Groundwater Recharge of Fractured Rock Aquifers in SE Australia Is Episodic and Controlled by Season and Rainfall Amount

Sustainable management of groundwater resources requires a comprehensive understanding of groundwater recharge; including when and under what conditions groundwater recharge occurs. However, recharge is one of the least understood hydrologic processes. Here we show how event-scale rainfall recharge thresholds vary over a year using a novel network of subterranean drip loggers installed in caves, mines, and tunnels to observe groundwater recharge events. These cannot be used to directly estimate groundwater recharge volumes, but instead detail temporal aspects, such as the rainfall amount required to trigger recharge. We describe how these thresholds vary over time and space from a range of geological, environmental, and climatic conditions. At our Southeast Australian sites, median rainfall recharge thresholds of 10–20 mm in 48 hr were needed to activate recharge. Rainfall events of this magnitude are infrequent, they are expected to change with climate change, and they are fundamentally important for informing groundwater recharge.