Key to evaluating the consequences of woody plant encroachment on water and carbon cycling in semiarid ecosystems is a mechanistic understanding of how biological and non-biological processes influence water loss to the atmosphere. To better understand how precipitation is partitioned into the components of evapotranspiration (bare-soil evaporation and plant transpiration) and their relationship to plant uptake of carbon dioxide (CO 2 ) as well as ecosystem respiratory efflux, we measured whole plant transpiration, evapotranspiration, and CO 2 fluxes over the course of a growing season at a semiarid Chihuahuan Desert shrubland site in south-eastern Arizona. Whole plant transpiration was measured using the heat balance sap-flow method, while evapotranspiration and net ecosystem exchange (NEE) of CO 2 were quantified using the Bowen ratio technique.Before the summer rainy season began, all water and CO 2 fluxes were small. At the onset of the rainy season, evapotranspiration was dominated by evaporation and CO 2 fluxes were dominated by respiration as it took approximately 10 days for the shrubs to respond to the higher soil moisture content. During the growing season, periods immediately following rain events (<2 days) were dominated by evaporation and respiration while transpiration and CO 2 uptake peaked during the interstorm periods. The surface of the coarse, well-drained soils dried quickly, rapidly reducing evaporation. Overall, the ratio of total transpiration to evapotranspiration was 58%, but it was around 70% during the months when the plants were active. Peak respiration responses following rain events generally lagged after the evaporation peak by a couple of days and were better correlated with transpiration. Transpiration and CO 2 uptake also decayed rather quickly during interstorm periods, indicating that optimal plant soil moisture conditions were rarely encountered. NEE of CO 2 was increasingly more negative as the growing season progressed, indicating a greater net uptake of CO 2 and greater water use efficiency due mainly to decreases in respiration.

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A sequential extraction procedure was used to fractionate Cd, Cu, Ni, and Zn in sludge‐treated soils into the designated forms of exchangeable, adsorbed, organically bound, carbonate, and residual. Samples were obtained from a soil column study where anaerobically digested sewage sludge in either liquid or air‐dried form was mixed into the top 15 cm of three reconstructed soil profiles and leached for 25 months with Colorado River water. The applied sewage sludge and uncontaminated soil samples were also sequentially extracted. The metals added into the soil had not moved out of the sludge‐soil layers during the course of leaching. The chemical forms present in the sludge‐soil layers at the termination of the leaching process were compared with those initially present in the soil and sludge. Most of each of the metals studied in the columns were found in the organically bound, carbonate, or residual forms, with the relative distribution among these forms depending on whether samples were taken in or below the sludge‐soil layer. Less than 36% of each metal in the sewage sludge was in the residual form, while, except for Cd, the soils contained > 65% of each metal studied in this form. It appeared that Cd, Ni, and Zn were all shifting to the residual form. The chemical forms of Cu had not changed significantly during the study. The occurrence of metals in the stable organically bound, carbonate, and residual forms in the sludge, coupled with a shift toward the more stable form (residual) after soil incorporation, contributed to the lack of metal movement in the soil profiles.

This paper investigates the performance of the National Centers for Environmental Prediction (NCEP) Noah land surface model at two semiarid sites in southern Arizona. The goal is to evaluate the transferability of calibrated parameters (i.e., direct application of a parameter set to a "similar" site) between the sites and to analyze model performance under the various climatic conditions that can occur in this region. A multicriteria, systematic evaluation scheme is developed to meet these goals. Results indicate that the Noah model is able to simulate sensible heat, ground heat, and ground temperature observations with a high degree of accuracy, using the optimized parameter sets. However, there is a large influx of moist air into Arizona during the monsoon period, and significant latent heat flux errors are observed in model simulations during these periods. The use of proxy site parameters (transferred parameter set), as well as traditional default parameters, results in diminished model performance when compared to a set of parameters calibrated specifically to the flux sites. Also, using a parameter set obtained from a longer-time-frame calibration (i.e., a 4-yr period) results in decreased model performance during nonstationary, short-term climatic events, such as a monsoon or El Niño. Although these results are specific to the sites in Arizona, it is hypothesized that these results may hold true for other case studies. In general, there is still the opportunity for improvement in the representation of physical processes in land surface models for semiarid regions. The hope is that rigorous model evaluation, such as that put forth in this analysis, and studies such as the Project for the Intercomparison of Land-Surface Processes (PILPS) San Pedro-Sevilleta, will lead to advances in model development, as well as parameter estimation and transferability, for use in long-term climate and regional environmental studies.

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