Surface water and energy fluxes in South America : an integrated approach based on remote sensing and flux measurements

South America is a water-abundant continent, home to the world's largest river basins and rainforest, which plays a crucial role in providing moisture to other regions of the continent through evapotranspiration (). is a crucial indicator of the earth's ecosystem functioning, linking the water, energy, and carbon cycles. Due to the great challenge of obtaining information based on in situ measurements, remote sensing data has become a great opportunity to obtain estimations. Based on measurements and estimations based on remote sensing data, this study aimed to evaluate the dynamics, patterns and controls of water and energy fluxes in South America, seeking to answer three main questions: i) can remote sensing data provide accurate information on the water balance?; ii) how do the factors controlling vary across different biomes and land use and land cover (LULC) conditions? and iii) can remote sensing models represent accurately patterns and its components under different LULC conditions? To answer the first question, we performed a water balance analysis, evaluating the uncertainties of precipitation and estimations using in situ measurements, and conducting an analysis to understand how much these uncertainties can be affected due to the basin’s scales. The results showed that due to the uncertainties related to each of the variable from remote sensing it is not yet possible to achieve the water balance closing. However, the approach proved to be a great alternative to evaluate the dynamics of water fluxes from small to large basins, especially in those where in situ measurement is still scarce. To seek to answer the second question, we evaluated the influence of biotic and abiotic factors on control processes, based on surface and aerodynamic conductances and the decoupling factor, at 20 flux measurement sites in South America. Through this analysis, different patterns of latent () and sensible () heat fluxes were verified, and different degrees of importance of biotic and abiotic controls on the process according to different LULC conditions. Finally, based on 11 flux measurement sites and four models (MOD16, GLEAM, PML and SSEBOP), we assessed the accuracy of estimates in the Amazon basin, and the representation of fluxes in forest, pasture, and soybean areas, in the Tapajós basin. The results showed that obtaining accurate estimates is still a major challenge in the Amazon basin, especially in humid and seasonally flooded sites. Significant discrepancies between the models and between measurements were found, and these discrepancies were even more significant when evaluated the individual components. However, even though each model did not perform significantly under all climatic and vegetation conditions, they present together a great opportunity to improve the accuracy of estimates, leading to an improved understanding of the impacts on water and energy fluxes due to human activities. Thus, these results demonstrate the potential and limitations of hydrological components obtained by remote sensing, especially for , and how LULC changes may modify this flux in South America.A América do Sul é um continente abundante em água, abrigando as maiores bacias hidrográficas e a maior floresta tropical do mundo, a floresta Amazônica. A Amazônia desempenha um papel crucial no fornecimento de umidade para outras regiões do continente por meio da evapotranspiração (). A é um indicador crucial do funcionamento do ecossistema terrestre, interligando os ciclos da água, energia e carbono. Devido ao grande desafio de obtenção de informações de por medições in situ, o uso de dados de sensoriamento remoto tem se mostrado uma grande alternativa para obter estimativas desta variável. Com base em dados medidos e estimados por sensoriamento remoto foi conduzido um estudo que visou analisar a dinâmica, os padrões e os controles dos fluxos de água e energia na América do Sul, buscando responder a três questões principais: i) os dados de sensoriamento remoto podem fornecer informações precisas sobre o balanço hídrico?; ii) como os fatores que controlam a variam em diferentes biomas e condições de uso e cobertura do solo (LULC)?; e iii) os modelos de sensoriamento remoto conseguem representar com acurácia os padrões de e das suas componentes em diferentes condições de LULC? Para responder a primeira pergunta realizou-se uma análise de balanço hídrico, na qual foi avaliada as incertezas das estimativas de precipitação e usando medições in situ, e uma análise do quanto essas incertezas podem ser afetadas devido ao efeito de escala das bacias analisadas. Os resultados mostraram que devido às incertezas relacionadas com cada uma das componentes estimadas por sensoriamento remoto ainda não é possível alcançar o fechamento do balanço hídrico. No entanto, a abordagem demonstrou ser uma grande alternativa para avaliar a dinâmica dos fluxos de água, de pequenas a grandes bacias, especialmente naquelas onde a medição in situ ainda é escassa. Para buscar responder a segunda pergunta analisou-se a influência dos fatores bióticos e abióticos no controle dos processos de , por meio da análise das condutâncias de superfície e aerodinâmica e do fator de desacoplamento em 20 locais de monitoramento de fluxo na América do Sul. Por meio desta análise verificou-se diferentes padrões dos fluxos de calor latente () e sensível (), além de diferentes graus de importância dos controles bióticos e abióticos sobre o processo de e de acordo com as diferentes condições de LULC. Por fim, com base em 11 locais de monitoramento de fluxo e quatro modelos de (MOD16, GLEAM, PML e SSEBOP), analisou-se a acurácia destas estimativas na bacia amazônica, e a representação dos fluxos de em áreas de floresta, pastagem e soja, na bacia do Tapajós. Os resultados mostraram que a obtenção de estimativas acuradas de ainda é um grande desafio na bacia Amazônica, principalmente em locais úmidos e sazonalmente inundados. Discrepâncias significativas entre os modelos e entre as medições foram encontradas, sendo estas discrepâncias ainda mais expressivas quando se analisou as componentes individuais de . No entanto, os resultados deste estudo demonstraram que apesar de cada modelo não apresentar um desempenho significativo em todas as condições climáticas e de vegetação, estes apresentam em conjunto, uma grande oportunidade para melhorar a acurácia das estimativas de , propiciando um aprimoramento na compreensão dos impactos nos fluxos de água e energia devido a atividades antrópicas. Deste modo, estes resultados enfatizam os potenciais e limitações das variáveis hidrológicos obtidas por sensoriamento remoto, especialmente para a , e como as mudanças LULC podem modificar este fluxo na América do Sul

