Interannual variability of evapotranspiration and vegetation productivity

Interannual variability of precipitation can influence components of the hydrological budget, affecting them directly and indirectly through adjustments in vegetation structure and function. We investigate the effects of fluctuations of annual precipitation on ecohydrological dynamics. Specifically, we use the advanced weather generator, AWE-GEN, to simulate 200 years of hourly meteorological forcing obtained by imposing four types of precipitation annual process with identical long-term mean. The generated time series force a mechanistic ecohydrological model, Tethys-Chloris. Simulations with perturbed precipitation variability are performed for four locations characterized by different vegetation cover and climate. The results indicate that long-term transpiration (T) and evapotranspiration (ET) fluxes as well as vegetation productivity expressed as Gross Primary Production (GPP) and Aboveground Net Primary Production (ANPP) are essentially unaffected by the imposed climate fluctuations. This finding supports the hypothesis of a relative insensitivity, except for water-limited environments, of interannual evapotranspiration and vegetation productivity to annual climatic fluctuations, which are mostly reflected in the fluxes of deep leakage and runoff. The occurrence of short periods of favorable meteorological conditions randomly taking place within the year was found to be a better explanatory variable for interannual variability of ET and ANPP than average annual or growing season conditions. The results indicated that local, single-site sensitivities are considerably smaller than those observed across climatic and vegetation spatial gradients and thus an important role of ecosystem reorganization in modifying ANPP and ET sensitivity in a changing climate is recognized.