Renata Gonçalves Aguiar

Patterns of water and heat flux across a biome gradient from tropical forest to savanna in Brazil

[1] We investigated the seasonal patterns of water vapor and sensible heat flux along a tropical biome gradient from forest to savanna. We analyzed data from a network of flux towers in Brazil that were operated within the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA). These tower sites included tropical humid and semideciduous forest, transitional forest, floodplain (with physiognomies of cerrado), and cerrado sensu stricto. The mean annual sensible heat flux at all sites ranged from 20 to 38 Wm 2 , and was generally reduced in the wet season and increased in the late dry season, coincident with seasonal variations of net radiation and soil moisture. The sites were easily divisible into two functional groups based on the seasonality of evaporation: tropical forest and savanna. At sites with an annual precipitation above 1900 mm and a dry season length less than 4 months (Manaus, Santarem and Rondonia), evaporation rates increased in the dry season, coincident with increased radiation. Evaporation rates were as high as 4.0 mm d 1 in these evergreen or semidecidous forests. In contrast, ecosystems with precipitation less than 1700 mm and a longer dry season (Mato Grosso, Tocantins

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.

Variability of Carbon and Water Fluxes Following Climate Extremes over a Tropical Forest in Southwestern Amazonia

The carbon and water cycles for a southwestern Amazonian forest site were investigated using the longest time series of fluxes of CO2 and water vapor ever reported for this site. The period from 2004 to 2010 included two severe droughts (2005 and 2010) and a flooding year (2009). The effects of such climate extremes were detected in annual sums of fluxes as well as in other components of the carbon and water cycles, such as gross primary production and water use efficiency. Gap-filling and flux-partitioning were applied in order to fill gaps due to missing data, and errors analysis made it possible to infer the uncertainty on the carbon balance. Overall, the site was found to have a net carbon uptake of ≈5 t C ha−1 year−1, but the effects of the drought of 2005 were still noticed in 2006, when the climate disturbance caused the site to become a net source of carbon to the atmosphere. Different regions of the Amazon forest might respond differently to climate extremes due to differences in dry season length, annual precipitation, species compositions, albedo and soil type. Longer time series of fluxes measured over several locations are required to better characterize the effects of climate anomalies on the carbon and water balances for the whole Amazon region. Such valuable datasets can also be used to calibrate biogeochemical models and infer on future scenarios of the Amazon forest carbon balance under the influence of climate change.

Partição do saldo de radiação em áreas de floresta amazônica e floresta de transição Amazônia - cerrado

In this work the seasonal behavior and the daily cycle of micrometeorological parameters and energy fluxes, in Amazonian forest (Rebio Jaru) and Transitional Amazonia-Cerrado forest (Maracai Farm) is analyzed. The measurements were done by equipments installed in the micrometeorological towers of LBA project (Large Scale Biosphere-Atmosphere Experiment in Amazonia), one located near from Ji-Parana city, RO (area of Amazonian forest) and other located near from Sinop city, MT (area of Transitional forest). The results analysis indicate the existence of a well defined seasonality with dry and wet seasons in both localities, which is reflected in the behavior of the precipitation, air humidity and temperature and, consequently, in the latent heat (λE) and sensible heat (H) fluxes. In the energy balance, the daily means show a different behavior of λE, in both areas, indicating more adaptation capacity of Amazonia forest to the dry period. The Rn/λE partition values showed similar seasonal behavior for both areas, with larger values for the wet season, decreasing for the dry season.

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.

