Implementing Standardized reference evapotranspiration and dual crop coefficient approach in the dssat cropping system model

Authors: DeJonge, Kendall; Thorp, Kelly

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/2017
Publication Date: 10/23/2017
Citation: Dejonge, K.C., Thorp, K.R. 2017. Implementing Standardized reference evapotranspiration and dual crop coefficient approach in the dssat cropping system model. Transactions of the ASABE. 10.13031/trans.12321.
DOI: https://doi.org/10.13031/trans.12321

Interpretive Summary: In this study, two major enhancements to the DSSAT-CSM model’s ET routines were evaluated: 1) addition of the ASCE Standardized Reference Evapotranspiration Equation, the most recent reference ET standard, and 2) addition of the FAO-56 dual crop coefficient approach to determine potential ET, which combined an evaporative coefficient (Ke) for potential evaporation with a dynamic basal crop coefficient (Kcb) for potential transpiration as a function of simulated leaf area index, LAI. Previously published data sets for maize in Colorado (5 years) and cotton in Arizona (7 years)were used to parameterize the model. Simulation results for crop yield, seasonal ETc, transpiration, and evaporation using the short grass reference (ETo) were compared among the three crop coefficient methods. Results showed that crop coefficient simulations with the new ETo-Kcb method better mimicked theoretical behavior as discussed in evapotranspiration standard documents such as FAO-56 and Manual 70. For example, spikes in the soil evaporation coefficient (Ke) due to irrigation and rainfall events were appropriately responsive throughout the season, and basal crop coefficients for transpiration were more directly responsive to simulated crop growth.

Technical Abstract: Methods for estimating reference evapotranspiration (ETo or ETr) and subsequent crop ET (ETc) via crop coefficient (Kc) and dual crop coefficient (Kcb, Ke) methods have been standardized for many years. The current version of the DSSAT Cropping System Model (CSM) has not been updated to fully implement these methods. In this study, two major enhancements to the model’s ET routines were evaluated: 1) addition of the ASCE Standardized Reference Evapotranspiration Equation so that both grass and alfalfa reference ET were properly calculated using the most recent reference ET standard and 2) addition of the FAO-56 dual crop coefficient approach to determine potential ET, which combined an evaporative coefficient (Ke) for potential evaporation (as a function of available energy for evaporation and fraction of wetted and exposed surface) with a dynamic basal crop coefficient (Kcb) for potential transpiration (as a function of simulated leaf area index, LAI). Previously published data sets for maize in Colorado (5 years) and cotton in Arizona (7 years) were used to parameterize the model. Simulations of ETo in Arizona and Colorado were compared to outputs from Ref-ET software, and simulated crop coefficients were contrasted among three crop coefficient methods: the current approach (Kcs), a previously published adjustment to the model’s Kc equation (Kcd), and a new dual Kc approach which follows FAO-56 explicitly (Kcb). Simulation results for crop yield, seasonal ETc, transpiration, and evaporation using the short grass reference (ETo) were compared among the three crop coefficient methods. A sensitivity analysis was conducted for the new ETo-Kcb method to demonstrate responses of crop yield and ETc to input parameters that control Kcb calculations and that define wetted surface area based on irrigation type. Results showed that crop coefficient simulations with the new ETo-Kcb method better mimicked theoretical behavior as discussed in evapotranspiration standard documents such as FAO-56 and Manual 70. For example, spikes in the soil evaporation coefficient (Ke) due to irrigation and rainfall events were appropriately responsive throughout the season, and basal crop coefficients for transpiration were more directly responsive to simulated crop growth. Simulated ETc and yield with ETo-Kcb were up to 4% higher and 28% lower, respectively, than that for the ETo-Kcs method, indicating that the seasonal ETc effects were minimal while daily ETc effects on yield were substantial. Future efforts require higher quality ETc data, such as that from lysimetry, for more comprehensive evaluation of daily ETc among the model’s ET methods. Use of FAO-56 concepts and existing ET standards were essential for identifying issues with the DSSAT-CSM’s ET routines and for guiding enhancements to the code, which has made the model much more conceptually relevant to irrigation and ET specialists.