In 2005, large sections of southwestern Amazonia experienced one of the most intense droughts of the last hundred years. The drought severely affected human population along the main channel of the Amazon River and its western and southwestern tributaries, the Solimões (also known as the Amazon River in the other Amazon countries) and the Madeira Rivers, respectively. The river levels fell to historic low levels and navigation along these rivers had to be suspended. The drought did not affect central or eastern Amazonia, a pattern different from the El Niño-related droughts in 1926, 1983, and 1998. The choice of rainfall data used influenced the detection of the drought. While most datasets (station or gridded data) showed negative departures from mean rainfall, one dataset exhibited above-normal rainfall in western Amazonia.The causes of the drought were not related to El Niño but to (i) the anomalously warm tropical North Atlantic, (ii) the reduced intensity in northeast trade wind moisture transport into southern Amazonia during the peak summertime season, and (iii) the weakened upward motion over this section of Amazonia, resulting in reduced convective development and rainfall. The drought conditions were intensified during the dry season into September 2005 when humidity was lower than normal and air temperatures were 3°-5°C warmer than normal. Because of the extended dry season in the region, forest fires affected part of southwestern Amazonia.

[1] The year 2010 featured a widespread drought in the Amazon rain forest, which was more severe than the "once-in-a-century" drought of 2005. Water levels of major Amazon tributaries fell drastically to unprecedented low values, and isolated the floodplain population whose transportation depends upon on local streams which completely dried up. The drought of 2010 in Amazonia started in early austral summer during El Niño and then was intensified as a consequence of the warming of the tropical North Atlantic. An observed tendency for an increase in dry and very dry events, particularly in southern Amazonia during the dry season, is concomitant with an increase in the length of the dry season. Our results suggest that it is by means of a longer dry season that warming in the tropical North Atlantic affects the hydrology of the Amazon Rivers at the end of the recession period (austral spring). This process is, sometimes, further aggravated by deficient rainfall in the previous wet season.

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Severe drought in moist tropical forests provokes large carbon emissions by increasing forest flammability and tree mortality, and by suppressing tree growth. The frequency and severity of drought in the tropics may increase through stronger El Niñ o Southern Oscillation (ENSO) episodes, global warming, and rainfall inhibition by land use change. However, little is known about the spatial and temporal patterns of drought in moist tropical forests, and the complex relationships between patterns of drought and forest fire regimes, tree mortality, and productivity. We present a simple geographic information system soil water balance model, called RisQue (Risco de Queimada -Fire Risk) for the Amazon basin that we use to conduct an analysis of these patterns for 1996-2001. RisQue features a map of maximum plant-available soil water (PAW max ) developed using 1565 soil texture profiles and empirical relationships between soil texture and critical soil water parameters. PAW is depleted by monthly evapotranspiration (ET) fields estimated using the Penman-Monteith equation and satellite-derived radiation inputs and recharged by monthly rain fields estimated from 266 meteorological stations. Modeled PAW to 10 m depth (PAW 10 m ) was similar to field measurements made in two Amazon forests. During the severe drought of 2001, PAW 10 m fell to below 25% of PAW max in 31% of the region's forests and fell below 50% PAW max in half of the forests. Field measurements and experimental forest fires indicate that soil moisture depletion below 25% PAW max corresponds to a reduction in leaf area index of approximately 25%, increasing forest flammability. Hence, approximately one-third of Amazon forests became susceptible to fire during the 2001 ENSO period. Field measurements also suggest that the ENSO drought of 2001 reduced carbon storage by approximately 0.2 Pg relative to years without severe soil moisture deficits. RisQue is sensitive to spin-up time, rooting depth, and errors in ET estimates. Improvements in our ability to accurately model soil moisture content of Amazon forests will depend upon better understanding of forest rooting depths, which can extend to beyond 15 m. RisQue provides a tool for early detection of forest fire risk.

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Various studies have shown that micro‐aggregated, strongly weathered tropical soils have different water retention properties than temperate‐region soils because of differences in mineralogy and weathering history. Hence, pedotransfer functions (PTFs), derived from data from temperate‐region soils (temperate PTFs) have limitations when applied to tropical soils. In the absence of PTFs specifically for tropical soils, temperate PTFs are being applied worldwide in global climate modeling exercises, regardless of their textural validity. We derived a PTF to predict the water retention parameters of the van Genuchten (1980) equation using data from more than 500 Brazilian soil horizons. A modified approach was adopted: Multiple regression was used to derive coefficients that relate the van Genuchten parameters to basic soil data, followed by re‐optimization of those coefficients by fitting the individual water content estimations to the measured data simultaneously. Pedotransfer functions were developed for four levels of availability of basic soils data and validated using independent data from 113 Brazilian soil horizons. Water retention curves were better predicted by the new PTFs than by two temperate PTFs tested: The root mean square deviation (RMSD) ranged from 3.78 to 5.84, compared with 9.08 and 10.44, respectively. The new PTFs performed better even when the comparison was restricted to the range of textural validity of the temperate PTFs. For the proposed PTF, RMSDs increased with increasing silt content, but decreased for the temperate PTFs. This reflects the differences in silt content between temperate and strongly weathered tropical soils.

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In 2005, southwestern Amazonia experienced the effects of an intense drought that affected life and biodiversity. Several major tributaries as well as parts of the main river itself contained only a fraction of their normal volumes of water, and lakes were drying up. The consequences for local people, animals and the forest itself are impossible to estimate now, but they are likely to be serious. The analyses indicate that the drought was manifested as weak peak river season during autumn to winter as a consequence of a weak summertime season in southwestern Amazonia; the winter season was also accompanied by rainfall that sometimes reached 25% of the climatic value, being anomalously warm and dry and helping in the propagation of fires. Analyses of climatic and hydrological records in Amazonia suggest a broad consensus that the 2005 drought was linked not to El Niñ o as with most previous droughts in the Amazon, but to warming sea surface temperatures in the tropical North Atlantic Ocean.