Rainfall recharge thresholds in a subtropical climate determined using a regional cave drip water monitoring network

Abstract

Quantifying the combination of climatic and hydrological conditions required to generate groundwater recharge is challenging, yet of fundamental importance for groundwater resource management. Here we demonstrate a new unsaturated zone physical method of determining rainfall-recharge thresholds in karst using a regional cave drip water monitoring network. For limestones of the Upper and Lower Macleay Valley, eastern Australia, set in a subtropical climate, we observe thirty-one cave drip water recharge events over a five-year monitoring period. Comparison to antecedent precipitation demonstrates a median observed recharge threshold of 76 mm / week precipitation (Lower Macleay) and 79 mm / week precipitation (Upper Macleay), with lower precipitation thresholds (down to 30 mm / week) possible. We use a simple water budget model to quantify soil and epikarst water storage volumes and to test hypotheses of the hydrological controls. Modelled soil and epikarst water storage capacities of about 65 mm (Lower Macleay) and 80 mm (Upper Macleay) confirm a correspondence between observed weekly precipitation thresholds and soil and epikarst capacities. However, discrepancies between observed and simulated recharge events helps elucidate the likely recharge processes including focussed recharge bypassing the soil and epikarst store, overflow and drainage between multiple karst stores, and tree water use from depth. Our observed recharge thresholds and modelled soil and epikarst storage capacities are comparable to recharge thresholds estimated across a range of water-limited environments globally. The method is readily applicable to any karst region where drip loggers can be installed in a cave system in close proximity to surface climate data.

Integrating field work and large-scale modeling to improve assessment of karst water resources

“…The different performances between grassland and forest point towards the very simplified representation of land cover in this preliminary model (Sarrazin et al 2018). While the weak performance at the German site, which is located at~1,450 m above sea level, is most probably due to the neglecting of snow processes in the model, the model deficiencies at the Austrian site could be due to general uncertainty of the gridded input data for this region as already discussed by Baker et al (2020).…”

“…The influence of antecedent soilmoisture conditions on recharge initiation could be revealed by considering a larger number of extracted soil-moisture events and their pre-event soil storages (Demand et al 2019). At those sites, where observations of groundwater, or of related fluxes like stream, discharge, spring discharge or drip in caves are available, methods to estimate recharge from soil-moisture observations by simple models (Baker et al 2020) or data-driven approaches can be explored (Arnold et al 2020). Those approaches may be supported by analysis of stable isotopes in soil water as already proven to be useful in non-karstic settings by Sprenger et al (2015).…”

Hydrogeochemical responses of cave drip water to the local climate in the Liangfeng Cave, Southwest China

“…Previous studies have conducted systematic monitoring that focused on the migration process of ecological environmental information in the entire cave system (Chen & Li 2018;Yin et al 2019;Baker et al 2020). The suspended aquifer in the epikarst provides a store of water that sustains percolation flow to speleothems in caves and to cave streams over extended dry periods.…”

Hydrogeochemical Responses of Cave Drip Water to the Local Climate in Liangfeng Cave, Southwest China

“…Hence, drip water is important for the scienti c interpretation of information recorded by stalagmites (Pape et al 2010;Feng et al 2014;Wu et al 2014). Previous studies have conducted systematic monitoring that focused on the migration process of ecological environmental information in the entire cave system (Chen et al 2018;Yin et al 2019;Baker et al 2020). Drip water is usually supplied by meteoric precipitation and a series of important hydrogeochemical processes occur in the epikarst and inside the cave.…”