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Heterogeneous landscapes common to arid and semi-arid regions are often characterized by a horizontal advection of sensible heat from dry surfaces to wet surfaces which modifies the evaporation and water balance of moist areas.

This study centered around a well-watered alfalfa field surrounded locally by arid surfaces, but within a large complex of irrigated fields where both local and regional scale advection were present. Latent and sensible heat flux densities were measured using eddy correlation at four different locations within the same field. Tethered balloons were used to obtain vertical profiles of temperature, humidity, wind speed and direction from the surface to heights of 50 m at the leading and downwind edges of the field.

Results show that sensible heat flux and turbulence intensity were strongly correlated indicating the dependence of sensible heat transfer on turbulence. Evaporation in excess of the equilibrium rate was governed by turbulence intensity and the advection of sensible heat. Growth of the local boundary layer above the alfalfa canopy as defined by temperature and humidity profiles averaged about 30 m over a 450 m traverse. The height of the developed local boundary layer was strongly correlated to the intensity of turbulence. Short-term evaporation rates were calculated from the upwind and downwind profiles of humidity and wind, using a modified vapor budget technique, and found to average approximately 10% lower than the measured evaporation rates. The contribution to evaporation from the advection of sensible heat was also calculated from the upwind and downwind profiles and found to range between 28-90% of the total evaporation.