Variability and Trends in Spring Runoff in the Western United States
نویسندگان
چکیده
In the western United States, over half of the water supply is derived from mountain snowmelt, with the snow acting as a natural reservoir, delaying runoff and providing runoff in the spring and summer when it is needed most. Interannual variability of both the magnitude and timing of spring runoff is tremendous, and western states have developed extensive reservoir systems to store water from wet years in order to weather droughts. However, important changes in snowpacks and runoff timing have been noted in recent decades. The fraction of annual streamflow that runs off during late spring and summer has declined by 10 to 25%. Warmer winters and springs have led to earlier snowmelt and a higher percentage of precipitation falling as rain rather than snow. Snowmelt runoff timing has advanced so that it arrives approximately one to three weeks earlier in 73% of mountainous catchments across western North America. Even conservative climate-change projections suggest that California could lose one third of its present day spring snowpack by the middle part of the 21 century, and other western states will follow suit. Hydrologists have long known that snow is a key component of the water budget, and yet most operational runoff models are based on historically derived empirical relations, rather than on spatially and temporally distributed physical processes. In a changing climate, the empirical relations may not remain valid, so more observational attention to the high altitude snowfields that supply so much of the region’s water supplies is needed to provide a basis for predicting the magnitude and timing of snow melt, sublimation and runoff.
منابع مشابه
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