Surface Albedo and Temperature Models for Surface Energy Balance Fluxes and Evapotranspiration Using SEBAL and Landsat 8 over Cerrado-Pantanal, Brazil.

The determination of the surface energy balance fluxes (SEBFs) and evapotranspiration (ET) is fundamental in environmental studies involving the effects of land use change on the water requirement of crops. SEBFs and ET have been estimated by remote sensing techniques, but with the operation of new sensors, some variables need to be parameterized to improve their accuracy. Thus, the objective of this study is to evaluate the performance of algorithms used to calculate surface albedo and surface temperature on the estimation of SEBFs and ET in the Cerrado-Pantanal transition region of Mato Grosso, Brazil. Surface reflectance images of the Operational Land Imager (OLI) and brightness temperature (Tb) of the Thermal Infrared Sensor (TIRS) of the Landsat 8, and surface reflectance images of the MODIS MOD09A1 product from 2013 to 2016 were combined to estimate SEBF and ET by the surface energy balance algorithm for land (SEBAL), which were validated with measurements from two flux towers. The surface temperature (Ts) was recovered by different models from the Tb and by parameters calculated in the atmospheric correction parameter calculator (ATMCORR). A model of surface albedo (asup) with surface reflectance OLI Landsat 8 developed in this study performed better than the conventional model (acon) SEBFs and ET in the Cerrado-Pantanal transition region estimated with asup combined with Ts and Tb performed better than estimates with acon. Among all the evaluated combinations, SEBAL performed better when combining asup with the model developed in this study and the surface temperature recovered by the Barsi model (Tsbarsi). This demonstrates the importance of an asup model based on surface reflectance and atmospheric surface temperature correction in estimating SEBFs and ET by SEBAL.

Ground and remote sensing-based measurements of leaf area index in a transitional forest and seasonal flooded forest in Brazil.

Leaf area index (LAI) is a key driver of forest productivity and evapotranspiration; however, it is a difficult and labor-intensive variable to measure, making its measurement impractical for large-scale and long-term studies of tropical forest structure and function. In contrast, satellite estimates of LAI have shown promise for large-scale and long-term studies, but their performance has been equivocal and the biases are not well known. We measured total, overstory, and understory LAI of an Amazon-savanna transitional forest (ASTF) over 3 years and a seasonal flooded forest (SFF) during 4 years using a light extinction method and two remote sensing methods (LAI MODIS product and the Landsat-METRIC method), with the objectives of (1) evaluating the performance of the remote sensing methods, and (2) understanding how total, overstory and understory LAI interact with micrometeorological variables. Total, overstory and understory LAI differed between both sites, with ASTF having higher LAI values than SFF, but neither site exhibited year-to-year variation in LAI despite large differences in meteorological variables. LAI values at the two sites have different patterns of correlation with micrometeorological variables. ASTF exhibited smaller seasonal variations in LAI than SFF. In contrast, SFF exhibited small changes in total LAI; however, dry season declines in overstory LAI were counteracted by understory increases in LAI. MODIS LAI correlated weakly to total LAI for SFF but not for ASTF, while METRIC LAI had no correlation to total LAI. However, MODIS LAI correlated strongly with overstory LAI for both sites, but had no correlation with understory LAI. Furthermore, LAI estimates based on canopy light extinction were correlated positively with seasonal variations in rainfall and soil water content and negatively with vapor pressure deficit and solar radiation; however, in some cases satellite-derived estimates of LAI exhibited no correlation with climate variables (METRIC LAI or MODIS LAI for ASTF). These data indicate that the satellite-derived estimates of LAI are insensitive to the understory variations in LAI that occur in many seasonal tropical forests and the micrometeorological variables that control seasonal variations in leaf phenology. While more ground-based measurements are needed to adequately quantify the performance of these satellite-based LAI products, our data indicate that their output must be interpreted with caution in seasonal tropical forests.

Patterns and drivers of evapotranspiration in South American wetlands.

Evapotranspiration (ET) is a key process linking surface and atmospheric energy budgets, yet its drivers and patterns across wetlandscapes are poorly understood worldwide. Here we assess the ET dynamics in 12 wetland complexes across South America, revealing major differences under temperate, tropical, and equatorial climates. While net radiation is a dominant driver of ET seasonality in most environments, flooding also contributes strongly to ET in tropical and equatorial wetlands, especially in meeting the evaporative demand. Moreover, significant water losses through wetlands and ET differences between wetlands and uplands occur in temperate, more water-limited environments and in highly flooded areas such as the Pantanal, where slow river flood propagation drives the ET dynamics. Finally, floodplain forests produce the greatest ET in all environments except the Amazon River floodplains, where upland forests sustain high rates year round. Our findings highlight the unique hydrological functioning and ecosystem services provided by wetlands on a continental scale.

