Evapotranspiration and turbulent transport in an irrigated desert orchard

Micrometeorological measurements were recorded in an irrigated pecan orchard for 2 weeks in the summer of 1996 near Las Cruces, NM. A vertical array of five sonic anemometers recorded three‐dimensional wind and temperature data within and above the orchard. The measured energy budget closure error was only 3.2% of net radiation, indicating freedom from local edge advection. The effects of regional (oasis) advection and unsteady winds on evapotranspiration (ET) were considered by comparing the observed latent heat flux values to estimates of ET using the Penman–Monteith and Advection–Aridity approaches. Penman–Monteith underestimated observed ET values by 82%. The Advection–Aridity modifications of potential evapotranspiration (PET) underestimated ET by 11%. Profiles of turbulence statistics demonstrated vertical heterogeneity of turbulence within the canopy. Directly above the canopy, momentum flux profiles showed little divergence. However, at a level of two times the tree heights, sensible heat flux profiles did show divergence, confirming the presence of “oasis” advection resulting from warm, dry air moving above the internal boundary layer. Upward convection from the hot soil surface between the trees diluted the oasis condition to the point where a weak upward sensible heat flux was observed during the midday periods when the soil was not shaded. Convection ratios, and exuberance ratios, generated from quadrant analyses of the heat and momentum flux events, showed that turbulent motions moved freely up and down within this canopy with little attenuation due to the open spaces between the trees.