Pierrat et al. (2026) Human contributions to evapotranspiration mitigate swings in dry-to-wet year transitions

Identification

• Journal: Communications Sustainability
• Year: 2026
• Date: 2026-01-09
• Authors: Zoe Pierrat, Rebecca Gustine, Anna Boser, Sophie Ruehr, Christine M. Lee, J. T. Reager, Kerry Cawse-Nicholson
• DOI: 10.1038/s44458-025-00002-w

Research Groups

• NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
• Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA.
• Bren School of Environmental Science & Management, University of California, Santa Barbara, CA, USA.
• Department of Environmental Science Policy and Management, University of California Berkeley, CA, USA.

Short Summary

This study quantifies how human interventions, primarily irrigation, stabilize California's total evapotranspiration during extreme shifts from drought to record rainfall. The findings reveal that while natural evapotranspiration fluctuates with precipitation, human activity maintains high water consumption in managed lands even during exceptionally wet years.

Objective

• To determine how total evapotranspiration (ET) in California responded to the transition from an extreme dry year (2022) to a record wet year (2023).
• To disentangle and quantify the relative contributions of natural environmental drivers versus human management (e.g., irrigation) in governing ET responses.

Study Configuration

• Spatial Scale: Statewide analysis of California, USA, with data aggregated to a 0.125° × 0.125° (approximately 150 $km^2$) grid.
• Temporal Scale: Summer growing season (May–September) for the years 2022 and 2023, compared against a historical baseline (2016–2021).

Methodology and Data

• Models used:
  • OpenET ensemble: Used to estimate total observed ET (natural + human).
  • NLDAS (National Land Data Assimilation System): Specifically the VIC and Mosaic land surface models, used to simulate "natural" ET (excluding human influences like irrigation).
• Data sources:
  • Satellite observations: Landsat, MODIS, VIIRS, and Sentinel-2 (via OpenET).
  • Vegetation Indices: VIIRS Normalized Difference Vegetation Index (NDVI).
  • Land Cover: National Land Cover Database (NLCD).
  • Climate Forcing: Gauge-based precipitation and surface meteorology reanalysis (NLDAS).
• Analytical Approach: Human-contributed ET was calculated as the difference between OpenET (total) and NLDAS (natural) estimates.

Main Results

• ET Stability: Despite a massive increase in precipitation in 2023 (141% of average), total statewide ET changed by less than 10% (a marginal increase of 3 ± 9 mm/month) compared to the 2022 drought year.
• Dry Year (2022) Dynamics: Human contributions accounted for 30% of statewide ET and 80% of ET in managed lands (cultivated crops and developed areas).
• Wet Year (2023) Dynamics: Natural ET increased significantly (by 17 ± 13 mm/month), while human-driven ET fell by approximately 30%.
• Persistent Human Signature: Even in the record wet year, the Fractional Human Impact (FHI) remained near 50% in managed areas, indicating that "boom" water years are insufficient to eliminate the human demand for supplemental water (blue water).
• Ecosystem Resilience: Evergreen forests showed high stability in ET between years, likely due to access to deep groundwater stores, whereas grasslands and shrublands showed high sensitivity to precipitation swings.

Contributions

• Methodological Framework: Establishes a scalable approach to separate natural and anthropogenic ET components without requiring proprietary or reported irrigation and crop data.
• Hydrological Insight: Demonstrates that human management acts as a "buffer" that mitigates the impact of climate volatility on the water cycle, which simultaneously explains why groundwater resources fail to recover quickly during wet years.
• Policy Relevance: Highlights the necessity of water-smart management even during high-precipitation years to address the chronic overdraw of California's water resources.

Funding

• NASA Postdoctoral Program at the Jet Propulsion Laboratory.
• National Aeronautics and Space Administration (NASA) contract with the California Institute of Technology (JPL).

Citation

@article{Pierrat2026Human,
  author = {Pierrat, Zoe and Gustine, Rebecca and Boser, Anna and Ruehr, Sophie and Lee, Christine M. and Reager, J. T. and Cawse-Nicholson, Kerry},
  title = {Human contributions to evapotranspiration mitigate swings in dry-to-wet year transitions},
  journal = {Communications Sustainability},
  year = {2026},
  doi = {10.1038/s44458-025-00002-w},
  url = {https://doi.org/10.1038/s44458-025-00002-w}
}

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