Applications of stable water and carbon isotopes in watershed research: Weathering, carbon cycling, and water balances

Research on rivers has traditionally involved concentration and flux measurements to better understand weathering, transport and cycling of materials from land to ocean. As a relatively new tool, stable isotope measurements complement this type of research by providing an extra label to characterize origin of the transportedmaterial, its transfer mechanisms, and natural versus anthropogenic influences. These new stable isotope techniques are scalable across a wide range of geographic and temporal scales. This review focuses on three aspects of hydrological and geochemical river research that are of prime importance to the policy issues of climate change and include utilization of stable water and carbon isotopes: (i) silicate and carbonate weathering in river basins, (ii) the riverine carbon and oxygen cycles, and (iii) water balances at the catchment scale. Most studies at watershed scales currently focus on water and carbon balances but future applications hold promise to integrate sediment fluxes and turnover, ground and surface water interactions, as well as the understanding of contaminant sources and their effects in river systems

Seasonal and Inter-annual Variation of Evapotranspiration in Amazonia Based on Precipitation, River Discharge and Gravity Anomaly Data

We analyzed seasonal and spatial variations of evapotranspiration (ET) for five Amazon sub-basins and their response to the 2015/16 El Nino episode using a recently developed water-budget approach. ET varied typically between similar to 7 and 10 cm/month with exception of the Xingu basin for which it varied between 10 and 15 cm/month. Outstanding features of ET seasonality are (i) generally weak seasonality, (ii) two ET peaks for the two very wet catchments Solimoes and Negro, with one occurring during the wet season and one during the drier season, and (iii) a steady increase of ET during the second half of the dry season for the three drier catchments (Madeira, Tapajos, Xingu). Peak ET occurs during the first half of the wet season consistent with leaf flush occurring before the onset of the wet season. With regards to inter-annual variation, we found firstly that for the Solimoes and Madeira catchments the period with large positive wet season anomalies (2012-2015) is associated with negative ET anomalies, and negative SIF (solar induced fluorescence) anomalies. Furthermore, we found negative ET of several cm/months and SIF (up to 50%) anomalies for most of the Amazon basin during the 2015/16 El Nino event suggesting down-regulation of productivity as a main factor of positive carbon flux anomalies during anomalously hot and dry conditions. These results are of interest in view of predicted warmer and more erratic future climate conditions.Peer reviewe

Potential hydrologic changes in the Amazon by the end of the 21st century and the groundwater buffer

