Temporally clustered streamflow events control focused groundwater recharge in drylands

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Abstract
Groundwater is the primary freshwater resource in many of the world’s drylands, sustaining millions of people and supporting agriculture and ecosystems where surface water is scarce or unreliable. Recharge in these regions is highly episodic and occurs mainly through ephemeral streams (i.e., focused recharge), yet the mechanisms that determine whether surface flows contribute to aquifer replenishment remain poorly constrained. A common assumption is that large floods dominate recharge, but evidence from long-term monitoring is limited and inconclusive. We combine a unique hydrogeological monitoring dataset from the arid zone (Fowlers Gap in western New South Wales, Australia) with numerical modelling of vadose zone processes to assess the controls on focused groundwater recharge. Our results show that even extreme floods that overtopped piezometers did not produce measurable recharge at the water table. In contrast, significant recharge occurred only during a temporal cluster of moderate flow events in 2022. Numerical simulations confirm that temporal flow clustering produces longer periods of ephemeral streamflow, which progressively wet the vadose zone, overcome evapotranspiration-driven moisture deficits, and increase relative hydraulic conductivity, enabling percolation to the water table. Isolated floods, by contrast, largely saturate only shallow sediments and water is subsequently lost to evapotranspiration. By explicitly incorporating evapotranspiration, our modelling provides a more realistic representation of dryland recharge dynamics and highlights the roles of antecedent conditions and vadose zone properties. These findings demonstrate that recharge is not governed by rainfall totals or intensity alone, but critically depends on the timing and sequence of storm events. The implications for climate change assessments and water management are substantial, as projected shifts toward more intense but less frequent rainfall may reduce opportunities for clustering and thereby limit groundwater replenishment. Process-based modelling and event-scale analyses are therefore essential for reliable recharge projections and sustainable groundwater management in drylands.
Funding Information
  • Australian Federal Government (NCRIS Groundwater)

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