This study discusses the climatological aspects of the most severe drought ever recorded in the semiarid region Northeast Brazil. Droughts are recurrent in the region and while El Nino has driven some of these events others are more dependent on the tropical North Atlantic sea surface temperature fields. The drought affecting this region during the last 5 years shows an intensity and impact not seen in several decades in the regional economy and society. The analysis of this event using drought indicators as well as meteorological fields shows that since the middle 1990s to 2016, 16 out of 25 years experienced rainfall below normal. This suggests that the recent drought may have in fact started in the middle-late 1990s, with the intense droughts of 1993 and 1998, and then the sequence of dry years (interrupted by relatively wet years in 2007, 2008, 2009 and 2011) after that may have affected the levels of reservoirs in the region, leading to a real water crisis that was magnified by the negative rainfall anomalies since 2010.

Drought is a natural and recurrent phenomenon. It is considered 'a natural disaster' whenever it occurs in an intensive manner in highly populated regions, resulting in significant damage (material and human) and loss (socioeconomic). This paper presents the efforts developed to monitor the impact of drought in the semiarid region of Northeast Brazil. In this scope, information from different sources is compiled to support the evaluation and identification of impacted municipalities, with the main objective of supporting emergency actions to mitigate their impact. In the semiarid region of Brazil there are frequent occurrences of dry periods during the rainy season, which, depending on the intensity and duration, can cause significant damage to family-farmed crops, with a farming system characterized by low productivity indices. However, rain-fed agriculture has great economic expression and high social importance due to the region is densely occupied, and contributes to the establishment of communities in the countryside. Specifically, in the present study, the methodology adopted to monitor the impact of agricultural droughts, including an analysis of the hydrological year 2015-2016, is presented, considering different water stress indicators for the identification of the affected municipalities and assessment of the methods and tools developed.

Canopy and aerodynamic conductances ( g _C and g _A ) are two of the key land surface biophysical variables that control the land surface response of land surface schemes in climate models. Their representation is crucial for predicting transpiration ( λ E _T ) and evaporation ( λ E _E ) flux components of the terrestrial latent heat flux ( λ E ), which has important implications for global climate change and water resource management. By physical integration of radiometric surface temperature ( T _R ) into an integrated framework of the Penman–Monteith and Shuttleworth–Wallace models, we present a novel approach to directly quantify the canopy-scale biophysical controls on λ E _T and λ E _E over multiple plant functional types (PFTs) in the Amazon Basin. Combining data from six LBA (Large-scale Biosphere-Atmosphere Experiment in Amazonia) eddy covariance tower sites and a T _R -driven physically based modeling approach, we identified the canopy-scale feedback-response mechanism between g _C , λ E _T , and atmospheric vapor pressure deficit ( D _A ), without using any leaf-scale empirical parameterizations for the modeling. The T _R -based model shows minor biophysical control on λ E _T during the wet (rainy) seasons where λ E _T becomes predominantly radiation driven and net radiation ( R _N ) determines 75 to 80 % of the variances of λ E _T . However, biophysical control on λ E _T is dramatically increased during the dry seasons, and particularly the 2005 drought year, explaining 50 to 65 % of the variances of λ E _T , and indicates λ E _T to be substantially soil moisture driven during the rainfall deficit phase. Despite substantial differences in g _A between forests and pastures, very similar canopy–atmosphere "coupling" was found in these two biomes due to soil moisture-induced decrease in g _C in the pasture. This revealed the pragmatic aspect of the T _R -driven model behavior that exhibits a high sensitivity of g _C to per unit change in wetness as opposed to g _A that is marginally sensitive to surface wetness variability. Our results reveal the occurrence of a significant hysteresis between λ E _T and g _C during the dry season for the pasture sites, which is attributed to relatively low soil water availability as compared to the rainforests, likely due to differences in rooting depth between the two systems. Evaporation was significantly influenced by g _A for all the PFTs and across all wetness conditions. Our analytical framework logically captures the responses of g _C and g _A to changes in atmospheric radiation, D _A , and surface radiometric temperature, and thus appears to be promising for the improvement of existing land–surface–atmosphere exchange parameterizations across a range of spatial scales.