Impacts on water resources

One of the most dramatic impacts of changing snow cover is on water resources. Snow cover in mountain regions provides critical water supplies, serving nearly one-sixth of the global population with freshwater for domestic, agricultural and industrial uses79. Much of the arid American West79 and Central Asia80 (Figure 4.10) depends heavily (about 75-85 per cent) on snow melt to supply water for municipalities and agriculture. Snow melt driven water resources are crucial for generation of hydroelectric power, particularly in the American West, Canada, and Europe81,82. The declining springtime snow cover in the Pacific Northwest of the United States and rising snowlines projected for many mountain areas, noted in the 'Trends and outlook' section above, threaten these critical water supplies.

Mountain snow cover typically develops in the autumn and grows to a maximum depth in early spring (Figure 4.11). As day length and sun angles increase, so do air temperatures, causing snow cover to warm and begin to melt. Snow cover balances the availability of water in mountain environments. Where winter precipitation falls as rain, surface runoff occurs almost immediately. In contrast,

Figure 4.10: Snow cover provides critical water supplies used for many purposes.

(a) Melting of prairie snow cover, seen here in Saskatchewan, Canada, provides spring ponds that are essential for recharge of groundwater and soilwater, for farm water supplies and as spring wetlands for waterfowl migrations through what is otherwise a semi-arid environment.

(b) Melting of alpine snow cover forms a small stream in the Rocky Mountains in British Columbia, Canada. This segment of the Rocky Mountains contains the headwaters of the Columbia River, which supplies water to a large area of western Canada and northwestern United States including many important irrigation and hydroelectric generation projects.

Photos: J. Pomeroy

Figure 4.10: Snow cover provides critical water supplies used for many purposes.

(a) Melting of prairie snow cover, seen here in Saskatchewan, Canada, provides spring ponds that are essential for recharge of groundwater and soilwater, for farm water supplies and as spring wetlands for waterfowl migrations through what is otherwise a semi-arid environment.

(b) Melting of alpine snow cover forms a small stream in the Rocky Mountains in British Columbia, Canada. This segment of the Rocky Mountains contains the headwaters of the Columbia River, which supplies water to a large area of western Canada and northwestern United States including many important irrigation and hydroelectric generation projects.

Photos: J. Pomeroy

Snow water equivalent (mm)

Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep

Figure 4.11: Mean monthly snow water equivalent, a common snow pack measurement, for Columbine Pass, Colorado from 1971 to 2000, showing a typical seasonal pattern in a mountain environment.

Source: Based on data from the U.S. National Resources Conservation Service

snow stores water during the winter and then melts in the spring and early summer, creating peak stream flows in the afternoon and an overall seasonal peak flow. In many semi-arid mountain environments, snow melt buffers the transition into the dry summer season. Mountain snow is also a key source of groundwater, since a significant portion of the snow melt enters the soil and drains downhill into valley sediments83. The timing, spatial distribution, and volume of snow melt are critical for determining how much water flows as surface runoff into rivers and lakes and how much becomes groundwater. Earlier snow melt across the western United States, for example, caused a one to four week earlier runoff for mountain rivers and longer periods of summertime low-flow84.

Continue reading here: Impacts on agriculture crops and animal husbandry

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