This study contributes to the discussions on the future of the Amazon rainforest under a projected warmer-drier climate from the perspectives of land hydrology. Using IPCC HadGEM2-ES simulations of the present and future Amazon climate to drive a land hydrology model that accounts for groundwater constraint on land drainage, we assess potential hydrologic changes in soil water, evapotranspiration (ET), water table depth, and river discharge, assuming unchanged vegetation. We ask: how will ET regimes shift at the end of the 21st century, and will the groundwater help buffer the anticipated water stress in some places-times? We conducted four 10 yr model simulations, at the end of 20th and 21st century, with and without the groundwater. Our model results suggest that, first, over the western and central Amazon, ET will increase due to increased potential evapotranspiration (PET) with warmer temperatures, despite a decrease in soil water; that is, ET will remain PET or atmospheric demand-limited. Second, in the eastern Amazon dry season, ET will decrease in response to decreasing soil water, despite increasing PET demand; that is, ET in these regions-seasons will remain or become more soil water or supply-limited. Third, the area of water-limited regions will likely expand in the eastern Amazonia, with the dry season, as indicated by soil water store, even drier and longer. Fourth, river discharge will be significantly reduced over the entire Amazon but particularly so in the southeastern Amazon. By contrasting model results with and without the groundwater, we found that the slow soil drainage constrained by shallow groundwater can buffer soil water stress, particularly in southeastern Amazon dry season. Our model suggests that, if groundwater buffering effect is accounted for, the future Amazon water stress may be less than that projected by most climate modelsFunding comes from NSF (NSF-AGS-1045110), US EPA (EPA-STAR-RD834190) and the European Commission FP7 (EartH2Observe). Computation used the NCAR's Computational and Information Systems Laboratory, sponsored by NSF and other agenciesS

Evapotranspiration seasonality across the Amazon Basin

Evapotranspiration (ET) of Amazon forests is a main driver of regional climate patterns and an important indicator of ecosystem functioning. Despite its importance, the seasonal variability of ET over Amazon forests, and its relationship with environmental drivers, is still poorly understood. In this study, we carry out a water balance approach to analyse seasonal patterns in ET and their relationships with water and energy drivers over five sub-basins across the Amazon Basin. We used in situ measurements of river discharge, and remotely sensed estimates of terrestrial water storage, rainfall, and solar radiation. We show that the characteristics of ET seasonality in all sub-basins differ in timing and magnitude. The highest mean annual ET was found in the northern Rio Negro basin (similar to 1497 mm year(-1)) and the lowest values in the Solimoes River basin (similar to 986 mm year(-1)). For the first time in a basin-scale study, using observational data, we show that factors limiting ET vary across climatic gradients in the Amazon, confirming local-scale eddy covariance studies. Both annual mean and seasonality in ET are driven by a combination of energy and water availability, as neither rainfall nor radiation alone could explain patterns in ET. In southern basins, despite seasonal rainfall deficits, deep root water uptake allows increasing rates of ET during the dry season, when radiation is usually higher than in the wet season. We demonstrate contrasting ET seasonality with satellite greenness across Amazon forests, with strong asynchronous relationships in ever-wet watersheds, and positive correlations observed in seasonally dry watersheds. Finally, we compared our results with estimates obtained by two ET models, and we conclude that neither of the two tested models could provide a consistent representation of ET seasonal patterns across the Amazon.Peer reviewe

Characterizing rainfall-runoff signatures from microcatchments with contrasting land cover characteristics in southern Amazonia

On the basis of interactions between landscape characteristics and precipitation inputs, watersheds respond differently to different climatic inputs. The objective of this study was to quantitatively characterize controls on runoff generation from two first order micro-catchments in the Amazonia region. The study investigated the variation of hydrological signatures at micro-catchment scale and related these to landscape and land cover differences and weather descriptors that control the observed responses. One catchment is a pasture cleared of all natural vegetation in the early 1980s, and the second catchment is a primary tropical forest with minor signs of disturbance. Water levels and meteorological variables were continuously monitored during the study period (December 2012–May 2013). Water level measurements were converted to discharge, evapotranspiration was quantified using Penman–Monteith equation and catchment pedohydrological properties were also determined. During the study period, mean total rainfall was 1200 mm, and direct runoff ratios were 0.29 and 0.12 for the pasture and forest catchments, respectively. Baseflow index was relatively high in the forest catchment (0.76) compared with pasture catchment (0.63). Results from this study showed that the pasture catchment had a 35% higher mean stream flow. Analysis of selected individual rainstorm events also showed peak discharges, which were attained much faster in the pasture catchment compared with the forest catchment. At both sites, rainfall-runoff responses were highly dependent on surface and subsurface flow generation. Overland flow was observed in the pasture site during intense rainfall events. The pasture catchment exhibited higher event water contribution than the forest catchment. Findings from this research suggest that shallow lateral pathways play a significant role in controlling runoff generation processes in the forest catchment, whereas infiltration excess runoff generation processes dominate in the pasture catchment. The findings in this study suggest that the conversion of forest to pasture may lead to important changes in runoff generation processes and water storage in these head water catchments

Enhanced chemical weathering of rocks during the last glacial maximum: a sink for atmospheric CO2?

