Understanding evapotranspiration trends and their driving mechanisms over the NLDAS domain based on numerical experiments using CLM4.5

Abstract

Previous studies documented a recent decline of the global terrestrial evapotranspiration (ET) trend, of which the underlying mechanisms are not well understood. Based on experiments using the Community Land Model version 4.5 driven with the North American Land Data Assimilation System phase-2 (NLDAS-2) forcing data, this study investigates the variation and changes of ET trends at the continental scale and the mechanisms underlying these changes. Simulations are conducted over the NLDAS domain including the contiguous U.S. and part of Mexico for the period of 1980-2014. Changes of ET trend are derived based on the two subperiods 1982-1997 and 1998-2008. The strongest signals of trend change, of either sign, are primarily located in dry regimes, where ET is limited by water rather than energy. Sensitivity experiments were performed to isolate the impact of some of the most influential factors on the changing ET trends. Results indicate that trends in wind speed and surface air temperature had negligible impact on the ET trend and its changes within the study domain, and the ET trend and its changes are dominated by changes in precipitation amount. Changes in precipitation characteristics including the frequency and intensity are suggested to have a secondary effect on the ET trend changes through modifying the partitioning of water between infiltration and runoff. These findings are further supported by correlation coefficients between ET and various driving factors. Results from this study may be region specific and therefore may not hold for ET trend changes over the rest of the globe.