Groundwater recharge from intense rainfall in Indonesia: evidence from East Kalimantan

Intense rainfall events play a critical role in groundwater recharge across tropical regions, including Indonesia. In this study, we employ stable isotope data (δ18O, δ2H) from precipitation and groundwater, along with the chloride mass balance (CMB) method, to investigate recharge dynamics. The results indicate a clear bias in groundwater recharge toward heavy rainfall. At 20 of the 27 sites across Indonesia, heavier rainfall events are consistently associated with more isotopically depleted signatures compared to less intense rainfall, reflecting the amount effect. Groundwater isotope data from 16 sites further support this pattern, consistently indicating recharge predominantly occurs during high-intensity precipitation. In East Kalimantan, groundwater isotopic compositions align more closely with daily intense rainfall events than with monthly averages, suggesting that recharge is primarily event-driven. We estimate that 12-28% (as an upper bound) of recharge in East Kalimantan originates from precipitation, with intense rainfall events (>75th percentile) contributing up to 57% of total recharge.