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Home > Hydrologic Effects > Temperature, Snowpack, and Runoff
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There is a high level of confidence in projections of warmer temperatures over most land surfaces. Unlike their projections of precipitation change, climate models are fairly consistent in predictions of regional surface temperature. Because temperature is central in determining the accumulation and melting of snow and ice, these scenarios are especially relevant to regions where snowpack or glacial runoff dominate the hydrology. In a warmer climate, it is very likely that a greater portion of winter precipitation will fall as rain rather than snow, especially in areas where winter temperatures are now only slightly below freezing. An increase in rain events would increase winter runoff but result in smaller total snowpack accumulations. Temperature also determines the timing of melt-off, and a warmer climate will likely result in an earlier melt season. Many regions are likely to see an increase in winter or spring flows and reduced summer flows. In fact, there is evidence that this is already occurring. Studies by Cayan et al. (2001) and Stewart et al. (2004) document the fact that the peak in spring runoff has been arriving earlier in the last few decades (Figure 1).
Figure 1. Centers of mass of yearly streamflow hydrographs in rivers throughout western North America, based on US Geological Survey streamflow gaging stations in the United States and and an equivalent Canadian streamflow
Warmer temperatures could increase the number of rain-on-snow events in some river basins, increasing the risk of winter and spring floods (Lettenmaier and Sheer 1991; Hughes et al. 1993). In currently glaciated basins, declining glacier reservoir capacity may eventually lead to an earlier peak of seasonal runoff and reduced late-summer streamflows. In some cases, increased melting of glacial ice can sustain summer streamflows in the near term but will deplete this source in the long run (Pelto 1993).
The loss of snow mass from sublimation (sublimation is the change of ice to water vapor, bypassing the liquid phase) is a critical part of basin-scale water budgets in snow-dominated regions, but is an understudied topic. In exposed landcover regions such as prairie and tundra, Pomeroy and Gray (1995) estimated sublimation loss of blowing snow to be 15-41 percent of annual snowfall. Sublimation attributed to radiative energy tends to be greater in areas with less cloud cover (e.g., the Southwestern US ), as sublimation is enhanced under direct sunlight, since photons of solar energy add the energy necessary for solid ice molecules to escape. On the eastern slopes of the Rocky Mountains, the warm and dry Chinook or “snow-eater” winds will quickly sublimate a snowpack, leading to unexpected reductions in basin water budgets. It is unclear how climate change could affect sublimation dynamics, since all three forces that contribute to sublimation (solar forcing, wind, and blowing snow) could change under anthropogenic warming. |
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