3 concentration in Equation 1. Recent studies have addressed temporal variations (enrichment) in snow melt δ18O throughout the melt event (Taylor, Feng, Williams, and McNamara, 2002), but implementation of methods to account for observed spatial variability are limited. Spatial variability in snowmelt isotopic chemistry is observed on the catchment scale in response to elevation, distance from the ocean, and latitude (Dansgaard, 1964; Ingraham, 1998; Ingraham and Taylor, 1986; Siegenthaler and Oeschger, 1980). Spatial variability in isotopic melt chemistry is also expected to vary on the meter-scale similar to melt volumes as observed by Williams, Sommerfeld, Massman, and Rikkers (1999). Small-scale controls on variability may include, snow redistribution in response to abrupt changes in vegetation and complicated pathways of melt through the snowpack. This thesis contributes to methods that define a new water input for Equation 1 during snowmelt events. Specific objectives of this thesis include 1) evaluating the spatial and temporal variation of snow melt δ18O in the snowpack in the Bogus Experimental Catchment (BEC), 2) demonstrating the wide range of possible outcomes for Equation 1 depending on the new water chemistry used, 3) developing a method of combining measured melt chemistry into a new water isotopic input for Equation 1 that
4 Figure 1.1. Typical δ18O enrichment of melt from an Alaska snowpack on the North Slope of the Brooks Range (Modified from Taylor et al., 2002). takes into account spatial and temporal variation, and 4) estimating new and old water contributions using the developed hydrograph separation technique. 1.1Project Scope The evaluation of isotopic variability in snowmelt and new methods of accounting for that variability in the BEC contributes to hydrograph separation research dating to 1967 (La Sala, 1967). Although steady improvements have been made in hydrograph separation techniques, there is a need to improve methods of obtaining a reasonable new water isotopic concentration for snow melt events. As snow melts, phase changes in the snowpack enrich melt. Factors such as elevation and aspect should affect large-scale spatial variability, while preferred pathways through the snowpack and vegetation should affect small-scale spatial variability. A very significant, but often overlooked source of error in new water chemistry is the time it takes melt in
5 the upper portion of the catchment to travel to the stream. Only by accounting for all of these factors can one be confident in a new water melt chemistry. The completion of a hydrograph separation study in the BEC during the 2003 melt season supports a large-scale investigation of cold season hydrologic pathways in the semiarid Dry Creek Experimental Watershed composed of rangelands and forests north of Boise, Idaho. Ongoing studies in the Dry Creek Experimental Watershed examine hill slope processes and their relations to watershed functions. Results from such studies will be incorporated into watershed models, which predict and evaluate water resources and flood hazards (Weiler and McDonnell, 2004).
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