On the divergence of potential and actual evapotranspiration trends: An assessment across alternate global datasets

Evapotranspiration is a key variable in hydrology, playing an important role in water and energy balance of the land surface. There has been speculation on the direction of trend in potential and actual evapotranspiration (PET and AET) resulting from rising global temperatures, in both observational and derived records representing the historical climate. In this study, PET and AET trends of eight different global model datasets where analyzed over two time periods: from 2003 to 2012 (short term) and from 1980 to 2012 (multidecadal), to identify regions where the trends coincide or differ and to study the reasons behind these changes. The short-term analysis showed considerable uncertainty exists on the detection and direction of significant trends on both PET and AET. There was little agreement among the datasets about the direction of the global trends. The multidecadal study showed much more consistent trends throughout the datasets, particularly in relation to positive significant PET trends. During this period, the global PET mean increased 0.091 mm month(-1) yr(-1), while the global AET rose at 0.045 mm month(-1) yr(-1). Much of the opposite PET/AET trends can be attributed to changes in the precipitation. Most of the regions which present these trends are water-limited and present strong correlations between AET and precipitation trends. Some energy-limited regions showed an increasing gap between PET and AET, suggesting the influence of additional variables controlling AET.