Modeling Evapotranspiration Using DSSAT and Eddy Covariance Measurements in Cotton and Corn.

Dorothy Menefee, Nithya Rajan and Song Cui

There is a new effort underway in the crop modeling community aimed at improving the ability of process-based simulation models to accurately simulate crop water use. Currently, evapotranspiration (ET) in crop models is estimated using methods such as the Penman-Monteith and soil water balance. Testing of simulations of ET by crop models is not widely done due to lack of availability of field-scale ET data. Since crop growth is highly dependent on water use, accurate simulation of ET is critical for using information gathered in modeling in agricultural decision-making. The objective of this study was to evaluate the accuracy of the Decision Support System for Agrotechnology Transfer (DSSAT) ET models for corn (Zea mays L.) and cotton (Gossypium hirsutum L.) in East-Central Texas. High frequency measurements of CO₂, water vapor and energy exchange were made using eddy covariance flux towers established in two near-by fields in Burleson County, TX in the 2017 growing season. These measurements were made using an open path eddy covariance system consisting of a CSAT-3A sonic anemometer (Campbell Scientific Inc., Logan, UT, USA) and LI-7500A infrared gas analyzer (LI-COR Biosciences, Lincoln, NE, USA). Using the eddy covariance method, half-hourly fluxes of latent heat fluxes (LE) and net ecosystem CO₂ exchange (NEE) were calculated as the covariance between fluctuations from the mean vertical wind speed and corresponding fluctuations of water vapor and CO₂ concentrations. Half-hourly LE values were converted to ET. Supporting meteorological and plant phenological data were also collected from these fields. The DSSAT- Cropping System Models (CSM) for cotton (CROPGRO plant growth module) and corn (CERES-Maize Plant Growth Module) were calibrated using plant phenological data. After calibration, model simulations of ET were compared with actual field measurements of ET. These results will be presented at the meeting.