Assessing evapotranspiration in rainfed and irrigated Alfalfa in the U.S. Southern Great Plains using eddy covariance measurements and OpenET products

Authors: Wagle, Pradeep; AFSHIN, MOGHANLOU - University Of Oklahoma; NISHAN, BHATTARAI - University Of Oklahoma; Northup, Brian; Moffet, Corey; Gunter, Stacey; RUDRA, BARAL - University Of Missouri

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2026
Publication Date: 1/20/2026
Citation: Wagle, P., Afshin, M., Nishan, B., Northup, B.K., Moffet, C., Gunter, S.A., Rudra, B. 2026. Assessing evapotranspiration in rainfed and irrigated Alfalfa in the U.S. Southern Great Plains using eddy covariance measurements and OpenET products. Agricultural and Forest Meteorology. 379. Article 111032. https://doi.org/10.1016/j.agrformet.2026.111032.
DOI: https://doi.org/10.1016/j.agrformet.2026.111032

Interpretive Summary: Investigating seasonal and interannual variations in evapotranspiration (ET) based on multiple years of flux data is crucial for a better understanding of the dynamics of water use by alfalfa in the Southern Great Plains (SGP) of the United States (U.S.). This study examined the dynamics of eddy covariance measured ET (ETEC) in rainfed and irrigated alfalfa in central Oklahoma. The results illustrated significant seasonal and interannual variations in ET, driven by meteorological conditions, vegetation dynamics, and water availability. The study demonstrated the potential of developing an empirical model using readily available satellite-derived enhanced vegetation index (EVI – greenness index) and solar radiation or air temperature data to monitor alfalfa ET across large areas effectively. We also compared ETEC with the newly available high-resolution (30 m) OpenET products across the western U.S. The OpenET products include outputs from six remote sensing-based ET models (METRIC, PTJPL, SIMS, SSEBop, SEBAL, and DisALEXI) and an ensemble product. While all OpenET models showed some discrepancies in their estimates of daily and total ET compared to the ETEC, they also show good potential for mapping ET from alfalfa fields in the SGP. Among the OpenET models, SIMS and SSEBop models performed most consistently across rainfed and irrigated alfalfa for estimating ET. The DisALEXI and SEBAL models showed weaknesses in irrigated alfalfa, while METRIC and PTJPL models performed poorly under all conditions.

Technical Abstract: Understanding the annual dynamics of water use by rainfed and irrigated alfalfa (Medicago sativa L.) can support the sustainable management of this important forage crop. Data on changes in evapotranspiration (ET) and plant growth patterns of alfalfa across years are scarce and are not well understood in the Southern Great Plains (SGP) of the United States (U.S.). The objectives of this study were to investigate the dynamics of eddy covariance (EC) measured ET (ETEC) and its controlling factors in rainfed and irrigated alfalfa and to compare daily magnitudes and annual dynamics of ETEC with OpenET products, a new high-resolution (30 m) ET data tool that provides several established remote sensing-based ET model products (METRIC, PTJPL, SIMS, SSEBop, SEBAL, and DisALEXI) across the western U.S. The ETEC showed significant seasonal and interannual dynamics in alfalfa’s water use that was driven by meteorological conditions, vegetation dynamics, and water availability. Warmer and wetter conditions in April were crucial for promoting initial alfalfa growth. Alfalfa’s water use (ET) mirrored its growth patterns throughout the year. Daily ET rates and cumulative ET at annual and seasonal scales were substantially lower than those reported for highly productive irrigated alfalfa in past studies. Satellite-derived enhanced vegetation index (EVI) and solar radiation (SR) explained 75% and 87% of variations in alfalfa ET at 8-day and monthly scales, respectively. It indicates the potential of developing an empirical model using readily available EVI and SR data to monitor alfalfa ET across large areas. When compared against ETEC, the performance of OpenET models varied widely, depending on field scenarios and criteria applied to model evaluations. Overall, SIMS and SSEBop demonstrated consistency and reliability in estimating ET for both rainfed and irrigated alfalfa. DisALEXI and SEBAL performed poorly in irrigated alfalfa. METRIC and PTJPL exhibited poor performances under both rainfed and irrigated conditions. By examining water use dynamics by alfalfa and the reliability of OpenET products, this study provides crucial information for effective water management practices for alfalfa in the SGP.