Khorami et al. (2026) Estimation of root zone water storage capacity (S) in natural ecosystems subject to high interannual climate variability

Identification

• Journal: Journal of Hydrology
• Year: 2026
• Date: 2026-03-19
• Authors: Maryam Tayebi Khorami, Gary Sheridan, Keirnan Fowler, Patrick N.J. Lane
• DOI: 10.1016/j.jhydrol.2026.135354

Research Groups

• School of Agriculture, Food, and Ecosystem Science, Faculty of Science, University of Melbourne, Melbourne, Victoria, Australia
• Department of Infrastructure Engineering, University of Melbourne, Melbourne, Victoria, Australia

Short Summary

This study evaluates two cumulative water deficit (CWD) methods for estimating root zone water storage capacity (Sr) in 105 Australian forested catchments, finding that a multi-year CWD approach yields significantly higher Sr estimates (median 25% higher) in regions with high interannual climate variability compared to the conventional single-year method.

Objective

• To determine where multi-year water-deficit sequences occur and how long they last.
• To assess the sensitivity of root zone water storage capacity (Sr) estimates to the length of consecutive deficit runs.
• To investigate how Sr estimates and their differences (ΔSr) vary with aridity and precipitation.

Study Configuration

• Spatial Scale: 105 natural forested catchments across diverse climate zones in Australia, with catchment areas ranging from 4.4 km² to 7732 km².
• Temporal Scale: Daily hydroclimatic data from 2000 to 2022 (22 years).

Methodology and Data

• Models used:
  • Single-year Cumulative Water Deficit (CWD𝑆𝑦) method: Resets cumulative deficits annually.
  • Multi-year Cumulative Water Deficit (CWD𝑀𝑦) method: Allows deficits to accumulate continuously over the full record.
  • Water balance equation for bias correction of actual evapotranspiration (AET).
• Data sources:
  • Precipitation (P): Australian Gridded Climate Data (AGCD), daily, 0.05° spatial resolution.
  • Streamflow (Q): CAMELS-AUS version 2 (v2) dataset (Fowler et al., 2025), daily.
  • Actual Evapotranspiration (AET): CMRSET (CSIRO MODIS Rescaled Evapotranspiration) product (Guerschman et al., 2022), monthly mean daily, approximately 30 m spatial resolution, with bias correction applied.
  • Catchment attributes: CAMELS-AUS v2, Australia’s State of the Forests Report – Synthesis 2023, Bureau of Meteorology Groundwater Dependent Ecosystems (GDEs) Atlas.

Main Results

• Root zone water storage capacity (Sr) estimates ranged from 113 mm to 664 mm for the single-year method (CWD𝑆𝑦) and from 113 mm to 1023 mm for the multi-year method (CWD𝑀𝑦).
• In catchments with high interannual climate variability, the median Sr was 25% higher under the multi-year approach, with the greatest divergence observed in southeastern Australia.
• 45 out of 105 catchments experienced two or more consecutive years where actual evapotranspiration (AET) exceeded precipitation (P), with the longest runs coinciding with the Millennium Drought (1997–2009).
• The mean difference (ΔSr = CWD𝑀𝑦 - CWD𝑆𝑦) was 102.3 mm, and the median difference was 66.2 mm, indicating that accounting for interannual carryover generally results in higher storage capacity estimates.
• In some catchments, ΔSr reached up to 598.6 mm, highlighting substantial interannual carryover.
• The divergence between methods (ΔSr) increased significantly with aridity (Spearman 𝜌 = 0.35, p < 0.001) and decreased with mean annual precipitation (MAP) (Spearman 𝜌 = -0.42, p < 0.001).
• The two methods converged (ΔSr near zero) in humid catchments (aridity index < 1; MAP > 1000 mm) where interannual carryover was negligible.

Contributions

• This study is the first to explicitly and systematically quantify how the choice between single-year and multi-year cumulative water deficit (CWD) formulations affects root zone water storage capacity (Sr) estimates across diverse climates.
• It clarifies the climatic conditions under which annual-independence assumptions in CWD methods are appropriate or lead to biased Sr estimates.
• The findings demonstrate that conventional annual-reset CWD implementations may significantly underestimate Sr in regions prone to sustained multi-year moisture deficits.
• The research recommends adopting multi-year cumulative formulations for Sr estimation in climates characterized by high interannual variability to provide a more complete estimate of plant-accessible storage.

Funding

• University of Melbourne Research Scholarship

Citation

@article{Khorami2026Estimation,
  author = {Khorami, Maryam Tayebi and Sheridan, Gary and Fowler, Keirnan and Lane, Patrick N.J.},
  title = {Estimation of root zone water storage capacity (S) in natural ecosystems subject to high interannual climate variability},
  journal = {Journal of Hydrology},
  year = {2026},
  doi = {10.1016/j.jhydrol.2026.135354},
  url = {https://doi.org/10.1016/j.jhydrol.2026.135354}
}