Rainfall recharge thresholds decrease after an intense fire over a near-surface cave at Wombeyan, Australia

Abstract

Quantifying the amount of rainfall needed to generate groundwater recharge is important for the sustainable management of groundwater resources. Here, we quantify rainfall recharge thresholds using drip loggers situated in a near-surface cave: Wildman's Cave at Wombeyan, southeast Australia. In just over 2 years of monitoring, 42 potential recharge events were identified in the cave, approximately 4 m below the land surface and comprising a 30° slope with 37 % bare rock. Recharge events occurred within 48 h of rainfall. Using daily precipitation data, the median 48 h rainfall needed to generate recharge was 19.8 mm, without clear seasonal variability. An intense experimental fire experiment was conducted 18 months into the monitoring period: the median 48 h rainfall needed to generate recharge was 22.1 mm before the fire (nCombining double low line 22) and 16.4 mm after the fire (nCombining double low line 20), with the decrease in rainfall recharge most noticeable starting 3 months after the fire. Rainfall recharge thresholds and the number of potential recharge events at Wildman's Cave are consistent with those published from other caves in water-limited Australia. At Wildman's Cave, we infer that soil water storage, combined with the generation of overland flow over bare limestone surfaces, is the pathway for water movement to the subsurface via fractures and that these determine the rainfall recharge threshold. Immediately after the fire, surface ash deposits initially retard overland flow, and after ash removal from the land surface, soil loss and damage decrease the available soil water storage capacity, leading to more efficient infiltration and a decreased rainfall recharge threshold.