Comparison of a Higher Order Closure Soil-Vegetation-Atmosphere Transfer Model with Observations from the CHATS Experiment

Tree crops are grown on a substantial portion of California's arable land and use a significant amount of water resources. A better understanding of orchard microclimate and evapotranspiration is needed as growers try to balance water use, yield, and fruit and nut quality amid periodic droughts and climate change. To identify key controlling environmental variables and physiological parameters, the within-canopy microclimate (air temperature, canopy temperature, and humidity), turbulent statistics, and fluxes of energy, momentum, and water vapor are simulated for a walnut orchard using UCD-ACASA, a higher-order-turbulence-closure soil-vegetation-atmosphere transfer model. The model output is compared with extensive micrometeorological measurements from sensors on a vertical profile tower during the Canopy Horizontal Array Turbulence Study (CHATS) field campaign. Measurements and simulation are for periods before and after leaf-out of the canopy and over a spectrum of atmospheric stability conditions. Sensitivity of fluxes and microclimate profiles to changes in vegetation parameters and environmental driving factors, including elevated CO2 concentrations, are tested as a preliminary step towards coupling ACASA to a tree model with the objective of better understanding current orchard evapotranspiration and possible impacts of climate change.