# Variability of Water Use Efficiency Associated with Climate Change in the Extreme West of Bahia
## 1 Introduction  
Over the years, the behavior of the vegetation under climatic and anthropogenic variations has been widely discussed [1, 2, 3]. Water use efficiency (WUE) and the ratio ofcarbon (C) assimilation to water (H\({}_{2}\)O) losses reflect the interaction between the C and H\({}_{2}\)O cycles. WUE is an indicator to quantify the coupling between the water and carbon cycles [4; 5]. The physiological responses of vegetation to environmental changes in terms of WUE [6] are fundamental because changes in the carbon and water cycles have an impact on this variable [7].  
That is, understanding the role of vegetation type in WUE behavior is essential, since each type presents different characteristics when conditioned to drought-motivated stress [8]. In this respect, we refer to tropical forests (e.g., Amazonia) in order to understand the role of vegetation type on the WUE on a regional scale as it relates to the water cycle [9], in the same way that the savannahs are conditioned for agricultural production [10].  
The analysis of WUE provides valuable information for the assessment of climate change impacts, deficit irrigation, and the management of strategies to promote ecosystem productivity [11]. Water use efficiency is the ratio between gross primary productivity (GPP) and evapotranspiration (ET) in a relatively homogeneous way [12]. Gross primary productivity corresponds to the process called photosynthesis and constitutes one of the main means of controlling the atmosphere--biosphere exchange of carbon dioxide (CO\({}_{2}\)) [13; 14; 15]. ET is defined as the sum of water lost by transpiration from crops and evaporation from the soil [16].  
WUE is conditioned by changes in environmental and climatic factors (air temperature, precipitation, and elevation) [4; 17; 18; 19]. These changes, in turn, can be influenced by rapid population growth and the modification of native vegetation, as these factors interfere with the amount of CO\({}_{2}\) in the atmosphere and, consequently, with the performance of the vegetation and its carbon-water cycle [20]. Regarding the climatic aspects (precipitation and air temperature variability), the climate of Northeast Brazil (NEB) is modulated by the El Nino-Southern Oscillation (ENSO) [21; 22], and the Atlantic Interhemispheric Sea Surface Temperature Gradient (AISTG) stands out [23; 24]. These modes of variability modify the circulation pattern and consequently the intensity and frequency of meteorological systems on regional and global scales, as well as on seasonal and annual scales, as observed mainly in the precipitation regime [25; 26; 27; 28].  
Changes in the precipitation regime influence the development of vegetation due to the close relationship between water availability and productivity [29; 30]. Plant growth varies according to the amount of water available in the environment, and although plants absorb water over their whole body surface, most of the supply comes from the soil [31; 32]. According to [33; 34], water availability has a strong influence on agricultural productivity, especially in the westernmost region of Bahia, a region of high-tech scientific and/or modern agriculture. This profile is the result of the expansion of monocultures motivated by the spread of globalized agribusiness in areas of the Brazilian Cerrado, which has stood out economically in the cultivation of grains, especially soybean, maize, and cotton [35; 36]. The present study aimed to map and quantify the spatio-temporal distribution of WUE based on its environmental interactions in the westernmost region of Bahia, NEB.