Insights into karst aquifer functioning from multi-method time series analysis of two adjacent tropical springs (Central Java, Indonesia)

Time Series Analysis (TSA) provides a statistical framework to assess the hydrodynamic behaviour of karst aquifers by analyzing rainfall-discharge relationships. Using TSA as the primary analytical method, this study investigates two neighbouring tropical karst springs—Kalisirah and Jumbleng—in the South Gombong karst area, Central Java, Indonesia. Despite their close proximity (615 m), the springs exhibit markedly different discharge behaviours. Seven TSA approaches were applied, including auto-correlation, cross-correlation, spectral density, phase function, cross-amplitude, gain, and coherence functions. The results reveal that Kalisirah Spring has a more responsive system with quicker release from the conduit and diffuse storage than Jumbleng Spring, indicating a more developed karst structure. Both springs exhibit similar fissure-flow responses. Additional methods—Master Recession Curve (MRC) analysis, recession constant calculation, and tracer testing—were used to validate the TSA findings. These methods confirmed a higher level of karstification in the Kalisirah system, with faster flow discharge, higher advection and dispersion values, and a more complex conduit network. This multi-method framework highlights the prominence of TSA in characterizing karst aquifer dynamics and emphasizes the importance of integrating physical validation methods to support TSA results. This study also provides better insight into the hydrology of tropical karst as a region and climate zone that are still underrepresented in global karst studies, and offers a replicable approach for sustainable water resources management in similarly complex hydrogeological settings.