## 4 Discussion  
In order to better understand the terrestrial impacts of drought, the response of water use efficiency (WUE) is investigated for different drought episodes. The range of WUE during drought is from 1 to 7 (g C/kg H\({}_{2}\)O), which in the south and southwest shows the maximum. Figure 5 shows WUE anomaly during the four major drought episodes of 2002, 2008, 2011 and 2012 across the CONUS. From the figure, it can be seen that WUE responds differently to various drought events for different biomes and climates. In general, the arid areas which faced severe drought show significant increase in WUE during the drought episodes. This is in agreement with previous studies [(30; 82)] indicating that plants in arid regions are highly adapted to the water scarcity associated with arid climate and have more resiliency to water deficits owing to a series of conservative water-use strategies [(77)]. Such an adaptation helps the vegetation in arid regions to reduce water loss and maintain vegetation growth. In cold regions, vegetation growth is mainly constrained by air temperature and solar radiation. The higher carbon uptake due to hotter weather that usually coincide with drought episodes [(83; 84)] may increase WUE in these regions [(30)]. Overall, comparing Figures 4 and 5, WUE is found to substantially increase in response to extreme droughs in each drought episode, indicating that if a region experiences severe drought (or worse), WUE is likely to increase during the drought episode. Figure 6 shows the relation between drought recovery time and drought duration and intensity for 2002, 2008, 2011 and 2012 drought episodes. For each year, the areas affected by drought are considered and the three characteristics are extracted and plotted against each other. For example, in 2012 drought when a region experienced a severe drought for 10 months, the expected drought recovery range was from 2.25 to 2.75 months. In general, a more severe drought episode is expected to result in longer recovery time compared to moderate droughs, which is approved by the results of Figure 6. Additionally, a longer drought episode increases the likelihood of protracted drought recovery. Comparing these four drought episodes reveals that in 2008, the area that experienced prolonged severe drought was relatively less than the other drought episodes and consequently, the affected  
Figure 5: Spatial distribution of WUE anomalies during 2002, 2008, 2011 and 2012 drought episodes. regions needed rather shorter recovery time. In 2011, the area affected by a prolonged drought episode shows a wider range of recovery time (compared to other years) and the recovery duration tends to be longer for the regions that experienced more severe droughts. In general, Figure 6 implies that drought duration, recovery and severity are positively correlated, meaning that a prolonged drought will generally result in longer drought recovery time. Similarly, the regions experiencing more severe drought episodes will require more time to recover from drought. To assess descriptive statistics of WUE changes and to better understand terrestrial response to agricultural drought, the WUE changes are investigated with drought severity. Figure 7 shows the WUE anomaly for each drought severity level for the 2002, 2008, 2011 and 2012 drought episodes over the affected areas. From the figure, a similar pattern can be found for the WUE changes in response to various drought episodes with different severities. In general, WUE shows sharper and higher positive anomalies when a region is affected by a more severe drought. During the 2002 drought episode, the areas affected by extreme or more intense drought showed an increase in WUE with a maximum value of 1.25 gC/kg H\({}_{2}\)O. The regions affected by moderate drought in the same year showed relatively lower changes of WUE with the minimum and maximum anomaly of \(-\)0.3 and 0.5, respectively. In the 2008 drought episode, more than 75% of the drought affected areas showed a positive anomaly for WUE.