Atmospheric carbon dioxide and moisture concentrations were measured with an infrared gas analyzer (IRGA) (LI-6262, LI-COR, Inc. Lincoln, Nebraska, USA). Measurements were made from 1997 through the present at the Kendall site. The meteorological data and Bowen ratio energy balance systems (BREB) (Model 023/CO2 Campbell Scientific Inc., Logan, Utah, USA) data are used to calculate carbon dioxide and evapotranspiration (ET) fluxes. The stored Bowen ration instrument data from the measurement site were transmitted by radio daily to our research station in Tombstone, AZ. From there, they were transferred through an Internet connection to Tucson, AZ. The data were then divided into 5-day increments and inserted into a Quattro1 Pro spreadsheet file which had all the formulations to calculate flux of soil heat, latent heat, sensible heat, evapotranspiration rates (ET), and CO2 rates on the 20-min time step of the data. All instrument and calculated data were graphed in the spreadsheet file and thoroughly reviewed for any instrument problems or data stream collection issues.

Carbon dioxide and water fluxes are important components of watershed function. In order to study carbon dioxide and water flux as they exist over the Walnut Gulch Experimental Watershed (WGEW), two sites were selected on the basis of their ecosystem composition, one site being dominated by shrubs and the other a grass dominated plant community. The grass site is identified as Kendall (109560800W, 314401000N; elevation; 1526 m). The soils at the Kendall site are a complex of Stronghold (coarse-loamy, mixed, thermic Ustollic Calciorthids), Elgin (fine, mixed, thermic, Ustollic Paleargids), and McAllister (fine-loamy, mixed, thermic, Ustollic Haplargids) soils, with Stronghold the dominant soil [NRCS Soil Survey, 2003]. Slopes range from 4 to 9%. The Stronghold surface A horizon (0-3 cm) contains 670 g kg1 sand, 160 g kg1 silt, and 170 g kg1 clay with 790 g kg1 coarse fragments >2 mm, 11 g kg1 organic carbon, and 7 g kg1 inorganic carbon. Vegetation is dominated by herbaceous plants, predominately black grama (Bouteloua eriopoda (Torr.) Torr.), sideoats grama (Bouteloua curtipendula (Michx.) Torr.), three-awn (Aristida sp.) and cane beardgrass (Bothriochloa barbinodis (Lag.) Herter). Vegetation canopy height at the grass site ranged from 0.4 to 0.7 m during the growing season.