Associated with neglecting the observed isotopic

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associated with neglecting the observed isotopic enrichment and show that the hydrograph separation underestimates old water contributions during early melt and overestimates old water during late melt. Therefore, measurements of snowmelt must be made repetitively throughout the event to sufficiently represent new water concentrations. 1.2.1.3 Spatial Variability of δ 18 O in Melt Water Many studies have implied spatial variations in isotopic chemistry by using more than one melt collection location. Moore (1989) collected meltwater from eight melt lysimeters during a melt event and reported daily averages between –20.05‰ and - 17.41‰ with standard deviations between 0.66‰ and 0.76‰. However, the study uses a grand mean from all lysimeters on all days as a constant new water value because temporal variations in chemistry and routing of new water could not be accounted for. This approach takes into account some spatial variation, but does not consider time variations in melt chemistry. Hooper and Shoemaker (1986) use isotopic chemistry from two melt lysimeters to represent new water in a 0.42 km 2 watershed. An average isotopic value is used when samples are taken from both locations at the same time. Spatial variability is reported to average 3.8‰ in deuterium between the two sampling points, which are approximately 300 m apart in distance and 220 m apart in elevation. Differences in
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10 isotopic values are attributed to rain events that occurred regularly during the melt event. Shanley, Kendall, Smith, Wolock, and McDonnell (2002) collect melt water from four locations with different aspects in a 0.41 km 2 basin. Small scale variability is reporte d to range between 1‰ and 3‰ in δ 18 O. Variability was accounted for by taking a daily arithmetic mean δ 18 O value stating that the variability is small compared to δ 18 O differences between meltwater and groundwater. The use of daily means is the first step in taking into account basin wide snowmelt chemistry over time, but very few collection locations are sampled in large basins. Melt δ 18 O is difficult to characterize because many factors can effect the melting processes in the snowpack and thus affect th e δ 18 O concentration of the snowmelt. Catchment scale factors such as elevation, aspect, wind redistribution, vegetation, and slope culminate with smaller scale factors such as melt pathways and local topography to give a significantly variable δ 18 O signal in time and space. 1.2.2 Assumption 6: Instantaneous Delivery of Snow Melt to the Stream . A spatially and temporally constant new water δ 18 O value does not require accounting for melt water travel times for different parts of the catchment. The constant new water δ 18 O value is simply used for the entire hydrograph event, even though the actual snowmelt period covers a relatively short fraction of the event hydrograph (Figure 1.2 a.). The horizontal dashed line in Figure 1.2 b represents the constant C n value to be used in the hydrograph separation if the measured melt δ 18 O is time constant. The constant δ 18 O value represents areas close to the stream early in the
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