## 1 Introduction  
The IPCC Fifth Assessment Report states that precipitation has generally increased over land in the Northern Hemisphere mid-latitudes [1]. Furthermore, there is substantial evidence to suggest that the hydrologic cycle has been intensifying with climate change [2; 3; 4]. These trends raise some questions. How is this hydrologic intensification being distributed across the cycle? To which components of the cycle has the additional precipitation been directed? What are the implications of these changes for the climate? Some researchers suggest that the additional water available from increasing precipitation may stimulate an increase in vegetation growth and subsequently lead to an increase in evapotranspiration (ET) [5; 6; 7]. This process may induce a negative feedback on the climate, providing an additional cooling mechanism that may reduce the magnitude of the climate-warming trend. Natural ET is comprised of canopy transpiration, interception loss (potential evaporation of precipitation intercepted by leaves on the canopy), and ground evaporation. The canopy-dependent component of ET responds to changes in vegetation structure [8; 9; 10], so that vegetation greening leads to an increase in ET, and to changes in atmospheric CO\({}_{2}\) concentration and environmental stresses through stomatal adjustment [11; 12]. Stressed vegetation tends to reduce its stomatal conductance and transpiration (e.g., [13]), leading to a warming effect, whereas unstressed vegetation facilitates the diffusion of water out of the plant during photosynthesis, and thus increases transpiration and cooling (e.g., [6]). ET can also be dictated in part by vapor pressure deficit (VPD) [14], defined as the saturation vapor pressure minus actual vapor pressure. The authors of [15] analyzed trends in the components of the hydrological cycle in 19 climate models simulating the effects of an increase in CO\({}_{2}\).The ensemble mean from these models indicates that, compared to their respective baselines over land, the global mean of runoff change would increase twice as fast as that of the precipitation. However, only a few of these simulations have considered the effects of vegetation-climate interactions. Conversely, the studies of [5; 6; 7] considered vegetation-climate interactions within climate models of different degrees of complexity in an environment with increased atmospheric CO\({}_{2}\) concentration, and all three studies accounted explicitly for an increase in vegetation structure through increase in vegetation Leaf Area Index (LAI). Additionally, the authors of [6] introduced plant physiological down-regulation--a process by which plants reduce their photosynthetic activity under abundant atmospheric CO\({}_{2}\). Under elevated CO\({}_{2}\), plants exhibit some down-regulationcharacterized by a reduction in the initial CO\({}_{2}\) enhanced rates of photosynthesis that result from a gradual decrease in the activity and/or amount of Rubisco, the enzyme in plant cells responsible for the first step in carbon fixation [16]. Down-regulation reduces the canopy conductance beyond the reduction caused by the radiative and physiological effects of increased CO\({}_{2}\)[12], leading thus to increased water availability, which is diverted, as an additional effect, to increase LAI beyond increases caused by climate changes and CO\({}_{2}\)-induced water use efficiency. Each study compared the effects of the vegetation feedback on climate to a conventional simulation where vegetation structure was not allowed to change. Results from all three simulations [5; 6; 7] suggested that an increase in vegetation density, or vegetation greening, will cause ET to increase. However, there was disagreement on the quantified significance of this potentially important feedback process and its global impact on the climate. In contrast to the models used in the analysis in [15], the study in [6] found the rate of change of precipitation to increase more than that of runoff.