Western Bahia, a region in Northeast Brazil highly dependent on water availability due to the large concentration of irrigated lands, has been suffering from decreasing precipitation in the last ...decades. The mean climate of Northeast Brazil and its variability have been linked to teleconnections with the Atlantic and Pacific sea surface temperatures, including the Atlantic Multidecadal Oscillation (AMO). This study investigates how large‐scale (AMO) and mesoscale (orographic) interactions are related to precipitation patterns in the region. Time series of precipitation and naturalized flows (corresponding to the flow that would occur in the river section if there were no anthropogenic actions) for the period 1981–2020 are analysed, indicating an average reduction of 11.5% in rainfall since the 1980s. The analyses show a significant (α = 0.05) rainfall reduction in the wettest months, the December and January bimester (DJ). A strong and significant negative correlation between AMO and precipitation (R = −0.62, α = 0.01) indicates a decrease in DJ rainfall when the AMO index is positive. The five rainiest and five driest DJ bimesters between 1981 and 2020 were selected to investigate the changes in atmospheric circulation. In the rainiest years, large‐scale mechanisms interact with the local topography to promote a major convection centre over Western Bahia. The driest years show suppressed convection over the topographic gradient and a change in the convection centre several hundred kilometres to the west. The continued increase in temperatures in the North Atlantic may perpetuate the decline in Western Bahia precipitation, putting pressure on water availability and impacting economic activities in the region.
How teleconnections associated to North Atlantic temperatures interact with the local topography and influence the precipitation regime in Wester Bahia, an important food production region in Northeast Brazil, affecting people's lives in the region.
The South American Monsoon System, which transports moisture from Amazonia to Central-West Brazil, is an important moisture source for the summer rainy season in this region. While local ...evapotranspiration also contributes to the atmospheric moisture supply, the balance between local and remote sources during the onset of the rainy season remains uncertain. Our research aimed to quantify the role of local evapotranspiration in initiating the rainy season in Central-West Brazil. By utilizing data from various sources, such as remote sensing (MODIS), modern reanalysis (ECMWF’s ERA5), and composite products of rainfall (CHIRPS), and analyzing them in a comparative way, we conclusively found that local evapotranspiration is the only relevant source of moisture to the atmosphere during the dry-to-wet season transition, preceding the establishment of the monsoon system.
Field observations and numerical studies revealed that large scale deforestation in Amazonia could alter the regional climate significantly, projecting a warmer and somewhat drier post‐deforestation ...climate. In this study we employed the CPTEC‐INPE AGCM to assess the effects of Amazonian deforestation on the regional climate, using simulated land cover maps from a business‐as‐usual scenario of future deforestation in which the rainforest was gradually replaced by degraded pasture or by soybean cropland. The results for eastern Amazonia, where changes in land cover are expected to be larger, show increase in near‐surface air temperature, and decrease in evapotranspiration and precipitation, which occurs mainly during the dry season. The relationship between precipitation and deforestation shows an accelerating decrease of rainfall for increasing deforestation for both classes of land use conversions. Continued expansion of cropland in Amazonia is possible and may have important consequences for the sustainability of the region's remaining natural vegetation.
Climate change and its effects on the hydrologic regime of the Amazon basin can impact biogeochemical processes, transportation, flood vulnerability, fisheries and hydropower generation. We examined ...projections of climate change on discharge and inundation extent in the Amazon basin using the regional hydrological model MGB-IPH with 1-dimensional river hydraulic and water storage simulation in floodplains. Future projections (2070–2099) were obtained from five GCMs from IPCC’s Fifth Assessment Report CMIP5. Climate projections have uncertainty and results from different climate models did not agree in total Amazon flooded area or discharge anomalies along the main stem river. Overall, model runs agree better with wetter (drier) conditions over western (eastern) Amazon. Results indicate that increased mean and maximum river discharge for large rivers draining the Andes in the northwest contributes to increased mean and maximum discharge and inundation extent over Peruvian floodplains and Solimões River (annual mean-max: +9 % - +18.3 %) in western Amazonia. Decreased river discharges (mostly dry season) are projected for eastern basins, and decreased inundation extent at low water (annual min) in the central (−15.9 %) and lower Amazon (−4.4 %).
