The Water Cycle - Water Science for Schools
Parts of the Water Cycle
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The Water Cycle: Snowmelt Runoff
If you live in Florida or on the French Riviera you might not wake up everyday wondering how melting snow contributes to the water cycle. But, in the world-wide scheme of the water cycle, runoff from snowmelt is a major component of the global movement of water. Of course, the importance of snowmelt varies greatly geographically, and in warmer climates it does not directly play a part in water availability. In the colder climates, though, much of the springtime runoff and streamflow in rivers is attributable to melting snow and ice.
Mountain snow fields act as natural reservoirs for many western United States water-supply systems, storing precipitation from the cool season, when most precipitation falls and forms snowpacks, until the warm season when most or all snowpacks melt and release water into rivers. As much as 75 percent of water supplies in the western states are derived from snowmelt.
During certain times of the year water from snowmelt can be responsible for almost all of the streamflow in a river. An example is the South Platte River in Colorado and Nebraska. Historically, the South Platte River was essentially "turned off" after the supply of water coming from melting snow was exhausted in late spring. Today, though, seepage of irrigation water from ditches and fields replenishes the alluvial aquifer (water-bearing deposit of sand and gravel left behind by a river) during spring and summer, and the aquifer slowly drains during fall and winter by discharging groundwater to the South Platte River. Indirectly, your buying a loaf of wheat bread in the grocery store helps to keep water flowing in the South Platte River all year long.
Contribution of snowmelt to streamflow
A good way to visualize the contribution of snowmelt to streamflow in rivers is to look at the hydrograph below, which shows daily mean streamflow (average streamflow for each day) for four years for the North Fork American River at North Fork Dam in California (USGS real-time streamflow data). The large peaks in the chart are mainly the result of melting snow, although storms can contribute runoff also. Compare the fact that minimum mean-daily streamflow during March of 2000 was 1,200 cubic feet per second (ft3), while during August streamflows ranged from 55-75 ft3.
Note that runoff from snowmelt varies not only by season but also by year. Compare the high peaks of streamflows for the year 2000 with the much smaller streamflows for 2001. It looks like a major drought hit that area of California in 2001. The lack of water stored as snowpack in the winter can affect the availability of water for the rest of the year. This can have an effect on the amount of water in reservoirs located downstream, which in turn can affect water available for irrigation and the water supply for cities and towns.
Snowmelt and flooding
The effect of snowmelt on potential flooding, mainly during the spring, is something that causes concern for many people around the world. Besides flooding, rapid snowmelt can trigger landslides and debris flows. In alpine regions like Switzerland, snowmelt is a major component of runoff. In combination with specific weather conditions, such as excessive rainfall on melting snow for example, it may even be a major cause of floods. In Switzerland, snowmelt forecasting is being used as a flood-warning tool to predict snowmelt runoff and potential flooding.
In some parts of the world, such as in Washington State in the Pacific Northwest of the United States, annual springtime flood events occur when rain falls on existing snowpacks, known as a "rain-on-snow event." Runoff during rain-on-snow events has been associated with mass-wasting of hillslopes, damage to riparian (areas alongside streams) zones, downstream flooding and associated damage, and loss of life. Some studies suggest that the amount of forest cover can have an influence on the magnitude of rain-on-snow events.
In January 1996, a combination of factors contributed to massive flooding in the northeastern United States. Heavy snowfall followed by a sudden thaw and heavy rain caused floods along rivers from New York through Pennsylvania to Virginia, producing water levels not seen since a major hurricane, Hurricane Agnes, hit the area in June 1972. Major rivers in Pennsylvania and the Potomac River were affected. The raging rivers, sometimes jammed with ice, caused a number of deaths and required many people to evacuate their homes. Ice blocks carried by the floodwaters exacerbated the damage done to buildings, bridges, and dams.
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