Evapotranspiration controls across spatial scales of heterogeneity

Abstract

AbstractIn semi‐arid regions where irrigation causes thermal surface heterogeneities, evapotranspiration 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 the diurnal variability of evapotranspiration. In this study, we combine data 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 use a latent heat tendency equation as a diagnostic tool to quantify the contributions of various surface‐ and boundary‐layer driven processes on surface latent heat flux. We define the following spatial scales based on surface characteristics: regional, landscape, and local. We find that, at the regional and landscape scales (1 km), the feedback mechanisms between the surface fluxes and the resulting boundary‐layer dynamics enhance the daily latent heat flux by 64% to 77%. Conversely, at the local scale (100 m), surface‐driven processes are most important for governing evapotranspiration. At the local scale, we find that the energy stored at the surface enhances evaporation in the late afternoon, which could explain the observed time lag between net radiation and latent heat flux. Furthermore, the combined effects of entrainment at the boundary‐layer top and advection of heat and moisture enhance daily latent heat by 27% at the regional scale to 43% at the local scale. This analysis also has implications for the land‐surface modelling community. It suggests that properly reproducing evapotranspiration to capture the relevant processes depends on capacity of the model to resolve the interaction of scales in surface heterogeneous conditions.