Implementing Standardized Reference Evapotranspiration and Dual Crop Coefficient Approach in the DSSAT Cropping System Model

Abstract

ABSTRACT. While methods for estimating reference evapotranspiration (ET o or ET r ) and subsequent crop ET (ET c ) via crop coefficient (K c ) and dual crop coefficient (K cb, DSSAT, Evaporation, Evapotranspiration, Maize, Reference crop ET, Standardization, Transpiration. vapotranspiration (ET), the combined result of soil surface evaporation and plant transpiration, is an important component of agricultural water management and landscape hydrology, particularly in the field of irrigation management. Adequate quantification of ET is imperative as demand for freshwater resources increases. Several U.S. states such as Colorado use quantification of ET as a "consumptive use" in water rights transfer legal cases. Many past studies have shown a direct physiological relationship between crop yield and ET (Doorenbos and Kassam, 1979;Hunsaker et al., 2015;Trout and DeJonge, 2017) and particularly yield and transpiration (Paredes et al., 2014;Steduto et al., 2012). Furthermore, accurate estimation of ET partitioning into evaporation (E) and transpiration (T) is paramount in irrigated and water-limited systems (Jensen and Allen, 2016;Kool et al., 2014;Pereira et al., 2015;Phogat et al., 2016). Ideally, these subcomponents of ET (i.e., E and T) should be evaluated independently rather than as a residual of each other (Kool et al., 2014). The Appendix reviews recent institutionally supported efforts to standardize procedures for ET quantification on several scales, which should be well known material for ET experts but is made available for readers who are unfamiliar with the subject. Full details of the FAO-56 dual crop coefficient methodology are available in FAO Irrigation and Drainage Paper No. 56 Mention of company or trade names is for description only and does not imply endorsement by the USDA. The USDA is an equal opportunity provider and employer.The standards were developed to establish benchmark ET equations, which represent the current state of the art in estimating ET.Over many decades, researchers have also developed complex cropping system models, which aim to comprehensively simulate the hydraulic processes, nutrient transformation and transport processes, and crop growth and development processes that occur in a cropping system. Such models have wide applicability for crop management, yield gap analysis, crop improvement, yield forecasting, synthesis of agronomic research, and assessment of policy (Boote et al., 1996). Because ET is often a large component of the water balance, simulation of the ET process is central to the calculations of these models, and accurate calculations of crop growth and yield depend on accurate ET calculations. However, the ET methods implemented among different models are often variable, and many do not incorporate the published ET standards as a simulation option. Generally, programming updates to the ET algorithms in the models have not kept pace with the development of new ET standards. Furthermore, preliminary results from a recent crop model intercomparison study demo...

Cotton Irrigation Scheduling Using a Crop Growth Model and FAO-56 Methods: Field and Simulation Studies

“…CSM-CROPGRO-Cotton with an updated ET methodology based on FAO-56 dual crop coefficients (DeJonge and Thorp, 2017) was used for in-season irrigation management during two Arizona cotton seasons. As compared to a well- [a] Seed cotton yield = fiber + seed.…”

“…As reported by Thorp et al (2014b) and DeJonge and Thorp (2017) Thorp (2017) further updated the model to include a dual crop coefficient method with K cb estimated from model-simulated LAI. The latter dual crop coefficient approach was used for in-season irrigation scheduling in 2015 and for posthoc simulation analysis of both growing seasons.…”

“…The updates improved the simulation of seasonal cotton ET in Arizona, which was underestimated by 15% with the model's original FAO-56 approach. An error in the model's wind speed adjustment equation for calculating ET o was later identified for cases of non-standard wind measurement height (e.g., Arizona meteorological network stations measure wind speed at 3 m rather than the standard 2 m height), which was circumvented by inclusion of the Walter et al (2005) standard ET o algorithm (DeJonge and Thorp, 2017). Thorp et al (2015) used the updated DSSAT-CSM model to analyze site-specific yield and water use for Arizona cotton, obtaining more reasonable crop coefficient parameters (EORATIO = 1.0 to 1.5) with an iterative optimization technique.…”

“…Thorp et al (2015) used the updated DSSAT-CSM model to analyze site-specific yield and water use for Arizona cotton, obtaining more reasonable crop coefficient parameters (EORATIO = 1.0 to 1.5) with an iterative optimization technique. DeJonge and Thorp (2017) further modified DSSAT-CSM to incorporate a dual FAO-56 crop coefficient approach, which brought the model more fully in compliance with FAO-56 methods. Efforts to improve DSSAT-CSM in these ways were, in part, spurred by failed attempts at using the model for in-season cotton irrigation management in 2007 in Arizona.…”

“…The main objective of this study was to compare DSSAT-CSM, including an updated FAO-56 ET routine (DeJonge and Thorp, 2017), with a well-tested FAO-56 spreadsheet method (Hunsaker et al, 2005a) for in-season cotton irrigation scheduling in central Arizona. While both irrigation scheduling tools incorporated FAO-56 dual crop coefficient procedures and the Walter et al (2005) standard algorithm for ET o calculations, they used different approaches to specify crop coefficients, calculate soil water processes, and consider effects of crop development and growth on water balances.…”