H12B-07
Vapor pressure deficit is as important as soil moisture in determining limitations to evapotranspiration during drought
Monday, 14 December 2015: 11:50
3024 (Moscone West)
Kimberly A Novick, Indiana University Bloomington, School of Public and Environmental Affairs, Bloomington, IN, United States, Christopher A Williams, Clark University, Worcester, MA, United States, Richard Phillips, Indiana University, Bloomington, IN, United States, Andrew C Oishi, USDA Forest Service Southern Research Station, Hot Springs National, AR, United States, Benjamin N Sulman, Princeton University, Atmospheric and Oceanic Sciences, Princeton, NJ, United States, Gil Bohrer, Ohio State University Main Campus, Civil, Environmental & Geodetic Engineering, Columbus, OH, United States and Darren L Ficklin, Indiana University Bloomington, Bloomington, IN, United States
Abstract:
The decoupling between potential evapotranspiration (PET) and actual evapotranspiration (AET) is a useful metric to characterize ecosystem hydrologic stress. As hydrologic stress evolves, PET increases following increases in incident radiation and vapor pressure deficit (VPD). AET, on the other hand, remains stationary or decreases due to declines in surface conductance imposed by decreasing soil water and stomatal closure under high VPD. Historically, it has been difficult to quantify the extent to which soil moisture as compared to VPD ultimately limits AET during hydrologic stress. Part of this difficulty relates to the strong correlation between soil moisture and VPD at timescales over which hydrologic stress evolves (weekly to monthly). Further, while it is relatively easy to manipulate soil moisture in experimental settings, manipulating VPD is much more difficult. Recently, the proliferation of eddy covariance flux sites has produced a rich collection of AET observations at fine timescales (i.e. hourly to daily) over which VPD and soil moisture are more decoupled. In this study, we leverage such data to quantify the extent to which soil moisture versus VPD constrains AET in more than 25 Ameriflux sites spanning a wide climate gradient. We found that AET was most significantly limited by soil moisture in dry sites where the annual PET was much higher than precipitation. VPD limitations to AET dominated in wetter sites, but even among the driest sites, they were of similar magnitude to soil moisture limitations. Our results highlight the critical, if at time underappreciated, role of VPD in determining ecohydrological functioning during periods of hydrologic stress. We also leverage these results together with future projections for VPD, soil moisture, and other relevant meteorological drivers to explore the extent to which the coherence between VPD and soil moisture, and their relative importance for limiting AET, may shift under future climate conditions.
