Fixed and variable components of evapotranspiration in a Mediterranean wild-olive - grass landscape mosaic

Abstract

Dry regions are typically characterized by heterogeneous ecosystems where trees are competing with the surrounding grasses for limited amount of water. In these regions, evapotranspiration (ET) is the leading loss term in the soil water budget, and its estimate, and the dynamic contribution of each ET component (i.e. tree and grass transpiration, and dry bare soil and wet surface evaporation), are still poorly quantified. In a typical heterogeneous Mediterranean ecosystem in Sardinia, we combined eddy-covariance estimates of ET with sap flux and energy balance estimates of wild-olive tree transpiration, a common tree species of the region, and with modeled evapotranspiration from the seasonal grass. Trees located in the southern edge of clumps, thus receiving more radiation, transpired more and showed a greater sensitivity to increasing vapor pressure deficit and soil moisture than trees located in clump centers or northern edges. Transpiration of the tree clumps in the footprint (Et), summed up with the modeled evapotranspiration components of the surrounding grass (mostly transpiration during the wet season and evaporation during the dry season), matched latent heat flux measurements, lending confidence in the estimates. Proper accounting for heterogeneity of sources within the eddy covariance footprint seems to have overcome potential errors from not preserving an important assumption of the method, the land-surface homogeneity, highlighting the methods reliability in such inhomogeneous ecosystem. Compared to ET, Et of wild olives was nearly constant over the hydrologic year, insensitive to variation in soil moisture and atmospheric conditions. In contrast, under favorable spring environmental conditions (radiation, vapor pressure deficit, and soil moisture), the pasture leaf area transpires at high rates, contributing to, and dominating the high ET during that season. Conversely, in dry periods, when evapotranspiration from the grass cover is dominated by low evaporation from the, principally, bare soil, Et dominants ecosystem ET.