Water sources: snow and ice

Snow’s Importance in Water Resources

Snow is precipitation in the form of ice crystals. When it falls to the ground and accumulates, it may be considered as water in storage. In Canada, about 36% of the total annual precipitation is in the form of snow. Much of Canada's annual precipitation comes as snow: in the North, 50%; in the Prairies, 25%; and on both coasts and in southern Ontario, as little as 5%.

Snow exerts a marked effect on the distribution of streamflow throughout the year. Instead of immediately infiltrating the soil or running off into stream channels as rainfall does, this water is first stored in the snowpack for several months. When the winter snowpack melts in the spring, it becomes a significant portion of the water available for streamflow. Because snow accumulates during periods when the evaporation loss is low, the relative contribution of its meltwaters to streamflow in some regions may be greater than the flow contributed by rainfall. 

A typical river hydrograph - from basin snowfall to river runoff

The relatively quick melting of snow in spring causes peak flows, sometimes resulting in floods. Some of the worst and most unpredictable flooding occurs when ice that has not yet melted is carried along in the swollen rivers until it jams, blocking the flow of water and creating a lake behind the jam with attendant flooding. When the ice jam breaks, a tremendous amount of water is suddenly released downstream, and more flooding may result.

On the other hand, snow supplies at least one third of the water used for irrigation in the world and is an important contributor to hydropower reservoirs. The fact that snow acts as water storage over the winter and provides soil moisture recharge in the spring is of particular importance to agricultural productivity in some regions.

Snow, Ice and Climate in the North

The cold northern climate slows down many processes in the hydrologic cycle. For example, in the Northwest Territories and Nunavut, where water bodies remain ice-covered for six to ten months of the year, there is little evaporation or precipitation occurring in winter due to the low moisture capacity of air at low temperatures. Runoff from winter snowfall is concentrated in the brief period of spring snowmelt, breakup, and flooding. Most of the highest streamflows in the Northwest Territories and Nunavut occur during spring runoff, except in the Mackenzie Mountains part of western Northwest Territories, where significant summer rainfall floods occur. Melting snow can also contribute to runoff for substantial parts of the summer -- for example it takes about two months for snowmelt to make its way through the Mackenzie River system.

The Yukon has an appreciably different climate. There the ice cover lasts from five to eight months, most of the precipitation is in winter, and evaporation is high. The runoff in the Yukon comes from both snowmelt and glacial melt. Glacial melt causes the characteristic August high water levels in the western Yukon rivers that drain off high mountains. This high water period is critical to the local ecosystems.

Glaciers - nature's frozen rivers

A huge quantity of fresh water is frozen in polar ice caps and in high mountain glaciers. Snow that is packed down over many years at high elevations becomes glacial ice, which slowly proceeds downslope like a frozen river, under the pull of gravity, and eventually melts to become part of streamflow at lower elevations. If the rate of melting is greater than the rate of accumulation, the glacier recedes; if it is less, the glacier advances.

Glaciers exert a direct influence on the hydrologic cycle by slowing the passage of water through the cycle. Like lakes and groundwater reservoirs, glaciers are excellent natural storehouses, releasing water when it is needed most. Glaciers, however, can release water when you need it least. Glacier-outburst floods, called jökulhlaups, can be devastating. Glacier-fed rivers reach their peak during hot summer weather.

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