The Agro-IBIS agroecosystem model was tested against 3 years of biometric data, soil temperature and moisture data, and eddy covariance measurements at the Mead, Nebraska AmeriFlux site. Three cropping systems managed with no-tillage were studied: (a) irrigated continuous maize, (b) irrigated maize-soybean rotation, and (c) rainfed maize-soybean rotation. The model satisfactorily represented crop growth, carbon (C) allocation, and phenology, as simulated biomass pools were generally within 10% of observations. However, daily net ecosystem production (NEP) was overestimated during the mid-summer by 1-4 g C m-2 day-1, and these systematic errors were attributed to underestimates (50-60%) of night-time ecosystem respiration (Re). In contrast, the model produced consistent overestimates of Re during the dormant season by 1-3 g C m-2 day-1, which led to a reduced seasonal cycle of Re. A majority of these inaccuracies were attributed to simplistic representations of heterotrophic and plant tissue respiration and their empirical dependence on temperature and soil moisture. Soil temperatures were overestimated by 4-10 °C in late winter through early summer and then again in late fall, coinciding with the period when vegetative cover was non-existent. Annual total net radiation was overestimated by 10%, and sensible and latent heat fluxes were overestimated from winter to early summer by 1-7 MJ m-2 day-1. Failing to account for the impacts of a crop residue layer on surface albedo and other physical properties is believed to have contributed to these inadequacies in simulated surface energy balance. We conclude that if modelers tune crop-biosphere models to agricultural FLUXNET data without accounting for the impacts of surface residue management, inconsistent estimates of large-scale C and water exchange with the atmosphere may result. This is extremely relevant to studies of the biogeophysical feedbacks to regional climate attributed to land management changes using coupled crop-climate models. Given the increasing adoption of conservation tillage, modelers should focus on including some representation of crop residue dynamics and aim to improve representation of leaf senescence and ecosystem respiration.
Bibliographical noteFunding Information:
The authors thank Shashi Verma, Andrew Suyker, and Tim Arkebauer for discussions and for making AmeriFlux and other associated data available to use for model validation. We extend gratitude to Bill Sacks for reading and commenting on an earlier version of the manuscript. This material is based upon work supported by the Department of Energy under Award Number DE-FC02-06ER64158, through the National Center for Climate Change Research (NICCR). This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability of responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the Unites States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
- Carbon flux
- Energy balance