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

Continuous measurements of standard meteorological variables using an automatic weather station and intermittent measurements of the surface energy balance, carbon dioxide flux, and momentum flux using Bowen ratio, eddy covariance, and sigma-T instrumentation were made for 13 months at a semi-arid Sonoran Desert site just west of Tucson, AZ. Weather observations demonstrate typical semi-arid Sonoran desert conditions, with frequent clear skies, high radiation, a large seasonal and diurnal temperature range, low relative humidity, and intermittent precipitation mainly of convective origin during a summer monsoon season. The substantial observational problems associated with surface flux measurements in this environment are reported. Comparisons between measured fluxes made simultaneously with different instrumental systems show acceptable agreement. Most of the incoming radiant energy leaves as sensible heat, and latent heat fluxes are always low, but transpiration is enhanced for about 10 days after rain. To investigate the influence of Crassulacean Acid Metabolism plants on carbon dioxide flux, measurements were sustained through the night. Carbon dioxide uptake is low, typically with peak daytime uptake in the order 0.25-1.0 μmol m^-2 s^-1 for the period for which data are available, and some carbon uptake persists even at night. The observations were used to validate and calibrate the surface energy balance simulated by the Biosphere- Atmosphere Transfer Scheme. Using the default 'semi-desert' soil and vegetation parameters specified in the National Center for Atmospheric Research Community Climate Model Version 2 resulted in a poor simulation of observations. However, using a set of site-specific parameters, including on-site observations to specify more realistic soil and vegetation characteristics, and optimized minimum surface resistance and plant wilting parameters, resulted in a substantial improvement in model performance. The site-specific parameters reflect the fact that the vegetation fraction is greater than assumed in the default parameter set, that leaf area index and minimum stomatal resistance are less, soils at the study site contain more clay, but that the plants' wilting point is lower than this clay fraction would imply. The modified, site-specific parameters more accurately describe the conservative character of the semi-desert vegetation and the moderate nature of its response to the seasonal water cycle.