Evapotranspiration Controls across Spatial Scales of Heterogeneity

In heterogeneous environments, evaporation is controlled not only by the land surface, but also by the interaction between the surface and the atmospheric boundary layer. The spatial scale of heterogeneity impacts the processes that drive evaporation. In this study, we combine a comprehensive observational data set across scales with a conceptual, coupled land-atmosphere model to study the drivers of evapotranspiration across spatial scales of heterogeneity for the Land Interactions with the Atmosphere in the Iberian Semi-arid Environment (LIAISE) field campaign. We aim to understand the role of spatial scales of irrigation-induced, thermal heterogeneity on the surface fluxes, and consequently, the development of the diurnal convective boundary layer. To do this, we have introduced a spatial scaling scheme based on observed surface characteristics to describe the LIAISE environment where heterogeneities range from the regional scale (~10s km) to the landscape scale (~1s km) to the local scale (~100s m). We find that the boundary layer observed in the LIAISE campaign is formed by the composite, regional land surface.

In this study, we use a latent heat tendency equation as a diagnostic tool to quantify the contributions of various surface- and boundary-layer-driven forcings and feedbacks on surface latent heat flux. This method allows us to quantify the forcings and feedbacks of latent heat (LE). The LE equation is validated against observations. We find that at the larger spatial scales (> 1 km), boundary-layer processes including the feedback mechanism resulting from surface warming, are relatively more important for enhancing surface evaporation compared to local spatial scales ( ~ 100 m) where surface-driven processes are most important for governing evapotranspiration. At the local scale, the time lag between latent heat and net radiation is mainly explained through boundary-layer processes. Boundary-layer warming through surface sensible heat enhances LE in the afternoons. Additionally, in heterogeneous areas, non-local drivers including advection of heat and moisture influence local evaporation in a heterogeneous environment. The role of warm air advection is more important for enhancing evaporation than dry air advection. Surface-driven processes control LE to a lesser extent at the local scale. However, energy stored in the soil enhances evapotranspiration in the late afternoon at the local scale. This could explain the observed time lag between the net radiation and latent heat flux. Our approach is not only useful to advance understanding and data interpretation but also as a diagnostic tool to evaluate the performance of land-surface models at the parameter level.