Rau et al. (2026) Temporally clustered streamflow events control focused groundwater recharge in drylands

Identification

• Journal: Environmental Research Letters
• Year: 2026
• Date: 2026-01-27
• Authors: Gabriel Rau, José Bastías Espejo, R Ian Acworth, Tony Bernardi, Martin S Andersen, Dylan Irvine, Mark O Cuthbert
• DOI: 10.1088/1748-9326/ae3e01

Research Groups

• Earth Sciences, School of Science, The University of Newcastle, Callaghan, Australia
• Centre for Integrated Resilience: Coasts, Water and Climate, The University of Newcastle, Callaghan, New South Wales, Australia
• National Centre for Groundwater Research and Training, Adelaide, Australia
• Department of Civil Engineering: Hydraulics, Energy and Environment, Universidad Politécnica de Madrid, Madrid, Spain
• Water Research Laboratory, School of Civil and Environmental Engineering, UNSW Sydney, Manly Vale, Australia
• Research Institute for the Environment and Livelihoods and Faculty of Science and Technology, Charles Darwin University, Casuarina, Northern Territory, Australia
• Faculty of Science and Technology, The University of Canberra, Canberra, Australian Capital Territory, Australia
• School of Earth and Environmental Sciences, Cardiff University, Cardiff, United Kingdom

Short Summary

This study combines long-term hydrogeological monitoring with numerical modeling to show that focused groundwater recharge in drylands is primarily controlled by temporal clusters of moderate streamflow events, rather than isolated large floods, due to the progressive wetting of the vadose zone that overcomes evapotranspiration.

Objective

• To assess the controls on focused groundwater (GW) recharge in drylands with thick vadose zones, specifically testing the hypothesis that recharge is driven by clusters of low- to medium-magnitude flows occurring in close succession, rather than by isolated large events.

Study Configuration

• Spatial Scale: Fowlers Gap in western New South Wales, Australia (arid zone). Numerical model domain: 2D axisymmetric, representing a 17 m vadose zone above a 10 m saturated layer, with results visualized at the streambed center and approximately 2 m from the bank (piezometer location).
• Temporal Scale: Field monitoring from May 2014–November 2016 and November 2021–November 2024 (with an instrument failure gap). Numerical modelling period: 21 November 2021–6 June 2023 (563 days). Analysis focuses on January 2022–November 2023.

Methodology and Data

• Models used:
  • Numerical model: Solves Richards’ equation for transient, variably saturated flow.
  • Constitutive relations: van Genuchten water retention function and Corey-type relative permeability relationship.
  • Software: PorousFlow module of the Multiphysics Object-Oriented Simulation Environment (MOOSE).
  • Evapotranspiration (ET) representation: Depth-limited root-uptake sink (PorousFlowHalfCubicSink) applied throughout the 17 m vadose zone, scaled by daily reference ET (ETo) and modulated by a cubic pressure-head function for moisture stress.
• Data sources:
  • Field monitoring: Groundwater levels from a piezometer transect (FG79 screened at 18–21 m depth) using a vented pressure transducer.
  • Streamflow: Creek stage recorded at 15 min intervals with a bubbler gauge.
  • Meteorological data: Daily rainfall from the Fowlers Gap weather station (Australian Government Bureau of Meteorology, Station No. 046128).
  • Evapotranspiration: Mean annual reference ET (ETo) estimated from daily BoM data (2014–2017) using the Penman–Monteith formulation.
  • Lithological data: Depth profile obtained during drilling for piezometer FG79.

Main Results

• Field observations over a decade showed that large, isolated floods (e.g., April 2022: 80.8 mm rainfall over 10 hours, 2.73 m stream peak) did not produce measurable groundwater recharge at the water table, despite overtopping piezometers.
• Significant groundwater recharge, indicated by an approximate 1.1 m rise in groundwater level at piezometer FG79, occurred only during a temporal cluster of moderate flow events between October and December 2022 (278 mm rainfall over 141 days, stream peaks 0.5–1.73 m).
• Numerical simulations confirmed that temporal flow clustering produces longer periods of ephemeral streamflow, which progressively wet the vadose zone, overcome evapotranspiration (ET)-driven moisture deficits, and increase relative hydraulic conductivity, enabling percolation to the water table.
• In contrast, isolated floods primarily saturate only shallow sediments, with infiltrated water subsequently lost to ET before reaching the water table.
• The groundwater level increase observed in 2022 was maintained for nearly 2 years after the recharge event.

Contributions

• Provides direct aquifer monitoring data and process-based numerical modelling evidence demonstrating that focused groundwater recharge in drylands with thick vadose zones is primarily governed by the temporal sequencing and duration of streamflow events, rather than the magnitude of isolated floods.
• Explicitly incorporates evapotranspiration (ET) to depth in the vadose zone model, offering a more realistic representation of dryland recharge dynamics and highlighting the critical roles of antecedent conditions and vadose zone properties in the balance between evaporative loss and cumulative wetting.
• Challenges the common assumption that large floods are the dominant drivers of recharge in drylands, establishing a threshold-like behavior dependent on the temporal clustering of flows.
• Emphasizes the necessity of process-based modelling and event-scale analyses for accurate recharge projections and sustainable groundwater management in drylands, particularly in the context of projected climate shifts towards more intense but less frequent rainfall.

Funding

• Australian Federal Government’s National Collaborative Research Infrastructure Strategy (NCRIS) Groundwater Infrastructure Program.

Citation

@article{Rau2026Temporally,
  author = {Rau, Gabriel and Espejo, José Bastías and Acworth, R Ian and Bernardi, Tony and Andersen, Martin S and Irvine, Dylan and Cuthbert, Mark O},
  title = {Temporally clustered streamflow events control focused groundwater recharge in drylands},
  journal = {Environmental Research Letters},
  year = {2026},
  doi = {10.1088/1748-9326/ae3e01},
  url = {https://doi.org/10.1088/1748-9326/ae3e01}
}