Observational and Modeling Constraints on Evapotranspiration and Water Vapor in the Upper Midwest, United States

Abstract:

Increases in atmospheric water vapor concentrations and convective precipitation over land provide evidence of intensification of the global hydrologic cycle in response to surface warming. The extent to which terrestrial ecosystems modulate these two components of the hydrologic cycle is important to understanding biophysical feedbacks in the climate system and the availability of water resources. Here, we use a multi-year oxygen-18 and deuterium isotope record of liquid water (precipitation, soil, and plants), atmospheric vapor, tall tower flux measurements, and Stochastic Time-Inverted Lagrangian (STILT) modeling to constrain the importance of evapotranspiration, and other source terms, in the humidification of the planetary boundary layer (PBL). Using an isotope tracer approach we estimated that mid-continental water vapor in the PBL can be derived from as much as 75% local evaporation during the growing season. This result is supported using an inverse modeling approach for cases of extreme dew-point events that have a strong agricultural fingerprint. The isotope observations of water vapor and precipitation were combined with a Monte-Carlo simulation to help constrain a mixing model to estimate the fraction of evaporated terrestrial water in precipitation. The results indicate that growing season precipitation has a median recycling signature of about 30% and is used to help diagnose recycling ratios in mesoscale models. Our land surface modeling results highlight that regional evaporation has changed little over the last 50 years and that the expansion of agricultural crops in the US Midwest has likely reduced the local annual contribution to atmospheric water vapor. These findings are consistent with observed increases in the regional stream-flow data. The compressed growing season of agricultural crops and their high transpiration rates may amplify precipitation intensity and runoff.