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Chloride derived from the atmosphere can be a valuable tracer in ecosystem and watershed processes. For these purposes and other environmental studies, it is important to establish temporal patterns and sources for in wet deposition. Weekly composite precipitation samples have been analyzed by the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) at West Point, NY during 1981–2003, although systematic contamination of precipitation significantly perturbed data for prior to 1998. Chloride and sodium ion seasonal wet deposition were highest in winter and lowest in summer through most of the record, probably as a result of more frequent marine-trajectory storms in winter. During 1998–2003, the period of highest quality data, the ratio of was significantly higher than average in summer and lower in winter. Higher summer occurred consistently throughout the record, often reaching values four times the seawater ratio. Based on the ratio of in seawater of annual wet deposition of during 1998–2003 was in excess of that for surface seawater. Additionally, a minor terrestrial dust component was approximated, which had the net effect of increasing annual excess wet deposition to ( or ) of the mean annual wet deposition at West Point ( or ). Consistent with plausible sources of non-seawater , we attribute excess wet deposition to HCl emission from coal fired generating stations, HCl emissions from domestic and industrial waste incineration and to HCl formation in the regional atmosphere from reactions of sea-salt aerosols with S and N acidic gases. [Copyright &y& Elsevier]

We present observations and simulations examining plant–water relations in a forested catchment characterized by strong topographic control over surface hydrology and stand structure. The system is dominated by xeric and mesic ecotypes of Quercus rubra L. Soil depths are typically very thin and the severity of seasonal water stress in these trees is determined largely by position along the local topographic gradient. Water balance related ecotypic differences in Q. rubra were measured during the 2000 growing season at an upland and lowland site. Significant site differences in stomatal conductance, sap flux density, and leaf to sapwood area ratios were observed. However, mid-day leaf water potentials and the leaf area-specific canopy hydraulic conductance were not significantly different despite a 20% increase in soil moisture content, an average 10 m increase in tree height, and a higher leaf area index at the lowland site. These results suggest a close coordination of tree morphology, stand structure, and the hydraulic conductance of the combined soil–root–leaf pathway that governs leaf-level water vapor exchange rates similarly across the topographic gradient. To place these stand-level observations in the context of catchment-scale water balance we linked the SPA canopy model with a 1-D soil column model and TOPMODEL hydrologic formulations. The SPA model was used to represent the canopy because of its specific inclusion of hydraulic constraints on transpiration and leaf water potential. The combined model is spatially explicit in the distribution of ecotype morphology, and calculates transpiration rates for TOPMODEL-derived saturated and unsaturated area fractions within the watershed separately. Model predictions of stomatal conductance, upland latent energy flux, stream discharge, and soil moisture content compared favorably with observations. Sensitivity analyses of canopy model parameters indicate stream discharge in this system is most sensitive to changes in the maximum leaf area index, the minimum leaf water potential, and belowground resistance. Discharge was least sensitive to changes in stem hydraulic conductivity and capacitance. Model results and observations are discussed with respect to adaptations to water stress, hydraulic controls on canopy water use, and ecosystem water use. [Copyright &y& Elsevier]