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Title: Investigating the source, transport, and isotope composition of water vapor in the planetary boundary layer

Author
item GRIFFIS, TIMOTHY - University Of Minnesota
item Baker, John
item WOOD, J - University Of Minnesota
item CHEN, Z - University Of Minnesota
item XIAO, K - University Of Minnesota
item WELP, L - Purdue University
item LEE, X - Yale University
item GOSKI, G - Dominican University Of California
item CHEN, M - University Of Minnesota
item NIEBER, J - University Of Minnesota
item SCHULZ, N - Yale University
item ZHANG, X - Princeton University

Submitted to: Atmospheric Chemistry and Physics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2016
Publication Date: 4/25/2016
Citation: Griffis, T.J., Wood, J.D., Baker, J.M., Lee, X., Xiao, K., Chen, Z., Welp, L.R., Schultz, N.M., Goski, G., Chen, M., Nieber, J.L. 2016. Investigating the source, transport, and isotope composition of water vapor in the planetary boundary layer. Atmospheric Chemistry and Physics. 16:5139-5157. doi:10.5194/acp-16-5139-2016.

Interpretive Summary: As the earth's surface warms, it is expected that there will be changes in the hydrologic cycle that could have important consequences. It is important to develop an understanding of how the earth's surface responds to and in turn affects these changes in the hydrologic cycle. We examined a number of lines of evidence, including long-term observations of precipitation, stream flow, evapotranspiration, isotopic composition of atmospheric water vapor, and model predictions to determine if evapotranspiration is changing and to see how the water vapor content of the atmospheric boundary layer is changing. Despite increasing temperatures and a lengthening growing season, there is no significant temporal trend in evapotranspiration over the past six decades. This surprising stability in evapotranspiration may be due to the compensating effects of rising carbon dioxide concentrations, which increases the water-use efficiency of plants, and promotes row crop agriculture at the expense of perennial vegetation. Modeling experiments indicate that expansion of row crops reduced regional annual evapotranspiration by 100 mm, in good agreement with observed increases in regional stream flow. The isotopic measurements that we made show that water vapor in the atmospheric boundary layer is strongly influenced by local evapotranspiration, but the lack of a temporal trend in evapotranspiration implies that the source of the observed long-term increase in atmospheric water vapor is primarily oceanic.

Technical Abstract: Increasing atmospheric humidity and convective precipitation over land provide evidence of intensification of the hydrologic cycle – an expected response to surface warming. The extent to which terrestrial ecosystems modulate these hydrologic factors is important to understanding feedbacks in the climate system and the availability of water resources. We synthesized long-term observations (eddy covariance, watershed water balance, tall tower boundary layer budgets, and isotopes) and modeling to explore trends in evapotranspiration (ET) and to evaluate its influence on planetary boundary layer (PBL) water vapor in the Upper Midwest, USA. There has been no clear temporal trend in ET over the past 60 years despite lengthening growing seasons, and substantial increases in air temperature and precipitation. Stable regional ET was attributed to the compensating effects of rising atmospheric CO2 and increases in row crop agriculture. Modeling experiments indicate that expanding agricultural land-use has reduced annual ET by 100 mm, consistent with observed increases in regional stream flow. Although conversion to agriculture has reduced annual ET, isotope tracer analyses reveal that growing season PBL water vapor can be derived from up to 75% local ET and that the precipitation recycling fraction within the region is about 30%, suggesting a potentially important link with convective precipitation. Although terrestrial ET can be a strong modulator of PBL water vapor, the lack of a significant temporal trend over the past 10 to 60 years implies that increases in annual precipitable mid-continental water vapor have been driven by oceanic evaporation and transport.