Climatic, hydrological and karst geological controls on groundwater recharge: the view from an Australian vadose zone cave observatory
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Date
2021-12-18
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American Geophysical Union (AGU)
Abstract
In water-limited environments, quantifying the timing and frequency of erratic rainfall recharge events and its climate forcing is of critical importance for groundwater resource management. In temperate semi-arid New South Wales, SE Australia (precipitation: 615 mm/year, pan evaporation: 1679 mm/year), since 2010 we have been using a limestone cave situated at 20 m below land surface, and just above the water table, as a vadose zone observatory of potential recharge approximated by drip rate observations. Complimented since 2018 by a soil moisture probe network and using the VarKarst karst-specialized recharge model, we investigate the climatic, hydrological and karst geological controls on recharge dynamics. We observe nineteen recharge events (07.2010 to 01.2021). They cluster into two periods (1) seven events between 08.2010 and 12.2010 during a La Niña (enhanced spring rainfall is typical in eastern Australia) and (2) seven events between 06.2016 and 10.2016 associated with a negative Indian Ocean Dipole (which is associated with wet winters and springs in southern Australia). Comparison with antecedent rainfall indicates a minimum of 40 mm rainfall over 14-days is required for recharge in winter, and >120 mm rainfall over 14-days in summer. We will use the karst recharge model to simulate the observed recharge events and to quantify the threshold behavior of the soil and vadose zone above the cave. Two recharge events have occurred since the establishment of the soil moisture network (03.05.2020, 29.07.2020). For those, we can analyze the influence of antecedent soil storage on the initiation of recharge and use this understanding for an evaluation of the simulated internal fluxes and storages of karst recharge model. Providing realistic results of both recharge and soil moisture observations, the model can be used as tool to predict the impact of past and future climate changes on groundwater renewal. Plain-language Summary Many arid and semi-arid regions rely on groundwater for water supply. Rainfall replenishes this groundwater resource, but how do we know when this replenishment (technically called recharge) occurs? We report the results from a unique Australian observatory – a cave situated in limestone and located just above the water table in a semi-arid climate region. Water percolating into the cave has been measured for over ten years, and we observe nineteen recharge events, most of which occur in wet climate phases of the climate phenomena La Niña or the Indian Ocean Dipole. Combined with measurements of climate and soil moisture, and a computational model of recharge, we quantify fluxes and storages of the unsaturated zone above the cave to better understand which conditions favor or hinder groundwater renewal. That way, we can estimate how potential future climate changes will affect future groundwater availability.
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Keywords
Climates, Hydrology, Caves, Geologic history, Ground water, Australia, Environment, New South Wales, Limestone, Soil mechanics, Rain water, Groundwater recharge
Citation
Baker, A., Berthelin, R., Hartmann, A. J., & Treble, P. C. (2021). Climatic, hydrological and karst geological controls on groundwater recharge: the view from an Australian vadose zone cave observatory. Paper presented at the AGU Fall Meeting 2021, New Orleans, Louisiana and Online, 13-17 December 2021. In AGU Fall Meeting Abstracts (Vol. 2021, H51D-03). Retrieved from: https://agu.confex.com/agu/fm21/meetingapp.cgi/Paper/834561