Long-Term Variability of Evapotranspiration and Vegetation Productivity: The Role of Temporal Scales

Abstract:

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 variability of annual precipitation on ecohydrological dynamics. Specifically, the Advanced WEather GENerator, AWE-GEN is used to simulate 200 years of hourly meteorological forcing obtained by imposing four types of precipitation annual process. The generated time series force a mechanistic ecohydrological model, Tethys-Chloris. Simulations are performed for four locations characterized by different vegetation cover and climate conditions. The results indicate that long-term transpiration (T) and evapotranspiration (ET) fluxes and vegetation productivity expressed as Gross and Above-ground Net Primary Production (GPP, ANPP) are essentially unaffected by the imposed climate fluctuations, showing a mild response only in the water-limited environment. This finding supports the hypothesis of interannual insensitivity of evapotranspiration and vegetation productivity to annual climatic fluctuations, which are mostly reflected in 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. Sensitivity of ET and GPP/ANPP to changes in mean precipitation was found to be much larger than sensitivity to changes in annual precipitation variability. As local, single-site sensitivities are considerably smaller than the sensitivities observed across climatic and vegetation gradients, an important role of ecosystem re-organization in modifying ANPP and ET sensitivity in a changing climate is highlighted.