage of diffuse flow experienced a drastic decrease in the incidence of flood peaks with varying ranges, namely at Kalisirah spring (9.06 - 31.16%); Jumbleng spring (14.30 - 60.73%); and Kalikarak spring (9.32 - 41.69%). As is the case with monthly percentages, the average percentage of diffuse flow during flood events in Kalisirah springs is the smallest (18.72%) when compared to Kalikarak springs (23.29%) and Jumbleng springs (43.34%). In short, the highest monthly diffuse flow fluctuations and the smallest percentage of diffuse flow during flood events indicate that aquifers that recharge Kalisirah springs have the most developed void sizes compared to the other two springs. Consequently, the spring with the lowest void development will have the most stable, reliable discharge for meeting both domestic and irrigation needs in this area. However, the differences between the three springs are not significant.