## 4 Discussion
Meanwhile, WUE anomaly is almost always (in 93% of areas) positive for severe to extreme droughts. WUE changes in the 2011 drought episode showed wider range and higher maxima compared to other drought episodes. The median of WUE anomaly for the regions affected by severe to extreme drought was 0.5 gC/kg H\({}_{2}\)O in 2011. The 2012 drought event showed similar results and more than 75% of the regions affected by drought indicate positive WUE anomaly in 2012. In general, Figure 7 reveals that WUE deviation and drought severity are positively correlated and a more severe drought increases the likelihood of positive WUE anomaly. Figure 6: The relation between drought recovery time (\(y\)-axis), drought duration (\(x\)-axis) and drought intensity (color bar) for the 2002, 2008, 2011 and 2012 drought episodes. To better understand the changes of WUE in relation to its components (ET and GPP), the distribution of relative anomalies of GPP, ET and WUE are plotted in Figure 8 for the 2002, 2008, 2011 and 2012 drought episodes. In the figure, the blue curves on the axes represent the distribution of the corresponding variable. The figure indicates that GPP and ET can have either positive or negative anomalies during drought. However, the distribution diagram reveals that negative anomaly occurs more often (i.e., the distributions are negatively skewed). Comparing the results of different years, WUE anomaly reaches higher values (darker colors) in 2002 and 2011, which can be attributed to higher severity of drought in these years. Previous studies found that drought causes substantial reduction in GPP (about 35% decrease) over most biome land covers, whereas it slightly enhances GPP (about 7% increase) in evergreen broadleaf forests and shrublands [75, 85]. In North America, a large reduction of GPP (over 50%) was found resulting in 51% decrease of net carbon uptake during the 2000-2004 drought in western North America [86, 87, 88]. Similarly, studies found decreasing (ranging from 1% to 28%) and increasing (ranging from 7% to 15%) WUE under drought stress, for different drought severities and land covers [1, 2, 86]. Overall, decomposing WUE in relation to its components in Figure 8 reveals that if the relative anomaly of ET is larger than that of GPP, WUE anomaly will be positive (shown in green color), whereas if GPP is higher than ET (lower half of the plot, shown in brown color), WUE anomaly will be negative. Evapotranspiration (ET) is an essential component of energy and water cycles and it is influential in multitude of earth processes, subsequently affecting meteorology and agriculture [88, 89]. Several studies have shown the role of evapotranspiration in developing and intensifying droughts [90, 91]. In some studies, potential evapotranspiration anomaly has been merely utilized as a measure of drought intensity and as a means for predicting drought [92]. To better understand the terrestrial impacts of drought on evapotranspiration (ET), the changes of ET are investigated for different drought episodes. Figure 9 shows the ET anomaly during the four major drought episodes of 2002, 2008, 2011 and 2012 across the CONUS. From the figure, it can be seen that ET tends to be below average in the areas affected by severe (or more intense) drought. This negative anomaly is found to be common in both dry and humid climates, which highlights that water availability is the dominant factor for evapotranspiration deviation [1, 93]. The highest changes of ET is found during the 2011 Texas drought, reducing ET about 100-200 mm. Evapotranspiration consists of three components: interception, soil surface evaporation and transpiration. Further study can shed light on different responses of evaporation and transpiration over different land covers during drought. Figure 7: The distribution of WUE anomaly in response to varied drought severities for the 2002, 2008, 2011 and 2012 drought episodes.