Montazar et al. (2026) Field-scale evaluation of OpenET for quantifying consumptive water savings under deficit irrigation in alfalfa

Identification

• Journal: Agricultural Water Management
• Year: 2026
• Date: 2026-04-08
• Authors: Aliasghar Montazar, John W. Shields, André Daccache, Mekonnen Gebremichael, Daniel H. Putnam
• DOI: 10.1016/j.agwat.2026.110337

Research Groups

• University of California Division of Agriculture and Natural Resources, UCCE Imperial County, Holtville, CA, USA
• United States Bureau of Reclamation, Boulder Canyon Operations Office, Boulder, NV, USA
• University of California Davis, Dept. of Biological and Agricultural Engineering, Davis, CA, USA
• University of California Los Angeles, Dept. of Civil and Environmental Engineering, Los Angeles, CA, USA
• University of California Davis, Dept. of Plant Sciences, Davis, CA, USA

Short Summary

This study evaluated OpenET satellite-derived actual crop evapotranspiration (ETc act) for quantifying consumptive water savings under summer deficit irrigation in alfalfa fields. It found that OpenET, particularly its ensemble product, reliably quantifies significant water savings (150–200 mm per deficit period, 40–50% reduction) in arid agricultural systems, validated by eddy covariance and soil moisture data.

Objective

• Evaluate the performance of OpenET satellite-derived ET under full irrigation using eddy covariance measurements as an independent benchmark.
• Assess OpenET behavior under intentional summer deficit irrigation by comparing modeled ET reductions with soil-moisture-derived root-zone water depletion.
• Quantify field-scale consumptive water savings associated with deficit irrigation using satellite-derived ET and assess interannual variability in model performance by distinguishing deficit-period ET from seasonal ET.

Study Configuration

• Spatial Scale: 20 grower-managed commercial alfalfa fields (14–32 hectares each) in California’s Imperial Valley. Rooting depth assumed 2.0 meters.
• Temporal Scale: Two-year field analysis (2024–2025) across two consecutive cropping seasons. Deficit periods ranged from 69 to 71 days in 2024 and 76 to 78 days in 2025.

Methodology and Data

• Models used: OpenET platform integrating eeMETRIC, SIMS, PT-JPL, SSEBop, DisALEXI, geeSEBAL, and an OpenET ensemble product.
• Data sources:
  • Satellite: OpenET platform (Landsat spatial resolution, 30 meters).
  • Observation (Ground-based):
    • Eddy covariance (EC) measurements for actual ET (ETc act) under full irrigation (sonic anemometer, net radiometer, soil heat flux plates, thermistor probes, EC5 soil moisture sensors).
    • TEROS-12 soil moisture sensors at 5, 20, 50, 80, 110, 140, 170, and 200 cm depths for root-zone soil-water depletion (SM-RZD) under deficit irrigation.
    • California Irrigation Management Information System (CIMIS) Meloland station (No. 87) for daily weather data (reference evapotranspiration, air temperature, solar radiation, relative humidity, wind speed).
    • Laboratory soil analyses (hydrometer method, pressure-plate apparatus) for soil texture, field capacity (FC), and permanent wilting point (PWP).

Main Results

• OpenET performance under full irrigation: The OpenET ensemble showed the lowest root mean square error (RMSE) of 0.7 millimeters per day (mm d⁻¹), mean absolute error (MAE) of 0.6 mm d⁻¹, and highest coefficient of determination (R²) of 0.87 compared to eddy covariance. Mean bias error (MBE) for the ensemble was 0.1 mm d⁻¹.
• OpenET performance under deficit irrigation: The OpenET ensemble most closely reproduced observed cumulative soil-water depletion (SM-RZD), with RMSE of 7.1 mm (2024) and 6.2 mm (2025), and MBE of -2.4 mm (2024) and 3.2 mm (2025).
• Consumptive water savings: Deficit irrigation reduced cumulative ETc act by 150–200 mm per deficit period, corresponding to 40–50% lower consumptive water use relative to full irrigation (p < 0.001). Specifically, mean savings were 156 mm (43% reduction) in 2024 and 202 mm (50% reduction) in 2025.
• ET, NDVI, and Kc act dynamics: Following irrigation cutoff, daily ETc act declined from 6–9 mm d⁻¹ to 1–3 mm d⁻¹. Normalized Difference Vegetation Index (NDVI) declined from approximately 0.75–0.90 to 0.25–0.40. Actual crop coefficient (Kc act) declined from approximately 0.8–1.1 to 0.2–0.4, indicating sustained soil-water limitation and reduced canopy function.
• Seasonal ET: Total seasonal ET in deficit-irrigated fields was approximately 231 mm (15%) lower than in fully irrigated fields.

Contributions

• This study presents the first field-scale evaluation of OpenET that leverages independent flux- (eddy covariance) and storage-based (soil moisture-derived root-zone depletion) observations to directly quantify consumptive water savings from intentional summer deficit irrigation in commercial alfalfa systems.
• It provides a credible and operational basis for verifying consumptive water savings from deficit irrigation in arid agricultural systems under real-world commercial conditions.
• The research demonstrates the robustness of ensemble-based ET products for deficit-irrigation verification, particularly in arid regions where intermittent rainfall during the deficit window can introduce uncertainty.
• It highlights the critical importance of evaluating modeled ET under water-limited conditions, rather than solely extrapolating performance from well-watered benchmarks.

Funding

• U.S. Department of the Interior, Bureau of Reclamation, under Grant No. R25AC00125.

Citation

@article{Montazar2026Fieldscale,
  author = {Montazar, Aliasghar and Shields, John W. and Daccache, André and Gebremichael, Mekonnen and Putnam, Daniel H.},
  title = {Field-scale evaluation of OpenET for quantifying consumptive water savings under deficit irrigation in alfalfa},
  journal = {Agricultural Water Management},
  year = {2026},
  doi = {10.1016/j.agwat.2026.110337},
  url = {https://doi.org/10.1016/j.agwat.2026.110337}
}