5.1.2 Assumptions 2 and 6: New Water is Constant in Time and Space, Instantaneous DeliveryResults show new water melt chemistry is temporally and spatially variable. The construction of a daily average enrichment curve accounts for temporal and spatial variability of new water. Daily melt chemistries from the entire basin are combined into a daily average enrichment curve because of an uneven distribution of melt sampling points in the basin with respect to distance from the stream, and a similarity of bucket chemistries in time. Figure 5.3 is a histogram of distance to stream distribution in the BEC and frequency of buckets within that distance class.
40 The similar pattern from individual snowmelt buckets consist of a steep enrichment of δ18O from March 12-18, a subsequent depletion from March 18-30, and another enrichment until snow was melted (Figure 4.4). Bucket W1 is located in theFigure 5.2. Plot of baseflow and combined input 18O showing no overlap. valley bottom, which maintained snow cover longer than the valley slopes. A daily average enrichment curve constructed including late melt chemistries from bucket W1 is constructed separately from the daily average enrichment curve used for the rest of the catchment. It is depicted as a dashed line extension onto the signal used for the rest of the basin in Figure 4.5. Construction of the new water chemical input recognizes that melt water entering the basin during the melt event does not all exit the basin immediately when the snow is melted. Instead the basin retards the flow of meltwater to the stream and it is delivered to the stream throughout the hydrograph recession. Several assumptions are made in constructing the new water isotopic input curve. Assuming a constant Darcy Velocity for melt water assumes that the hill slope is
41 saturated from the first day of melt to the last day of the melt event, and that flow is moving through a uniform porous soil with no preferred pathways. There is also a point in time that decreases in the hydraulic gradient from soils draining, and soil tension slow Figure 5.3. Histogram showing distribution of buckets compared to basin area. down and eventually stop the movement of melt water to the stream. The hydrograph separation was cut off at August 17th2003 to account for the slowing of the flow to the stream because the stream returned to baseflow. At this point, the new water stored in the basin would become old water to be lost from the basin by evaporation and transpiration, or released to the stream during baseflow or another event. By weighting the daily average input curves exclusively with fraction of basin area, no attempt is made to weight inputs by volume. By not taking volume into account, the assumption is made that melt volume is equal in time and space during the melt period.
42 5.1.3 Assumption 3 and 4: Old Water Chemistry is Constant in Time and Space, Soil Water Contributions are NegligibleA constant old water 18O value is commonly used when post event stream concentrations return to preevent concentrations after the event (Dincer et al., 1970);
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