Modeling Evapotranspiration Using DSSAT and Eddy Covariance Measurements in Texas Cotton

Cotton (Gossypium hirsutum L.) is an important commodity crop throughout the southern United States, with the largest portion of production in Texas. Projected decreases in rainfall and increases in drought frequency associated with climate change make understanding cotton evapotranspiration (ET) and water use efficiency (WUE) an important area of research. The equipment required to make accurate ET measurements is often not used in agricultural research. The use of modeling systems to estimate ET is a growing trend in agricultural research; however these models are often not calibrated with real-time field data. The objective of this study is to compare ET measurements from an eddy covariance system to modeled estimates using the Decision Support System for Agrotechnology Transfer (DSSAT)’s CROPGRO cotton model for East-Central Texas. High frequency measurements of CO₂, water vapor and energy exchange were made using an eddy covariance flux tower established in February 2017 in a production cotton field in Burleson County, TX. These measurements were made using an open path eddy covariance system consisting of: a CSAT-3A sonic anemometer (Campbell Scientific Inc., Logan, UT, USA), LI-7500A infrared gas analyzer (LI-COR Biosciences, Lincoln, NE, USA), meteorological instruments, and soil sensors. Using the eddy covariance method, half-hourly fluxes of latent heat (LE), sensible heat (H) and net ecosystem CO₂ exchange (NEE) were calculated as the covariance between fluctuations from mean vertical wind speed and corresponding water vapor and CO₂ flux. Supporting meteorological and plant phenological data were also collected. The DSSAT CROPGRO model was calibrated using phenological data. After calibration, modeled simulations of ET were compared with actual field measurements of ET and calculations of WUE were made. The model showed a degree of agreement with the measured values [RMSE, R²], indicating that the CROPGRO Cotton ET model can be useful tool for agricultural management decisions for cotton producers.