APPLICATION OF MODIS EVAPOTRANSPIRATION PRODUCT FOR A PASTURE AREA IN THE WESTERN AMAZON

The processes of land use and occupation generate interventions in the natural ecosystems making them susceptible to reactions, such as changes in the processes that govern water cycling, emphasizing the importance of monitoring the evapotranspiration behavior. In this sense, the objective of this study was to verify the applicability of the evaporation product originated by the MODIS sensor to a pasture area, from 2003 to 2010, at Fazenda Nossa Senhora in the municipality of Ouro Preto do Oeste - Rondonia. Were used evapotranspiration data from the MODIS (Terra / Aqua) sensor, estimated by MOD16 algorithm, and micrometeorological tower located in the pasture area, generated by eddy covariance system. It was verified that for ET Eddy x ET MOD16 (Quality control – QC 0/8) data set, ET MOD16 (QC 0/8) data showed evapotranspiration values above those of ET Eddy and with a greater amplitude. A linear correlation between the study datasets was not identified, however, seasonal variations are captured by product, showing good approximation with ET Eddy data, especially in the transition periods.

CO2 FLUXES IN A WET TROPICAL FOREST AREA IN WESTERN AMAZON IN YEAR OF EL NIÑO

The objective this paper was to quantify the hourly variability of CO 2 fluxes in the year of the El Nino, in 2015, in a tropical rain forest in the Western Amazon. Data of CO 2 fluxes collected at 63.5 m height in a tower of the Large-Scale Biosphere–Atmosphere Experiment in Amazonia in the Jaru Biological Reserve, composed of the Open Ombrophilous forest, were analyzed. Data from the dry-wet period were used, which by convention, the negative and positive values indicate absorption and emission of CO 2 , respectively. The CO 2 uptake in the forest had higher levels between 9:30 a.m. and 4:00 p.m., with mean values between -19 and -13 μmol m -2 s -1 , however the peak absorption occurs at 1:00 p.m. with -30 μmol m -2 s -1 , considering that at this time the suns rays are most intense. In average, the Rebio Jaru absorbs more than the areas of the transition Amazonia-Cerrado, indicating that the physiological activities in Ombrophilous forest require a greater amount of carbon. The average daily behavior of CO 2 fluxes showed higher concentrations of absorption during the day than emissions overnight, behaving as a carbon sink, corroborating the importance of the forest in assimilation of the atmospheric carbon.

SOIL HEAT FLUX MODELED FROM TEMPERATURE DATA AT TWO DEPTH LEVELS IN A RAINFOREST IN WESTERN AMAZONIA

The global climate is dependent of ecological balance of forests, especially tropical. The heat flux in the soil is an important factor in studies of energy balance representing the main form of energy exchange between soil and atmosphere. The aim of the present work was to estimate soil heat flux using soil temperature measurements at two depth levels in a tropical forest in the Western Amazon, in order to obtain coherent data for both the use of the values and for the filling of failures in database. Had been used data on temperature and soil heat flux collected in a micrometeorological tower belonging to the towers network of the Large Scale Biosphere-Atmosphere Program in the Amazon, located in the Jaru Biological Reserve. The estimated data presented 94% agreement with the measured data, the two have similar behaviors that allow the use in filling of failures in a demonstrative way. However, there is a delay in the estimated values of the heat flux in the soil in relation to the measured one, which interferes in the result of the model, provoking more studies to improve it.

SEASONAL AND INTER-ANNUAL VARIABILITY OF MICROCLIMATE IN FOREST AREA IN SOUTHWESTERN AMAZONIA

The Amazon is a recognized global ecosystem, due to its high biodiversity and the magnitude of the mass and energy exchanges performed. In this study it sought to analyze the seasonal and interannual variations of microclimate in a site of tropical forest in southwestern Amazon. For this purpose, net radiation data were used, air temperature, relative humidity and wind speed collected in a micrometeorological tower belonging to the LBA, located in Jaru Biological Reserve, from 2004 to 2010. The results showed that existence of well-defined seasonal patterns was verified, with variations between wet, wet-dry, dry, dry-humid periods for all variables in question. Yet, through analysis of the interannual variations were observed warming of the increment signs and decreased moisture in the locality. This observation, although patchy, deserves special attention, since changes in the microclimate in the Amazon region may have yet unknown consequences.

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.