Graciela R. Fischer

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.

Previsão do número de dias de chuva para a metade sul do Rio Grande do Sul utilizando a Temperatura da Superfície do Mar (TSM)

NUMBER OF RAINFALL DAyS FORECASTING FOR THE SOUTHERN HALF OF RIO GRANDE DO SUL USING THE SEA SURFACE TEMPERATURE (SST) This study presents a climate forecasting model of the Number of Rainfall Days (NRD) for some meteorological stations in Rio Grande do Sul using the Sea Surface Temperatures (SST) as the predictable variables. Two sets of data were used in this research: the monthly data of NRD, which were obtained from 5 meteorological stations located in the southern half of Rio Grande do Sul, in the period of 1982 until 2005; and the SST data, measured in the same period. This series was divided in two periods: the dependent period is from 1982 to 2002, and it was used to determine the predictable equations and the regression coefficients; and the independent period, which is from 2003 to 2005, and was used to validate the model. The SST data were employed to establish the relations between the variables through the regression analysis. Good results were obtained in the prediction of the NRD for the regions and all the months analyzed. The predictable and observed data had a very similar distribution of the variables. Although there was some predictable values that differed from the observed ones, but these differences were not significant. The higher differences between the foreseen and the observed values occurred in the independent period.

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.

METEOROLOGICAL VARIABLES IN FOREST AND PASTURE AREA IN THE WESTERN AMAZON IN YEARS OF EXTREME EVENTS

Deforestation in the Amazon comes from historical contexts which date from the colonization of the region. Several studies have identified climate change from the conversion of forests into agricultural systems. So, this study aimed to identify possible differences in the behavior of the specific humidity, air temperature and net radiation due to conversion of forest to pasture, using data corresponding to extreme event years, once these have occurred in short period of time. Therefore, it was used measures of the variables of a tropical rain forest area and a pasture area, where there are two towers belonging to the Large Scale Biosphere-Atmosphere Experiment in Amazonia - LBA. The measurements correspond to the years of 2005 and 2010, which occurred drought events, and 2009, a year that occurred a flood period. For each variable were calculated hourly means of each site, separated by year and by period. It was found that the years with dry events showed greater variability in all variables, especially in the dry period of the mentioned years, being the pasture the most affected area. Understanding this dynamic enables better planning in agricultural activities in the region.

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.