The impact of increases in drought frequency on the Amazon forest's composition, structure and functioning remain uncertain. We used a process- and individual-based ecosystem model (ED2) to quantify the forest's vulnerability to increased drought recurrence. We generated meteorologically realistic, drier-than-observed rainfall scenarios for two Amazon forest sites, Paracou (wetter) and Tapajós (drier), to evaluate the impacts of more frequent droughts on forest biomass, structure and composition. The wet site was insensitive to the tested scenarios, whereas at the dry site biomass declined when average rainfall reduction exceeded 15%, due to high mortality of large-sized evergreen trees. Biomass losses persisted when year-long drought recurrence was shorter than 2-7 yr, depending upon soil texture and leaf phenology. From the site-level scenario results, we developed regionally applicable metrics to quantify the Amazon forest's climatological proximity to rainfall regimes likely to cause biomass loss > 20% in 50 yr according to ED2 predictions. Nearly 25% (1.8 million km ) of the Amazon forests could experience frequent droughts and biomass loss if mean annual rainfall or interannual variability changed by 2σ. At least 10% of the high-emission climate projections (CMIP5/RCP8.5 models) predict critically dry regimes over 25% of the Amazon forest area by 2100.

Abstract. In Amazon forests, the relative contributions of climate, phenology, and disturbance to net ecosystem exchange of carbon (NEE) are not well understood. To partition influences across various timescales, we use a statistical model to represent eddy-covariance-derived NEE in an evergreen eastern Amazon forest as a constant response to changing meteorology and phenology throughout a decade. Our best fit model represented hourly NEE variations as changes due to sunlight, while seasonal variations arose from phenology influencing photosynthesis and from rainfall influencing ecosystem respiration, where phenology was asynchronous with dry-season onset. We compared annual model residuals with biometric forest surveys to estimate impacts of drought disturbance. We found that our simple model represented hourly and monthly variations in NEE well (R 2 = 0.81 and 0.59, respectively). Modeled phenology explained 1 % of hourly and 26 % of monthly variations in observed NEE, whereas the remaining modeled variability was due to changes in meteorology. We did not find evidence to support the common assumption that the forest phenology was seasonally light-or water-triggered. Our model simulated annual NEE well, with the exception of 2002, the first year of our data record, which contained 1.2 MgC ha −1 of residual net emissions, because photosynthesis was anomalously low. Because a severe drought occurred in 1998, we hypothesized that this drought caused a persistent, multi-year depression of photosynthesis. Our results suggest drought can have lasting impacts on photosynthesis, possibly via partial damage to still-living trees.

<p><strong>Abstract.</strong> In Amazon forests, the relative contributions of climate, phenology, and disturbance to net ecosystem exchange of carbon (NEE) are not well understood. To partition influences across various timescales, we use a statistical model to represent eddy covariance-derived NEE in an evergreen Eastern Amazon forest as a constant response to changing meteorology and phenology throughout a decade. Our best fit model represented hourly NEE variations as changes due to sunlight, while seasonal variations arose from phenology influencing photosynthesis and from rainfall influencing ecosystem respiration, where phenology was asynchronous with dry season onset. We compared annual model residuals with biometric forest surveys to estimate impacts of drought-disturbance. We found that our simple model represented hourly and monthly variations in NEE well (R<sup>2</sup> = 0.81, 0.59 respectively). Our model also simulated annual NEE well, with exception to 2002, the first year of our data record, which contained 1.2 MgC ha<sup>−1</sup> of residual net emissions, because photosynthesis was anomalously low. Because a severe drought occurred in 1998, we hypothesized that this drought caused a persistent, multi-year depression of photosynthesis. We did not find evidence to support the common assumption that droughts or disturbances affected this region during 2005 or 2010, nor that the forest phenology was seasonally light- or water-triggered. Our results suggest drought can have lasting impacts on photosynthesis, possibly via partial damage to still-living trees.</p>

This work compares methods of climate measurements, such as those used to measure evapotranspiration, precipitation, net radiation, and temperature. The satellite products used were compared and evaluated against flux tower data. Evapotranspiration was validated against the SSEBop monthly and GLEAM daily and monthly products, respectively, and the results were RMSE = 24.144 mm/month, NRMSE = 0.223, r2 = 0.163, slope = 0.411; RMSE = 1.781 mm/day, NRMSE = 0.599, r2 = 0.000, slope = 0.006; RMSE = 36.17 mm/month, NRMSE = 0.401, r2 = 0.002, and slope = 0.026. Precipitation was compared with the CHIRPS data, K67 was not part of the CHIRPS station correction. The results for both the daily and monthly comparisons were RMSE = 18.777 mm/day, NRMSE = 1.027, r2 = 0.086, slope = 0.238 and RMSE = 130.713 mm/month, NRMSE = 0.706, r2 = 0.402, and slope = 0.818. The net radiation validated monthly with CERES was RMSE = 75.357 W/m2, NRMSE = 0.383, r2 = 0.422, and slope = 0.867. The temperature results, as compared to MOD11C3, were RMSE = 2.829 °C, NRMSE = 0.116, r2 = 0.153, and slope = 0.580. Comparisons between the remote sensing products and validation against the ground data were performed on a monthly basis. GLEAM and CHIRPS daily were the data sets with considerable discrepancy.

