New paper alert! Our new study, led by Margaret Shanafield (Flinders University) and published in Environmental Research Letters, uses Australia’s National Groundwater Recharge Observing Network (NGROS) to monitor recharge responses across 16 caves, mines and tunnels spanning a continent-sized climate gradient. Our paper represents the first continental-scale assessment of event-based groundwater recharge thresholds, and demonstrates that understanding when rain falls can be more important than understanding how much rain falls.
We found that groundwater recharge thresholds, (that is, the amount of rainfall needed to generate recharge), were remarkably similar across Australia. Despite differences in climate, geology and vegetation, annual rainfall thresholds required to trigger recharge were statistically indistinguishable at most sites. Instead, we found that seasonality mattered far more than climate, geology and vegetation. The strongest control on recharge was when the rainfall occurred. In winter-dominated rainfall regions, essentially the southern part of Australia, recharge was concentrated in winter and spring, while summer and autumn rainfall rarely generated recharge, even during large rain events.
With several years of data to analyse, we also observed that the rainfall recharge thresholds are not fixed. Rainfall events exceeding the “recharge threshold” frequently failed to generate recharge during drought years because soils and shallow fractures remained too dry. Conversely, smaller rainfall events could generate recharge once subsurface storage had been replenished.
In short, hydrological memory matters. We showed that antecedent moisture conditions strongly influence whether rainfall becomes recharge. This highlights the importance of subsurface “memory” and suggests that simple rainfall-based recharge estimates can be misleading. Using continental-scale field observations, we show how winter rainfall first replenishes soil and fracture storage before recharge can occur, creating a seasonal “window” for groundwater replenishment.
What does this mean for the future. Clearly, predicting future groundwater availability under climate change requires more than annual rainfall projections. Changes in rainfall timing, drought frequency, and seasonality may be just as important as changes in total rainfall.
#Groundwater #Hydrology #WaterResources #ClimateChange #Recharge #Drought #EnvironmentalScience #Australia
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