During approximately 80% of its growing season, lowland flooded irrigated rice ecosystems in southern Brazil are kept within a 5–10-cm water layer. These anaerobic conditions have an influence on the partitioning of the energy and water balance components. Furthermore, this cropping system differs substantially from any other upland nonirrigated or irrigated crop ecosystems. In this study, daily, seasonal, and annual dynamics of the energy and water balance components were analyzed over a paddy rice farm in a subtropical location in southern Brazil using eddy covariance measurements. In this region, rice is grown once a year in low wetlands while the ground is kept fallow during the remaining of the year. Results show that the energy budget residual corresponded to up to 20% of the net radiation during the rice-growing season and around 10% for the remainder of the year (fallow). The energy and water balance analysis also showed that because of the high water table in the region, soil was near saturation most of the time, and latent heat flux dominated over sensible heat flux by up to one order of magnitude in some cases. The estimate of evapotranspiration ET using the crop coefficient multiplied by the reference evapotranspiration KcETo and the Penman–Monteith equation ETPM, describing the canopy resistance through leaf area index (LAI) obtained by remote sensing, represent well the measured evapotranspiration, mainly in the fallow periods. Therefore, using a specific crop parameter like LAI and crop height can be an easy and interesting alternative to estimate ET in vegetated lowland areas.
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.
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