EVAPORATION AND THE DEVELOPMENT OF THE LOCAL BOUNDARY-LAYER OVER AN IRRIGATED SURFACE IN AN ARID REGION

Heterogeneous landscapes common to arid and semi-arid regions are ofte n characterized by a horizontal advection of sensible heat from dry su rfaces to wet surfaces which modifies the evaporation and water balanc e of moist areas. This study centered around a well-watered alfalfa fi eld surrounded locally by arid surfaces, but within a large complex of irrigated fields where both local and regional scale advection were p resent. Latent and sensible heat flux densities were measured using ed dy correlation at four different locations within the same field. Teth ered balloons were used to obtain vertical profiles of temperature, hu midity, wind speed and direction from the surface to heights of 50 m a t the leading and downwind edges of the field. Results show that sensi ble heat flux and turbulence intensity were strongly correlated indica ting the dependence of sensible heat transfer on turbulence. Evaporati on in excess of the equilibrium rate was governed by turbulence intens ity and the advection of sensible heat. Growth of the local boundary l ayer above the alfalfa canopy as defined by temperature and humidity p rofiles averaged about 30 m over a 450 m traverse. The height of the d eveloped 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 va por budget technique, and found to average approximately 10% lower tha n the measured evaporation rates. The contribution to evaporation from the advection of sensible heat was also calculated from the upwind an d downwind profiles and found to range between 28-90% of the total eva poration.