## 3 Results  
The carbon and water cycles have very strong relation and it implies that a disturbance in each component of WUE (i.e., GPP or ET), which can be caused by a hydrological extreme event, may impact the carbon cycle as well. In other words, drought is an intermittent disturbance in the water cycle, which can significantly impact the terrestrial carbon cycle [(73; 75)]. During 2000 to 2014, the average WUE over the Contiguous US (CONUS) is 1.95 gC/kg\(\cdot\)H\({}_{2}\)O and shows great spatial variations (Figure 2). The dry ecosystems of California, Nevada, Arizona, New Mexico, Utah and west Texas indicate high values of WUE ranging from 2.4 gC/kg\(\cdot\)H\({}_{2}\)O to 4 gC/kg\(\cdot\)H\({}_{2}\)O. Whereas, WUE is generally less than 1.6 gC/kg\(\cdot\)H\({}_{2}\)O in the Midwestern US. At the biome level (shown in Figure 3), EBF and Shrublands show the largest WUE and Cropland and DNF indicate the lowest WUE. According to Figures 2 and 3, arid ecosystems indicate the highest WUE (3.2 gC/kg\(\cdot\)H\({}_{2}\)O), followed by the coastal regions that show comparable WUE values (2.2 gC/kg\(\cdot\)H\({}_{2}\)O). The observed differences in WUE among biomes and ecosystems have been well documented by previous studies [(3; 31; 76)]. Caused by heterogeneities in both environmental conditions and plant physiological characteristics, the drivers controlling the spatial pattern of WUE are determined by elevation, latitude, plant morphology and climate conditions [(61; 77)]. Figure 2: Spatial distribution of mean water use efficiency (WUE) over the contiguous US (CONUS) for the study period (2000–2014). Figure 4 shows the drought severity and drought recovery duration over the CONUS for 2002, 2008, 2011 and 2012 drought episodes. In 2002, the western US faced more severe drought and some regions in Utah, Colorado, Arizona and southern California and Nevada experienced extreme drought. Accordingly, drought recovery for these regions took longer time and for the regions that experienced extreme drought condition, the minimum drought recovery is found 3 months. On the other hand, eastern US regions (e.g., North and South Carolina and Virginia) experienced severe drought and the drought recovery duration for these regions was relatively shorter. In 2008, the severe and extreme drought extent was less than 2002. California, Wisconsin and Washington were among the states that experienced severe drought in 2008. In California and Washington, the areas that were not covered with cropland biome indicate longer drought recovery. Meanwhile, Wisconsin is covered with more cropland biome and drought recovery was relatively shorter for it in 2008. In North Dakota, Nevada, Utah and Montana, most of which are covered with grasslands, the regions affected by severe drought show longer drought recovery duration. Additionally, Figure 4 shows that Texas, Oklahoma, Kansas, New Mexico and Arizona faced an extreme drought episode in 2011 [76; 77] and the drought recovery duration is found to be over 3-4 months in the region. The 2012 drought was one of the worst drought episodes in recent years which had catastrophic impacts and caused $40 billion damage, mostly due to agricultural losses [78; 79; 80]. Almost two-thirds of the nation dealt with drought on September 2012, according to the US Drought Monitor [81]. Figure 4 shows that the central and Midwest states were impacted with severe and extreme drought in 2012. Drought recovery duration is found to be between 2-3 months for most of the region, which is actually a markedly long period, since the drought initiated during the growing season, lasted for several months and then recovered 2-3 months after termination, which might have even affected the following year's harvest and crop yield. Figure 3: Spatial distribution of land cover over the CONUS and the average WUE of each biome during 2000–2014. The lines indicate \(\pm\)1 standard deviation for each case.