With the growing demands for food and biofuel, new technologies and crop management systems are being used to increase productivity and minimize land-use impacts. In this context, estimates of productivity and the impacts of agriculture management practices are becoming increasingly important. Numerical models that describe the soil–surface–atmosphere interactions for natural and agricultural ecosystems are important tools to explore the impacts of these agronomical technologies and their environmental impacts. However, these models need to be validated by considering the different soil and environmental conditions before they can be widely applied. The process-based terrestrial agricultural version of the Integrated Biosphere Simulator (IBIS) model (Agro-IBIS) has only been calibrated and validated for North American sites. Here, the authors validate the Agro-IBIS results for an experimental soybean site in southern Brazil. At this site, soybean was grown under two different management systems: no tillage (NT) and conventional tillage (CT). The model results were evaluated against micrometeorological, soil condition, and biomass observations made during the soybean growing season. The leaf area index (LAI) was underestimated, approaching the values obtained in the CT crop system, with higher error in the leaf senescence period. The model shows higher skill for daily averages and the diurnal cycle of the energy balance components in the period of high LAI. The soil temperature and moisture were robustly simulated, although the latter is best correlated with the observations made at the CT field. The ecosystem respiration is highly underestimated, causing an overestimation of the cumulative net ecosystem exchange (NEE), particularly at the end of the crop cycle.

The water balance in agricultural cropping systems is dependent on the physical and hydraulic characteristics of the soil and the type of farming, both of which are sensitive to the soil management. Most models that describe the interaction between the surface and the atmosphere do not efficiently represent the physical differences across different soil management areas. In this study, the authors analyzed the dynamics of the water exchange in f Corresponding author: Debora Regina Roberti, debora@ufsm.br the agricultural version of the Integrated Biosphere Simulator (IBIS) model (Agro-IBIS) in the presence of different physical soil properties because of the different long-term soil management systems. The experimental soil properties were obtained from two management systems, no tillage (NT) and conventional tillage (CT) in a long-term experiment in southern Brazil in the soybean growing season of 2009/10. To simulate NT management, this study modified the top soil layer in the model to represent the residual layer. Moreover, a mathematical adjustment to the computation of leaf area index (LAI) is suggested to obtain a better representation of the grain fill to the physiological maturity period. The water exchange dynamics simulated using Agro-IBIS were compared against experimental data collected from both tillage systems. The results show that the model well represented the water dynamics in the soil and the evapotranspiration (ET) in both management systems, in particular during the wet periods. Better results were found for the conventional tillage management system for the water balance. However, with the incorporation of a residual layer and soil properties in NT, the model improved the estimation of evapotranspiration by 6%. The ability of the Agro-IBIS model to estimate ET indicates its potential application in future climate scenarios.

Resumo

RESUMONeste trabalho são analisados resultados da evapotranspiração (ET) e do índice de área foliar (IAF) obtidos durante a estação de cultivo (2009/2010) para os sistemas de plantio direto (PD) e plantio convencional (PC) no ciclo da cultura da soja, no município de Cruz Alta, Rio Grande do Sul (RS). INTRODUÇÃOO conhecimento da água perdida por evapotranspiração é fundamental para se conhecer o balanço hídrico de certa região. A partir da disponibilidade hídrica, pode-se então determinar se essa região é indicada para o cultivo de determinada espécie vegetal ou se é necessário o uso de irrigação.A complexidade na mensuração da (ET) se deve principalmente à difícil diferenciação aos processos de evaporação e transpiração que ocorrem na natureza (Allen et al., 1998). Em uma área agrícola, por exemplo, a evolução no crescimento da cultura determina qual processo contribuirá mais representativamente para a ET.A técnica de covariância dos vórtices tem sido aplicada cada vez mais nos estudos relacionados às perdas por evapotranspiração em cultivos de soja (Suyker e Verma, 2009, Singer et al., 2010. Desta forma o conhecimento da ET em diferentes sistemas de manejos pode avaliar se existem diferenças significativas nas perdas de água entre os mesmos.

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