Volk et al. (2026) Assessing and correcting bias in gridded reference evapotranspiration over agricultural lands across the contiguous United States

Identification

• Journal: Agricultural Water Management
• Year: 2026
• Date: 2026-07-17
• Authors: John Volk, Christian Dunkerly, Sayantan Majumdar, Justin Huntington, Blake Minor, Yeonuk Kim, Charles Morton, Peter M. ReVelle, Ayşe Kiliç, Forrest Melton, Richard G. Allen, Christopher Pearson, A. J. Purdy, Todd G. Caldwell
• DOI: 10.1016/j.agwat.2026.110647

Research Groups

• Desert Research Institute, Reno, NV, USA
• University of Nebraska-Lincoln, Lincoln, NE, USA
• NASA Ames Research Center, Moffett Field, CA, USA
• California State University, Monterey Bay, Seaside, CA, USA
• (ret) University of Idaho, Kimberly, ID, USA

Short Summary

This study assesses and corrects bias in gridded reference evapotranspiration (ETo) over agricultural lands across the contiguous United States. The results show that gridMET ETo systematically overestimates ETo by 10–20% at most cropland sites, while pockets of underestimation appear in some arid western regions.

Objective

• Investigate biases in gridded reference evapotranspiration (ETo) data across irrigated agricultural areas of the contiguous United States.
• Evaluate the effectiveness of ETo bias correction for agricultural water management applications.
• Quantify the influence of corrected gridMET ETo on OpenET RSET predictions.

Study Configuration

• Spatial Scale: Continental-scale analysis over the contiguous United States (CONUS).
• Temporal Scale: Daily and monthly time steps from 1979 to present.

Methodology and Data

• Models used: Penman-Monteith reference evapotranspiration framework developed by the Food and Agriculture Organization of the United Nations (FAO) and the American Society of Civil Engineers’ Environmental & Water Resources Institute (ASCE-EWRI).
• Data sources: GridMET dataset, a ~4 km resolution daily gridded meteorological dataset that covers CONUS starting from 1979.

Main Results

• GridMET ETo has a positive relative bias to agricultural weather station measurements across the CONUS.
• Mean annual bias in the Eastern U.S. shows higher homogeneity with an overestimation of gridMET ETo typically around 5–16% of measured ETo.
• The humid subtropical region in the Southeast and the mediterranean climate region near the West Coast showed the highest overestimation in gridMET ETo.
• Seasonally, biases tended to be highest in the colder months of October through February.

Contributions

• This study provides a comprehensive assessment of bias in gridded reference evapotranspiration (ETo) data across irrigated agricultural areas of the contiguous United States.
• The results highlight the importance of correcting ETo bias for accurate agricultural water management applications.
• The study demonstrates the effectiveness of using independent micrometeorological dataset to validate bias-corrected gridMET ETo.

Funding

• This research was supported by the National Aeronautics and Space Administration (NASA) through the OpenET project.

Citation

@article{Volk2026Assessing,
  author = {Volk, John and Dunkerly, Christian and Majumdar, Sayantan and Huntington, Justin and Minor, Blake and Kim, Yeonuk and Morton, Charles and ReVelle, Peter M. and Kiliç, Ayşe and Melton, Forrest and Allen, Richard G. and Pearson, Christopher and Purdy, A. J. and Caldwell, Todd G.},
  title = {Assessing and correcting bias in gridded reference evapotranspiration over agricultural lands across the contiguous United States},
  journal = {Agricultural Water Management},
  year = {2026},
  doi = {10.1016/j.agwat.2026.110647},
  url = {https://doi.org/10.1016/j.agwat.2026.110647}
}