Soil moisture and vegetation controls on evapotranspiration in a heterogeneous Mediterranean ecosystem on Sardinia, Italy

TLDR

Micrometeorological evapotranspiration measurements are difficult to interpret in heterogeneous landscapes. The study presents a method to estimate evapotranspiration in heterogeneous ecosystems as the residual of the energy balance using surface temperature, a footprint model, and high‑resolution Quickbird imagery. Data were collected by an eddy‑correlation tower at Orroli, Sardinia, covering woody vegetation, grass, and bare soil, and the authors implemented two two‑source models (2SR and tile) that use the imagery and footprint model to compute ET. Woody vegetation maintained near‑potential transpiration even under severe drought, whereas grass was less tolerant and the transition to bare soil after rains markedly reduced evapotranspiration.

Abstract

Micrometeorological measurements of evapotranspiration (ET) can be difficult to interpret and use for validating model calculations in the presence of land cover heterogeneity. Land surface fluxes, soil moisture (θ), and surface temperatures ( T s ) data were collected by an eddy correlation‐based tower located at the Orroli (Sardinia) experimental field (covered by woody vegetation, grass, and bare soil) from April 2003 to July 2004. Two Quickbird high‐resolution images (summer 2003 and spring 2004) were acquired for depicting the contrasting land cover components. A procedure is presented for estimating ET in heterogeneous ecosystems as the residual term of the energy balance using T s observations, a two‐dimensional footprint model, and the Quickbird images. Two variations on the procedure are successfully implemented: a proposed two‐source random model (2SR), which treats the heat sources of each land cover component separately but computes the bulk heat transfer coefficient as spatially homogeneous, and a common two‐source tile model. For 2SR, new relationships between the interfacial transfer coefficient and the roughness Reynolds number are estimated for the two bare soil–woody vegetation and grass–woody vegetation composite surfaces. The ET versus θ relationships for each land cover component were also estimated, showing that that the woody vegetation has a strong tolerance to long droughts, transpiring at rates close to potential for even the driest conditions. Instead, the grass is much less tolerant to θ deficits, and the switch from grass to bare soil following the rainy season had a significant impact on ET.