Evapotranspiration: spatial variability in semiarid terrain at the sub-daily scale

An experimental campaign was organised in summer 2021 in the Eastern Ebro basin, in the frame of the LIAISE Initiative (Land surface Interactions with the Atmosphere over the Iberian Semi-arid Environment). Amongst other measurements, the surface energy budget were obtained over different surfaces, including irrigated crops and orchards, rainfed areas and a lake, all them within a radius of 10 km.

For dry weather conditions, the values of the latent heat flux (or Evapotranspiration, ET) were very different depending on the surface, varying from more than 500 W/m2 over an irrigated corn field to less than 30 W/m2 over dry bare soil. These different values implied also different sensible heat fluxes, with the irrigated surfaces showing thermal stable stratification in the afternoon well before sunset due to the cooling effect of evaporation.

The contrast between the irrigated and the rainfed areas is very well-marked induced mesoscale circulations and local transport of humidity, which were overruled by the sea breeze arrival in the afternoon. Over the semi-arid rainfed areas, ET proceeded very irregularly in time and challenged the fundamental hypotesis of stationarity for the computation of the turbulent fluxes. Over all the well-watered surfaces, classical estimates of Potential ET, such as Penman-Monteith or Priestley-Taylor, overestimated the observed values.