Humidification of the planetary boundary-layer in the mid-continental US and the anti-transpirational effect of crops

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 and land use change modify these two hydrologic factors is important towards understanding biophysical feedbacks in the climate system, and the availability of water resources. Here, we use a multi-year oxygen and deuterium isotope record of water (precipitation, soil, plant, and atmospheric vapor), regional climate modeling, and satellite observations to help constrain the role of terrestrial vegetation in the humidification of the planetary boundary layer. Using three different isotope tracers, we show that mid-continental water vapor in the planetary boundary layer can be derived from as much as 60% local evaporation during the growing season. Paradoxically, our modeling results illustrate that regional evapotranspiration has changed little over the last 50 years, and that the expansion of agricultural crops in the US Midwest has had an anti-transpirational effect — causing a lower annual contribution to atmospheric water vapor. These findings are consistent with observed increases in regional stream-flow data. Our long-term isotope data and modeling analyses indicate that the recent increase in mid-continental water vapor is largely of oceanic origin, which was presumably amplified by enhanced evaporation.