## 4 Conclusions  
Based on a century-scale observational analysis over large hydrological units of the Eastern United States, we find that increase in precipitation led to an increase in evapotranspiration during the twentieth century, possibly, in part, through vegetation greening. We observed that precipitation increased during our study period (1901-2008), and more so than runoff across most hydrological units and averaged over the large study area. We determined that changes in water storage were negligible and, taking into account anthropogenic water use, estimated that ET had a positive trend during this time period. We also showed a statistically significant correlation between increasing NDVI and (P-Ru) trends, indicating that this vegetation greening as measured by NDVI may have been partially responsible for the positive ET trend.  
Projected increases in global temperature, and accompanying regional increases in land precipitation, are supported by observations [1]. An increase in land precipitation will increase soil moisture; and where vegetation growth was previously water-limited, canopy greening will occur. Where this happens, evapotranspiration will also increase, leading to surface air temperature cooling and supporting the negative feedback in climate simulations with elevated CO\({}_{2}\) as suggested in [5,6]. This claim is reinforced by results from the Intergovernmental Panel on Climate Change [1], which shows no significant trend to increases in surface temperature in the Mississippi River basin and areas east over the past 100 years. The same results show warming has occurred in the northeast US region roughly corresponding to hydrological units 1 and 2, however. In agreement with our observations, modeling studies [58] indicate that in central North America, vegetation greening is essentially due to increase in precipitation. Our analysis lends weight to the notion that, in tandem with external factors such as agriculture intensification [59], the additional water available from increasing precipitation associated with an increase in CO\({}_{2}\) and climate change, stimulates vegetation growth ([6] and references therein). This effect introduces a negative feedback with important implications for the climate and the carbon cycle. The feedback increases carbon sequestration through vegetation growth and enhances evapotranspiration rates, thus cooling the air.  
This process has been modeled under increased CO\({}_{2}\) conditions through various studies [5; 6; 7]. The results showed that the magnitude of the projected cooling depended partly on the treatment of the vegetation physiological activity under increased CO\({}_{2}\) concentration and the associated climate change, and partly on the nature of the locally dominant surface-atmosphere interaction, both of which are highly model dependent. This study establishes an observational basis for this hypothesis, which should be included in all climate models projecting the future state of climate under an increased CO\({}_{2}\) concentration.  
We consider this study to be a preliminary observational analysis and a more detailed analysis of the relationship between excess water and vegetation greening is warranted. Although further work is necessary to solidify conclusions, results presented in this study suggest that changes in the state of vegetation may already be playing a role in the continental water and energy budget as atmospheric CO\({}_{2}\) increases, possibly slowing the course of climate trends.