a rainfall-type model validated in several basins in South America, including in rivers of the Amazon Basin. The inputs of the model are climatological, rainfall, relief, and soil cover data. The objective of this study is to test the sensitivity of the model in extreme scenarios of soil use and occupation, changes in precipitation and mean air temperature. The case study was carried out in the Curuá-Una river basin, located southeast of Santarém-Pará. The MapWindow-GIS software and the IPH-Hydro Tools plug-in were used in the preprocessing, and the MGB-IPH plugin was processed. The results showed that the MGB-IPH has sensitivity to changes in soil use, precipitation, and mean air temperature. In the sensitivity test of soil use and occupation the results showed that low vegetation and anthropization increase the maximum peaks of flow; In periods of Amazon flood with low rainfall occurrence, the low vegetation scenario has a higher flow rate; And the forest scenery prevents intense floods. In the tests of changes in the precipitation regime, a 50% decrease in rainfall reduced the flow by 32% and a 50% increase in rainfall increased the flow by 218.6%. However, in the tests of increase of the mean air temperature the results did not show significant responses in the flow regime, however, this scenario, added with the increase and / or decrease of the precipitation regime, presented as attenuator for both floods and droughts. Teste de sensibilidade do Modelo Hidrológico de Grandes Bacias (MGB-IPH)em cenários de mudanças extremas no uso e ocupação do solo e, regime de precipitação e temperatura média do ar R E S U M O O Modelo Hidrológico Conceitual Distribuído de Grandes Bacias do Instituto de Pesquisas Hidráulicas (MGB-IPH) é um modelo do tipo chuva-vazão validado em diversas bacias da América do Sul, inclusive, em rios da bacia amazônica. As entradas do modelo são dados climatológicos, relevo e cobertura do solo. Pretende-se neste estudo testar a sensibilidade do modelo em cenários extremos de uso e ocupação do solo e, alterações na precipitação e temperatura média do ar. O estudo de caso foi realizado na bacia do rio Curuá-Una localizado a sudeste de Santarém-Pará. No pré-processamento utilizou-se o software MapWindow-GIS e o plugin IPH-Hydro Tools e no processamento utilizou-se o plugin MGB-IPH. Os resultados mostraram que o MGB-IPH possui sensibilidade às mudanças no uso do solo, precipitação e temperatura média do ar. No teste de sensibilidade de uso e ocupação do solo os resultados mostraram que vegetação baixa e antropização aumentam os picos máximos de vazão; em períodos de cheia amazônica com ocorrência de baixa precipitação o cenário vegetação baixa possui maior vazão; e o cenário floresta evita cheias intensas. Nos testes de mudanças no regime de precipitação a diminuição de 50% de chuvas reduziu a vazão em 32% e, o aumento de 50% de chuvas aumentou a vazão em 218,6%. Já nos testes de aumento da temperatura média do ar os resultados não mostraram respostas significativas no regime de vazão, porém este c...

Flux measurements of latent heat, sensible heat, momentum, and CO2 were performed from 15 to 26 June 2015 on the reservoir of the hydroelectric plant Curuá-Una (PA). The flux system is located upstream of the main channel of the reservoir and installed at 3 m above the water surface on a floating structure. The hydroelectric plant Curuá-Una was the first plant built in the Amazonia and it is in operation for almost 40 years. During installation, the vegetation around the river channel was not removed, which led to large emissions of greenhouse gases into the atmosphere. The wind speed was important to maintain turbulent mixing mechanically. Latent heat flux showed significant correlation with the wind velocity (r = 82%). As a result of the combined effect of turbulent mixing generated thermally and mechanically, the latent and sensible heat fluxes were positive throughout the investigation period and the atmospheric surface layer remained unstable. The CO2 flow was predominantly negative (84%), characterizing the reservoir as a CO2 sink.

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RESUMOO contraste térmico entre o rio Tapajós e FLONA Tapajós aliado às ocorrências de ventos alísios fracos faz com que ocorra a brisa no sentido rio-floresta, alterando significativamente as características termodinâmicas da CLA local. Os resultados mostram que existe um horário padrão de término da brisa de rio com mais de duas horas, de duração até às 18 horas, com início variável. As sondagens com balão cativo não apresentam padrões característicos, ocorrendo ou não brisa do rio e, em outras, duas camadas de ar acopladas com características bem definidas com informações do rio e floresta. INTRODUÇÃOA estrutura termodinâmica da Camada Limite Atmosférica (CLA) é definida por características da superfície. Na Amazônia, com a predominância de rios e lagos, deve-se levar em consideração as diferenças de capacidade térmica apresentada entre corpos d'água e a superfície que o rodeia. Assim, forçantes térmicos, devido à presença de rios, induzem o comportamento climático e influenciam nas condições da atmosfera local (SILVA DIAS et al., 2004;LU et al., 2005).O presente trabalho avaliou os padrões de ocorrência da brisa do rio Tapajós (ventos de oeste) e sua influência nas características da CLA local por meio de sondagens.