Energy balance in the DSSAT-CSM-CROPGRO model

Highlights

• The energy balance option (EBL) implemented in the DSSAT- CROPGRO model.

• The EBL was tested against measured soybean biomass and daily energy fluxes.

• Evapotranspiration was consistently simulated by the EBL and with the FAO-56 (FAO).

• Significant improvement was achieved using the EBL for simulate the soil temperature.

• EBL option connected to leaf-level and canopy-based gas exchange.

Abstract

One potential way to improve crop growth models is for the models to predict energy balance and evapotranspiration (ET) from first principles, thus serving as a check on “engineered” ET methodology. In this paper, we present new implementations and the results of an energy balance model (EBL) developed by Jagtap and Jones (1989) and then implemented in DSSAT's CROPGRO (CG-EBL) model by Pickering et al. (1995) as a linked energy balance-photosynthesis model that has not been field-tested until now. The energy balance code computes evapotranspiration and other energy balance components, as well as a canopy air temperature, based on three sources (sunlit leaves, shaded leaves, soil surface). Model performance was evaluated with measured biomass and energy fluxes from two sites in Nebraska, namely, the US-Ne2 irrigated maize-soybean rotation field and the US-Ne3 rainfed maize-soybean rotation field, which are part of the Ameriflux eddy covariance network (https://ameriflux.lbl.gov/sites). After implementing new aerodynamic resistances and the stomatal conductance model of the Ball–Berry–Leuning, crop growth, evapotranspiration and soil temperature were simulated well by the EBL model. The EBL improved ET predictions slightly over the often-used FAO56 method [Penman–Monteith (Allen et al., 1998)] for 4 of the 5 years evaluated for both irrigated and rainfed conditions. Further, a significant improvement was achieved using EBL for the simulation of soil temperature at the various depths compared to STEMP, the original subroutine in DSSAT for simulating soil temperature. Compared to the other available DSSAT methods, the EBL explicitly simulates the impacts of crop morphology, physiology and management on the crop's environment and energy and mass exchange, which in turn directly affect the water use and irrigation requirements, phenology, photosynthesis, growth, sterility, and yield of the crop.

Introduction

Crop growth models are widely used tools to assess the likely effects of global change on future agricultural productivity, as well as being a crop management aid for present-day farmers. Although these models are improving rapidly, there are still some important processes that are not incorporated or which are generically parameterized. For example, most models simulate the potential crop transpiration as the product of potential evapotranspiration (ETo) and an empirical crop energy-extinction coefficient (k_ep) that reduces potential transpiration as a function of crop leaf area index (LAI). One of the limitations of this approach is the decoupling of the transpiration and carbon assimilation by a crop canopy. Another example is that almost all such models “grow” the crops at air temperature, usually measured at a nearby weather station, whereas the actual temperature of crop canopies can deviate several degrees from air temperature (e.g., Jackson et al., 1981, Allen et al., 2003), especially in arid environments. Therefore, one potential way to improve such models is to first have them compute the carbon assimilation and transpiration (largely regulated by the stomatal opening) and the temperature of the crop canopy. Then they can “grow” the crop at that canopy temperature.

Computing the crop canopy temperature, however, generally requires much more coding effort and an accounting of the significant fluxes of energy flowing to and from the crop and soil surfaces. For crops, usually the latent heat of water vapor transfer, i.e. evapotranspiration or water use, is the major energy flux for the dissipation of the net radiation. Therefore, solving for energy balance of a crop canopy provides a method to explicitly compute the impacts of crop morphology and physiology and management on the crop environment and energy and mass exchange. These energy exchanges directly affect the water use and irrigation requirements, phenology, photosynthesis, sterility, growth, and yield.

In addition to the value of simulating a temperature that is closer to the crop's actual temperature, there remains considerable uncertainty in simulation of crop evapotranspiration (e.g., Kimball et al., 2019), possibly because of multiple approaches/equations for simulating potential evapotranspiration and the allocation between actual crop transpiration and soil evaporation. Developing an energy balance approach for simulating crop and soil evaporation and soil temperature is an enhancement over the multiplicity of available ET prediction equations. The energy balance approach in this paper is an alternative that does not rely on any of those formulations, but rather it is based on first principles of soil-crop-atmosphere energy balance and conductivities of the crop and soil to losses and gains of sensible heat and latent heat (water evaporation).

The Decision Support System for Agrotechnology Transfer (DSSAT) family of crop growth models is widely used with thousands of downloads each year (Hoogenboom et al., 2019). One important model within the DSSAT family of Cropping System Models is CSM-CROPGRO (hereafter CROPGRO), which can simulate the growth of many legumes, some non-legumes such as cotton, and some forage crops (Boote et al., 1998; Pequeno et al., 2014, 2018). CROPGRO is a mechanistic model that predicts leaf-to-canopy carbon assimilation on an hourly time step.  The CROPGRO model uses daily weather data for input but simulates hourly diurnal weather patterns in an internal loop. Photosynthesis is computed within the hourly loop following a simplification of the Farquhar leaf rubisco-kinetics approach combined with hedge-row canopy geometry to compute light absorption and photosynthesis of sunlit and shaded leaves (Boote and Pickering, 1994; Pickering et al., 1995).

CROPGRO computes photosynthesis and energy fluxes in a subroutine called ETPHOT that was written by Pickering et al. (1995) and placed in the CROPGRO source code more than two decades ago. However, the energy balance capability of that code in CROPGRO is likely to be surprising to many current and potential users because it has been a hidden non-advertised feature in the DSSAT V3.5 release (Boote et al., 1998), which was subsequently inadvertently uncoupled during code “modularization” in the early 2000s.  Nevertheless, the code has remained there, but it has only been used for leaf to canopy assimilation, ignoring the coupled energy balance. The energy balance code within ETPHOT computes evapotranspiration (ET) and other energy balance components, as well as a canopy temperature, following a three-source (sunlit leaves, shaded leaves, soil surface) model initially developed by Jagtap and Jones (1989; Fig. 1). While the photosynthesis portion of ETPHOT has been well tested and used for many years, the ET and energy balance portion has not. Therefore, we had three objectives for this study: (1) resurrect the energy balance code within ETPHOT, (2) present some changes that improve the model, and (3) test the ability of the energy balance model (EBL) model to simulate crop ET by comparison with eddy covariance field data on soybean collected as part of the Ameriflux network at Mead, Nebraska, USA.