Surface flux measurement and modeling at a semi-arid Sonoran Desert site

Abstract

Continuous measurements of near-surface weather variables using an automatic weather station and intermittent measurements of surface energy, momentum and carbon dioxide fluxes using Bowen ratio, eddy covariance, and sigma-T systems were collected for 13 months at a semi-arid Sonoran Desert site near Tucson, Arizona. Comparisons between measured fluxes made simultaneously with different instrumental systems show acceptable agreement. To investigate the influence of Crassulacean Acid Metabolism plants on carbon dioxide flux, measurements were sustained through the night. Observations were analyzed to characterize the typical magnitude of diurnal and seasonal variations in surface energy and carbon dioxide exchanges for this vegetation type and were then used to validate and calibrate the surface energy balance simulated by the Biosphere-Atmosphere Transfer Scheme. Using the standard "semi-desert" soil and vegetation parameters specified in the National Center for Atmospheric Research Community Climate Model version 2 gave a poor description of surface energy exchange. However, a combination of site-specific soil and vegetation parameters, and a simple optimization to modify the value of minimum surface resistance and plant wilting parameters, substantially improved the model performance. The site-specific parameters reflect the fact that the vegetation fraction is greater than assumed in the standard parameter set, leaf area index and minimum stomatal resistance are less, soils at the study site contain more clay, but the plants' wilting point is lower than this clay fraction would imply. These modified parameters more accurately describe the conservative character of the semidesert vegetation and the moderate nature of its response to the seasonal water cycle.