It has been proposed that increased rates of chemical weathering and the related drawdown of atmospheric CO2 on the continents may have at least partly contributed to the low CO2 concentrations during the last glacial maximum LGM.. Variations in continental erosion could thus be one of the driving forces for the glacialrinterglacial climate cycles during Quaternary times. To test such an hypothesis, a global carbon erosion model has been applied to a LGM scenario in order to determine the amount of CO2 consumed by chemical rock weathering during that time. In this model, both the part of atmospheric CO2 coming from silicate weathering and the part coming from carbonate weathering are distinguished. The climatic conditions during LGM were reconstructed on the basis of the output files from a computer simulation with a general circulation model. Only the predicted changes in precipitation and temperature have been used, whereas the changes in continental runoff were determined with an empirical method. It is found that during the LGM, the overall atmospheric CO2 consumption may have been greater than today by about 20%., mainly because of greater carbonate outcrop area related to the lower sea level on the shelves. This does not, however, affect the atmospheric CO2 consumption by silicate weathering, which alone has the potential to alter atmospheric CO2 on the long-term. Silicate weathering and the concomitant atmospheric CO2 consumption decreased together with a global decrease of continental runoff compared to present-day both by about 10%.. Nevertheless, some uncertainty remains because the individual lithologies of the continental shelves as well as their behavior with respect to chemical weathering are probably not well enough known. The values we present refer to the ice-free continental area only, but we tested also whether chemical weathering under the huge ice sheets could have been important for the global budget. Although glacial runoff was considerably increased during LGM, weathering under the ice sheets seems to be of minor importance

Predicting Water Cycle Characteristics from Percolation Theory and Observational Data.

The fate of water and water-soluble toxic wastes in the subsurface is of high importance for many scientific and practical applications. Although solute transport is proportional to water flow rates, theoretical and experimental studies show that heavy-tailed (power-law) solute transport distribution can cause chemical transport retardation, prolonging clean-up time-scales greatly. However, no consensus exists as to the physical basis of such transport laws. In percolation theory, the scaling behavior of such transport rarely relates to specific medium characteristics, but strongly to the dimensionality of the connectivity of the flow paths (for example, two- or three-dimensional, as in fractured-porous media or heterogeneous sediments), as well as to the saturation characteristics (i.e., wetting, drying, and entrapped air). In accordance with the proposed relevance of percolation models of solute transport to environmental clean-up, these predictions also prove relevant to transport-limited chemical weathering and soil formation, where the heavy-tailed distributions slow chemical weathering over time. The predictions of percolation theory have been tested in laboratory and field experiments on reactive solute transport, chemical weathering, and soil formation and found accurate. Recently, this theoretical framework has also been applied to the water partitioning at the Earth's surface between evapotranspiration, ET, and run-off, Q, known as the water balance. A well-known phenomenological model by Budyko addressed the relationship between the ratio of the actual evapotranspiration (ET) and precipitation, ET/P, versus the aridity index, ET0/P, with P being the precipitation and ET0 being the potential evapotranspiration. Existing work was able to predict the global fractions of P represented by Q and ET through an optimization of plant productivity, in which downward water fluxes affect soil depth, and upward fluxes plant growth. In the present work, based likewise on the concepts of percolation theory, we extend Budyko's model, and address the partitioning of run-off Q into its surface and subsurface components, as well as the contribution of interception to ET. Using various published data sources on the magnitudes of interception and information regarding the partitioning of Q, we address the variability in ET resulting from these processes. The global success of this prediction demonstrated here provides additional support for the universal applicability of percolation theory for solute transport as well as guidance in predicting the component of subsurface run-off, important for predicting natural flow rates through contaminated aquifers