5 contributions to streamflow from surface storage

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5. Contributions to streamflow from surface storage are negligible. Also implied by the hydrograph separation equation is an instantaneous delivery assumption, which assumes that melt chemistry measured anywhere in the catchment will affect the stream chemistry immediately. The instantaneous delivery assumption is referred to as assumption 6 in this thesis. Two problem assumptions with respect to snowmelt isotopic hydrograph separation that are addressed in this thesis are: 1) both the spatial and temporal variability of assumption 2, and 2) instantaneous delivery implication of assumption 6. 1.2.1 Assumption 2: New water is Temporally and Spatially Constant . Variability in the isotopic chemistry of snowmelt originates from phase changes in water. 18 O is an isotope of water that occurs far less often than 16 O. The δ 18 O of precipitation depends on the history of that water in terms of origin and temperature during phase changes. Since 18 O is a heavier atom, water molecules with 18 O form stronger bonds with other water molecules. Water molecules with 16 O will therefore preferentially melt from ice and evaporate from water leaving a parent phase enriched in 18 O. Colder environments facilitate larger degrees of fractionation because the
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8 molecular structure of water is more organized and more specific to allowing 16 O into it. When water is warmer, the molecular structure is less specific, and allows 18 O to be interchangeable with 16 O. Negative concentrations of δ 18 O reflect the fact that most terrestrial water originates by evaporating from the ocean, thus being depleted in 18 O compared to the parent oceanic water. Methods used to quantify the new water chemistry during snowmelt events have evolved steadily. Little attention was paid to melt chemistry as early studies used an average isotopic value from snow cores collected on one day. Now, studies are beginning to account for complex temporal and spatial isotopic variations in melt water. 1.2.1.1 Snow Cores Early studies collected and melted bulk snow cores to represent the new water chemistry, which was assumed constant in time and space (Rodhe, 1981); (Bottomley, Craig, and Johnston, 1986). However, Taylor, et al. (2002) and Hooper and Shoemaker (1986) conducted field experiments indicating a significant difference in isotopic content between meltwater and snow samples. It is therefore necessary to collect melt samples to characterize new water chemistry. 1.2.1.2 Temporal Variability of δ 18 O Meltwater Melt chemistries collected over time from one location are reported by Martinec (1975) and Dincer, Payne, Florkowski, Martinec and Tongiorgi (1970). Both studies assume a constant mean chemistry value for the new water and state that there is little areal and time variations about the mean value. Genereux (1998) and Hooper and Shoemaker (1986), however, observed an isotopic signal that varies significantly over
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9 time. Taylor, Feng, Kirchner, Osterhuber, Klaue, and Renshaw (2001) conduct laboratory and field experiments on temporal variati ons in δ 18 O and observe an isotopic enrichment during snowmelt (Figure 1.1). Taylor et al. (2002) quantify the errors
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