Streamflow event clustering and vadose-zone memory as coupled controls on focused groundwater recharge in dryland catchments

Groundwater recharge is one of the least constrained hydrological fluxes in dryland catchments, particularly where thick vadose zones, ephemeral streamflow, and high evaporative demand decouple rainfall from aquifer replenishment. Recharge is commonly attributed to rare, high-magnitude floods, yet this perspective rarely accounts for event sequencing and vadose-zone memory. Here, we synthesise multi-year hydrometric observations and process-based simulations from an ephemeral dryland stream in the arid zone of Australia (Fowlers Gap, NSW) to show that temporal clustering of moderate streamflow events can enable focused recharge where thick vadose zones impose strong percolation thresholds. Field data indicate that several historic floods produced substantial vadose-zone wetting but no sustained groundwater response, whereas sequences of closely spaced, moderate flows generated delayed, yet persistent water-table rise beneath the channel.

Numerical simulations demonstrate that event clustering progressively wets the vadose zone, suppresses evapotranspiration losses, and non-linearly increases unsaturated hydraulic conductivity, resulting in sufficient flow via the streambed to measurably recharge the aquifer. These results show that groundwater recharge in dryland catchments emerges from the interaction between event sequencing, vadose-zone properties, transmission losses, and evapotranspiration, rather than from rainfall or streamflow magnitude alone. Under projected climate change, shifts toward more intense but less frequent rainfall may reduce recharge by disrupting event clustering, even where total precipitation remains unchanged. Explicitly accounting for vadose-zone memory and event sequencing is therefore essential for recharge estimation, model calibration, and dryland water-resource assessments.