Leonardo J. G. Aguiar

Modeling the photosynthetically active radiation in South West Amazonia under all sky conditions

Accurate estimates of photosynthetically active radiation (PAR) are critical for the development of realistic models of plant productivity. However, in many areas such as the vast Amazon region of South America, there have been few empirical studies of PAR. Here, we analyzed the relationship between PAR and broadband solar irradiance (Rs) and formulated models to estimate PAR in two experimental sites (pasture and forest) in the Brazilian Amazon. Three different models of increasing complexity were developed based on information from Rs (model 1), Rs and clearness index (kt; model 2), and Rs, kt, and water vapor pressure (model 3). Estimates of PAR were generated for each season and for the entire year. All models had very high determination coefficients and indices of agreement for both pasture and forest sites. This strongly supports the use of Rs and kt to produce robust estimates of PAR. The results obtained by annual models were close than that found by seasonal models, demonstrating that a single annual model is able to estimate PAR, albeit with lower accuracy.

SIMULATION OF THE WATER USE EFFICIENCY FOR THE CULTURE OF SOYBEAN USING THE CROPGRO MODEL

Simulacao da eficiencia do uso da agua para a cultura da soja utilizando o modelo CROPGRO. O presente trabalho teve como objetivo simular, usando o modelo CROPGRO, a eficiencia do uso da agua pela cultura da soja e comparar com resultados experimentais da Rede Sulflux (Rede Sul Brasileira de Fluxos Superficiais e Mudancas Climaticas) em Cruz Alta, RS, no ano agricola de 2009/10. O modelo CROPGRO-Soybean superestimou a eficiencia do uso da agua da cultura da soja, para as condicoes, caracteristicas do periodo e fatores considerados. The present study aimed to simulate the water use efficiency by soybean culture, using CROPGRO model, and compare with Sulflux Network experimental results (Brazilian South Network of Surface Flows and Climate Change) in Cruz Alta, RS, in the agricultural year 2009/10. The CROPGRO - soybean model overestimated the water use efficiency of soybeans for the conditions, period features and factors considered.

Energy balance closure in the Southwest Amazon forest site—a statistical approach

Energy balance closure is the main indicator of the quality of energy flux measurements obtained by the eddy covariance technique. Many researchers use a simple linear regression model between energy balance components to evaluate closure. However, these studies typically fail to verify the appropriateness of the statistical assumptions of regression analysis, which can lead to erroneous conclusions if the model is not satisfactory. Thus, the aim of this study was to calibrate and validate simple and robust with bootstrap and cross-validation linear regression models to verify the efficiency of energy balance closure in the Amazon ecosystem. Measurements of net radiation and latent, sensible, and ground heat fluxes were made from January to December 2008 in a tropical rain forest area in South West Amazonia in an experimental site belonging to the network of flux towers of the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). The results demonstrated that simple linear regression models are not appropriate for analyzing energy balance closure. However, robust linear regression models with bootstrap and cross-validation improved the fit to the data. Despite the better fit, there was an increase in energy balance closure residuals suggesting that the eddy covariance technique is underestimating the values of energy fluxes in Amazon forest areas more than what were reported in previous researches.

Mudanças no uso da terra e o particionamento de energia no sudoeste da Amazônia

The significant rate of deforestation in the Amazon region has become a global concern. This process consists substantially in the conversion from forest to agricultural systems, which is the activity that drives the local economy. This new ecosystem formed requires studies that elucidate its operation, seeking to understand how this region interacts with the atmosphere. To investigate the behavior of energy balance components in a grassland ecosystem, a study was conducted at Fazenda Nossa Senhora (FNS), in Ouro Preto do Oeste, Rondonia. This study examined net radiation and sensible, latent and soil heat fluxes patterns, in the periods: wet, wet-dry, dry and dry-wet from 2000 to 2005. Measurements of sensible and latent heat fluxes were performed using the eddy covariance method. The results of these analyses showed that the partitioning of energy in the grassland were 77.8, 69.6, 50.6 and 75.2%, respectively in the wet, wet-dry, dry and dry-wet periods, for the latent heat flux and 22.5, 31.2, 46.7 and 24.7% for the sensible heat flux, indicating a higher vulnerability of grassland to seasonal changes.

INFLUÊNCIA DAS FRIAGENS NOS FLUXOS DE DIÓXIDO DE CARBONO DE UMA FLORESTA TROPICAL NO SUDOESTE DA AMAZÔNIA

No ano de 2010, foram realizadas medicoes de temperatura e umidade relativa do ar, radiacao fotossinteticamente ativa e fluxo de dioxido de carbono (CO 2 ) atraves da tecnica de correlacao de vortices turbulentos com o objetivo de analisar a influencia de um evento de friagem na absorcao de CO 2 em uma floresta tropical no sudoeste da Amazonia. A friagem ocasionou reducao na absorcao de CO 2 .

EFEITOS DA NEBULOSIDADE NA ABSORÇÃO DE CARBONO EM UMA CULTURA DE MILHO EM CRUZ ALTA, RS

Foram realizadas medicoes no periodo de 11 de novembro de 2010 a 19 de fevereiro de 2011 de variaveis meteorologicas e fluxo de dioxido de carbono (CO 2 ), atraves da tecnica de covariância dos vortices turbulentos, com o intuito de analisar a influencia da nebulosidade na absorcao do C em uma cultura de milho em Cruz Alta, RS. Observou-se maior absorcao de C a niveis intermediarios de cobertura de nuvens.