title: Stomatal response to humidity and CO2implicated in recent decline in US evaporation.
abstract: Evapotranspiration, defined as the total flux of water from the land surface to the atmosphere, is a major component of the hydrologic cycle and surface energy balance. Although evapotranspiration is expected to intensify with increasing temperatures, long-term, regional trends in evapotranspiration remain uncertain due to spatially and temporally limited direct measurements. In this study, we utilize an emergent relation between the land surface and atmospheric boundary layer to infer daily evapotranspiration from historical meteorological data collected at 236 weather stations across the United States. Our results suggest a statistically significant (α = 0.05) decrease in evapotranspiration of approximately 6% from 1961 to 2014, with a significant (α = 0.05) sharp decline of 13% from 1998 to 2014. We attribute the decrease in evapotranspiration mostly to declines in surface conductance, but also to offsetting changes in longwave radiation, wind speed, and incoming solar radiation. Using an established stomatal conductance model, we explain the changes in inferred surface conductance as a response to increases in carbon dioxide and, more recently, to an abrupt decrease in atmospheric humidity.
authors.affiliation: Department of Earth and Environment, Boston University, 675 Commonwealth Ave., Boston, MA, 02215, USA.
authors.forename: Angela J
authors.initials: AJ
authors.lastname: Rigden
authors.affiliation: Department of Earth and Environment, Boston University, 675 Commonwealth Ave., Boston, MA, 02215, USA.
authors.forename: Guido D
authors.initials: GD
authors.lastname: Salvucci
mesh_terms: D002245:Carbon Dioxide; D006813:Humidity; D014481:United States; D014867:Water; D058438:Water Cycle; D014887:Weather
chemical_list: D014867:Water; D002245:Carbon Dioxide
keywords: biosphere-atmosphere interaction; evaporation; evapotranspiration; relative humidity; stomatal conductance; surface conductance