Abstract
The physical hydroclimate system of the Amazon functions on several spatial and temporal scales. Large-scale processes control the main seasonal patterns of atmospheric circulation and ...rainfall. Seasonal variability in solar forcing, associated with the low rainforest albedo, provides energy for continental heating, convection, and the onset of the South American monsoon. Mesoscale processes cause localized circulations such as river breeze and deforestation breeze. We assessed the impact of different deforestation scenarios for the mid-century last decade rainy season. Here we describe a yet unreported synoptic-scale circulation that delays the rainy season onset in southern Amazonia. This model-predicted circulation is driven by extensive (ca. 40%) deforestation patterns and may last as long as two months. This persistent anomalous circulation may result in a rainy season onset delay of 30–40 d compared to the historical period. Like other synoptic-scale phenomena, differences in surface heating drive this circulation. Given the unabated deforestation trends, the consequences for local ecosystems, agriculture, and power generation of delayed rainy season onset associated with this circulation may be difficult to revert.
Considerable uncertainty surrounds the fate of Amazon rainforests in response to
climate change.
Here, carbon (C) flux predictions of five terrestrial biosphere models (Community
Land Model version ...3.5 (CLM3.5), Ecosystem Demography model version 2.1 (ED2),
Integrated BIosphere Simulator version 2.6.4 (IBIS), Joint UK Land Environment
Simulator version 2.1 (JULES) and Simple Biosphere model version 3 (SiB3)) and a
hydrodynamic terrestrial ecosystem model (the
Soil–Plant–Atmosphere (SPA) model) were evaluated against
measurements from two large-scale Amazon drought experiments.
Model predictions agreed with the observed C fluxes in the control plots of both
experiments, but poorly replicated the responses to the drought treatments. Most
notably, with the exception of ED2, the models predicted negligible reductions
in aboveground biomass in response to the drought treatments, which was in
contrast to an observed c. 20% reduction at both sites.
For ED2, the timing of the decline in aboveground biomass was accurate, but the
magnitude was too high for one site and too low for the other.
Three key findings indicate critical areas for future research and model
development. First, the models predicted declines in autotrophic respiration
under prolonged drought in contrast to measured increases at one of the sites.
Secondly, models lacking a phenological response to drought introduced bias in
the sensitivity of canopy productivity and respiration to drought. Thirdly, the
phenomenological water-stress functions used by the terrestrial biosphere models
to represent the effects of soil moisture on stomatal conductance yielded
unrealistic diurnal and seasonal responses to drought.
As the largest river basin on Earth, the Amazon is of major importance to the world's climate and water resources. Over the past decades, advances in satellite‐based remote sensing (RS) have brought ...our understanding of its terrestrial water cycle and the associated hydrological processes to a new era. Here, we review major studies and the various techniques using satellite RS in the Amazon. We show how RS played a major role in supporting new research and key findings regarding the Amazon water cycle, and how the region became a laboratory for groundbreaking investigations of new satellite retrievals and analyses. At the basin‐scale, the understanding of several hydrological processes was only possible with the advent of RS observations, such as the characterization of "rainfall hotspots" in the Andes‐Amazon transition, evapotranspiration rates, and variations of surface waters and groundwater storage. These results strongly contribute to the recent advances of hydrological models and to our new understanding of the Amazon water budget and aquatic environments. In the context of upcoming hydrology‐oriented satellite missions, which will offer the opportunity for new synergies and new observations with finer space‐time resolution, this review aims to guide future research agenda toward integrated monitoring and understanding of the Amazon water from space. Integrated multidisciplinary studies, fostered by international collaborations, set up future directions to tackle the great challenges the Amazon is currently facing, from climate change to increased anthropogenic pressure.
Plain Language Summary
The Amazon basin is the largest river basin in the world, characterized by complex hydrological processes that connect high rates of precipitation, extensive floodplains, dense tropical forests, complex topography, and large variations in freshwater storage and discharge. It plays a key role in the water, energy, and carbon cycles and interacts with the global climate system. Earth observations have played a major role in supporting research in Amazon hydrology, and the characterization of several hydrological processes was only possible with the help of remote sensing data. The basin is now facing great risk under current climate change and increased anthropogenic pressure and the resulting environmental alterations require a better understanding of the overall basin's water cycle across scales. We review the strengths and limitations of observations from satellites in the context of the current and upcoming hydrology‐oriented satellite missions, and we make recommendations for improving satellite observations of the Amazon basin water cycle, along with an interdisciplinary and stepwise approach to guide research for the next decades.
