Evapotranspiration : Atmospheric boundary layer interactions across scales of surface heterogeneity

Summary

Although, irrigated land accounts for only 20% of agricultural land area, it produces 40% of food and consumes more than 90% of the total consumptive water use globally (Decker et al., 2017; Falkenmark & Lannerstad, 2005). Irrigation in drylands creates thermal heterogeneities in the land surface, which have implications for both water resources and the development of regional weather patterns. In this thesis, we investigate both how irrigation-induced surface heterogeneity influences evapotranspiration in a semi-arid region and how the spatial scale of heterogeneity controls the feedbacks between the land surface and the atmospheric boundary layer (ABL). To address these topics, we combine the observational data from the Land surface Interactions with the Atmosphere over the Iberian Semi-Arid Environment (LIAISE) experiment with both a conceptual land-atmosphere model and a high-resolution turbulence-resolving atmospheric model (Chapter 2 of this thesis). We adopt a multi-scale approach to analyze the overlapping spatial scales of heterogeneity ranging from the regional scale (~10 km) to the landscape scale (~1 km) to the local scale (~100 m). Using the conceptual model, we find that the partitioning of energy depends on the spatial scale of heterogeneity, and the observed boundary-layer properties are satisfactorily replicated by a composite of the regional scale land surface (Chapter 3). Processes linked to the surface heterogeneity, including advection of a warm/dry air mass and entrainment of air from the free atmosphere into the atmospheric boundary layer, increase the daily evapotranspiration from 27 to 43% depending on the spatial scale (Chapter 4). When we increase the model complexity to study how the scales of heterogeneity interact in the atmosphere, we discover that although the influence of the surface heterogeneities lessens in the middle of the ABL, they induce atmospheric heterogeneity in the top of the ABL. Furthermore, the length scale of spatial variability of the boundary-layer height is ~800 m, which implies that the landscape scale is most responsible for the development of the ABL (Chapter 5). Finally, we study the drivers of turbulent flux divergence in the surface layer by combining real and virtual tower measurements. We find that in the atmospheric surface layer the latent heat flux divergence does not equal the local-scale moisture advection (Chapter 6). These findings are relevant for quantifying and understanding the fate of water in water-limited areas. By studying evapotranspiration and its controls, we quantify the influence of processes that are most important for driving land-atmosphere interactions – and therefore plant water consumption – in irrigated semi-arid regions. Our multi-scale approach serves as a proxy of future climate scenarios where both semi-arid regions and irrigated croplands within them are expected to expand. Finally, by investigating the influence of spatial scale on the processes that govern the bidirectional feedbacks between the surface and the atmosphere, we identify emergent properties that may become relevant as weather and climate models move to higher resolutions.