# Drought Propagation in Brazilian Biomes Revealed by Remote Sensing
## 1 Introduction  
The Earth's surface has warmed over the past decades [1; 2], and the last few years have been described as the warmest on record [3]. A changing climate can lead to unprecedented climate extremes due to changes in frequency, intensity, spatial extent, duration, and timing of droughts and floods [4; 5]. Drought can lead to major impacts on agriculture, hydropower production, and water supplies [6; 7; 8]. In 2020, Brazil experienced one of the worst water crises in almost a century [9], and in 2021-2022, it continued to experience drought events in its south and southeast regions [10], as shown by the Brazilian's Drought Monitoring System ([[https://monitordesecas.ana.gov.br/](https://monitordesecas.ana.gov.br/)]([https://monitordesecas.ana.gov.br/](https://monitordesecas.ana.gov.br/)) (accessed on 12 December 2022)). Furthermore, climate change is projected to lead to more intense and recurrent drought events in some regions of Brazil [11]. Brazil is particularly vulnerable to droughts events, especially agricultural and electrical production in eastern Brazil, since irrigated agriculture uses 32% of the country's water, and hydropower provides more than 70% of Brazil's energy generation [6]. Considering the importance of agriculture (irrigated and rainfed) to the Brazilian economy (e.g., soybean crops), drought events cause an increase in water use for irrigation to avoid crop failure due to climatic conditions. [PERSON] et al. [7] identified agricultural regions within Brazil, primarily in the southern and northeastern states, where crop yields have been climaticallysensitive to moisture deficits during the past decade. In southern Brazil, one of the worst droughts ever was registered in the austral summer of 2020, which affected several sectors, such as agriculture, hydropower generation, and water supply to the population [12]. In the Amazon, reduced water vapor in the atmosphere, evapotranspiration (_ET_), and precipitation (_P_) [13] can impact agriculture, river transport, and hydroelectric generation, triggering forest fires, tree mortality, and changes in carbon emissions [14]. Droughts have been grouped into four types [15; 16; 17]: (i) meteorological drought, related to anomalous \(P\) deficiency; (ii) agricultural drought, associated with deficit in soil moisture; (iii) hydrological drought, related to negative anomalies in surface and subsurface water; and (iv) socioeconomic drought, when the phenomenon leads to socioeconomic impacts. However, these definitions can be updated as the knowledge of drought risk expands. In addition, these drought types are all progressive stages of the same drought propagating through the hydrological cycle [18]. These events can be assessed by changes in hydrological fluxes, such as \(P\), and land surface water fluxes, including _ET_ and runoff, and storage, including soil moisture and groundwater [19]. The depletion of soil moisture storage is related to complex hydrological processes connecting the atmosphere, surface, and soil or geological formations, including _ET_, infiltration and percolation to groundwater, and runoff to streams [20]. These variables can be used to assess different drought types; for instance, \(P\) for meteorological drought, soil moisture for agricultural drought, and streamflow, reservoir, or groundwater levels for hydrological drought [16; 21]. The expression \"drought propagation\" has been used to denote changes in the drought signal as it moves through the terrestrial part of the hydrological cycle (from anomalous meteorological conditions to hydrological drought) [20]. A variety of characteristics of drought propagation (e.g., the response time scale, lag time, and propagation rate) can be defined to depict the propagation process [22]. Additionally, determining the mechanisms of drought propagation is necessary in a multi-indicator approach [23].