Soil CO2 dynamics in a tree island soil of the Pantanal: the role of soil water potential.

The Pantanal is a biodiversity hotspot comprised of a mosaic of landforms that differ in vegetative assemblages and flooding dynamics. Tree islands provide refuge for terrestrial fauna during the flooding period and are particularly important to the regional ecosystem structure. Little soil CO2 research has been conducted in this region. We evaluated soil CO2 dynamics in relation to primary controlling environmental parameters (soil temperature and soil water). Soil respiration was computed using the gradient method using in situ infrared gas analyzers to directly measure CO2 concentration within the soil profile. Due to the cost of the sensors and associated equipment, this study was unreplicated. Rather, we focus on the temporal relationships between soil CO2 efflux and related environmental parameters. Soil CO2 efflux during the study averaged 3.53 µmol CO2 m⁻² s⁻¹, and was equivalent to an annual soil respiration of 1220 g C m⁻² y⁻¹. This efflux value, integrated over a year, is comparable to soil C stocks for 0-20 cm. Soil water potential was the measured parameter most strongly associated with soil CO2 concentrations, with high CO2 values observed only once soil water potential at the 10 cm depth approached zero. This relationship was exhibited across a spectrum of timescales and was found to be significant at a daily timescale across all seasons using conditional nonparametric spectral Granger causality analysis. Hydrology plays a significant role in controlling CO2 efflux from the tree island soil, with soil CO2 dynamics differing by wetting mechanism. During the wet-up period, direct precipitation infiltrates soil from above and results in pulses of CO2 efflux from soil. The annual flood arrives later, and saturates soil from below. While CO2 concentrations in soil grew very high under both wetting mechanisms, the change in soil CO2 efflux was only significant when soils were wet from above.

Growth rhythm of Vochysia divergens Pohl (Vochysiaceae) in the Northern Pantanal / Ritmo de crescimento de Vochysia divergens Pohl (Vochysiaceae) no Pantanal Norte

The Pantanal is the largest wetland in the world, characterized by high biodiversity, but large areas have been invaded by Vochysia divergens, a flood- adapted pioneer and native tree of the riparian forest and Brazilian Savanna. Seasonality in rainfall causes annual rhythms in the tree's physiology, in which climatic stress factors induce a cambial dormancy in trees and, in consequence, growth zones in the trunk. Current analysis evaluates the seasonal variation of the diameter increase of V. divergens specimens in a seasonal floodable forest of the Northern Brazilian Pantanal. Field sampling was conducted between January and December 2012. Air temperature, relative humidity and precipitation were measured at a micrometeorological station, water level was measured by graded tape, and soil water content was measured by a portable TDR sensor. Diameter growth was monitored by dendrometric bands and aboveground litter production was sampled by litter traps. The annual diameter increase was 4.0 mm. There was correlation between diameter increment and climate and phenology, with the highest increase during the flooding period and the lowest during the dry season and reproductive period. Climate and phenology affected the growth rhythm of V. divergens.

O Pantanal é a maior area úmida do mundo, com alta diversidade de plantas e animais, mas vastas áreas são invadidas por Vochysia divergens Pohl (Vochysiaceae), que é uma árvore pioneira bem adaptada à inundação e nativa de matas ripárias e cerrado. A sazonalidade da chuva leva a ritmos anuais na fisiologia das árvores em que fatores de estresse climático induzem à dormência cambial e, consequentemente às zonas de crescimento no caule. Assim, o objetivo deste estudo foi avaliar a variação sazonal do incremento diamétrico de indivíduos de V. divergens em uma floresta sazonalmente inundável no pantanal norte. As coletas ocorreram de janeiro a dezembro/2012. A temperatura do ar, umidade relativa e precipitação foram obtidas por estação micrometeorológica, o nível de água no solo por trena graduada e o conteúdo de água no solo por sensor TDR. O incremento diamétrico foi monitorado por bandas dendrométricas e a produção de liteira por coletores. O incremento diamétrico anual foi 4,0 mm. Houve correlação do incremento diamétrico com clima e fenologia, com maior incremento na inundação e menor na estação seca e no período reprodutivo da espécie. Portanto, o ritmo de crescimento de V. divergens foi afetado pelo clima e fenologia.