Key Points
Integrated view of scientific advances in Amazon hydrology with remote sensing
Expected progress to understand the water cycle, aquatic ecosystems, and environmental changes with upcoming hydrology‐oriented missions
Need to translate advanced knowledge from remote sensing to support water management and environmental governance
Western Bahia, Brazil, is a classic example of a region where intense irrigation growth has led to acute water stress situations in a few small basins. The water stress problem has the potential to ...grow regionally. However, there are currently no systematic field measurements of water withdrawn from rivers or groundwater to supply irrigation systems. In this work, we merge remote sensing and river gauge data to assess both the amount of water used for irrigation in Western Bahia and also its consequences for regional water stress, identifying water conflict situations and assessing water security. Remote sensing products used include time series of the normalized difference vegetation index, evapotranspiration, and rainfall. Field data include time series of river discharge and calibration data for crop status and actual evapotranspiration. From calibrated remote sensing products, three-day water balances were calculated for each center pivot using computations of irrigation depth and water uptake for irrigation, both individually at the center-pivot scale and integrated regionally. From these regional integrations, a simple water-use diagnostic indicated that three sub-basins presented the most critical conditions for water conflicts. An in-depth analysis of these sub-basins shows that, despite the high water stress, water use for irrigation has been steadily increasing, pushing the water use to its limits. This work demonstrates that the use of remote sensing products together with field data is a powerful tool for diagnosing water conflict situations. The limitations of this work relate to the absence of field data to validate the water uptake estimated and to the lack of additional long-term and high-quality river flow stations to provide diagnostics for all small basins in the region.
Western Bahia is a critical region in Brazil’s recent expansion of agricultural output. Its outstanding increase in production is associated with strong growth in cropland area and irrigation. Here ...we present analyses of Western Bahian historical changes in land use, including irrigated area, and suitability for future agricultural expansion that respects permanent protection areas and the limits established by the Brazilian Forest Code in the Cerrado biome. For this purpose, we developed a land use and land cover classification database using a random forest classifier and Landsat images. A spatial multicriteria decision analysis to evaluate land suitability was performed by combining this database with precipitation and slope data. We demonstrate that between 1990 and 2020, the region’s total agricultural area increased by 3.17 Mha and the irrigated area increased by 193,480 ha. Throughout the region, the transition between the different classes of land use and land cover followed different pathways and was strongly influenced by land suitability and also appears to be influenced by Brazil’s new Forest Code of 2012. We conclude that even if conservation restrictions are considered, agricultural area could nearly double in the region, with expansion possible mostly in areas we classify as moderately suitable for agriculture, which are subject to climate hazards when used for rainfed crops but are otherwise fine for pastures and irrigated croplands.
In Western Bahia, one of the most active agricultural frontiers of the world, cropland area and irrigated area are increasing at fast rates, and water conflicts have been happening at least since ...2010. This study makes a hydroclimatic analysis of the water resources in Western Bahia, from both supply and demand viewpoints. Time series of precipitation for the period 1980–2015 and river discharge for the period 1978–2015 are analyzed, indicating a significant reduction of up to 12% in rainfall since the 1980s, and a reduction in river discharge in all stations studied, in both the rainy season and the dry season. Combined with that, irrigated area has increased over 150-fold in 30 years, and in the most irrigated regions, has increased by 90% in the last eight years only. Seven regions in Western Bahia have been identified where the potential for water use conflicts is critical. Moreover, the combination of reduced availability and increased demand of water resources indicates that, if current trends are maintained, conflicts over water may become more frequent in the next years or decades. A short-term alternative to avoid such conflicts is to largely avoid irrigation during the months with low discharge. However, a monitoring system in which the availability and demand of water resources for irrigation are actually measured and monitored, is the safest path to provide water security to this region.
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