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Precipitation & Water Supply
Image of some clouds and sea mist around the Olympic Mountains, WA.  This image links to a more detailed image.Water Supply Factors such as snow melt and rainfall are obvious contributors to the water supply of a given area. In the coastal regions of the Northwest, a less obvious yet highly significant source of water is fog drip. Right: Clouds and sea mist around the Olympic Mountains, WA. Photo: Ed Shay

As chilled moist ocean air is pulled inland, its moisture transforms into a layer of low-lying fog. The fog passes through the trees, and the sharp slender needles of the conifers provide numerous points for condensation. The resulting droplets drip from the needles to the ground beneath adding valuable moisture to the watershed.

An example of the significant role fog drip plays in contributing to water supply comes from data generated by the Portland Bureau of Water Works. The average annual rainfall for Portland is approximatly 40 inches while average total runoff for the Bull Run watershed, Portland's primary water source, is 135 inches. The research project conducted within the Bull Run Municipal watershed area and quoted below demonstrates that a substantial portion of this increased water supply comes from fog drip associated with an old-growth forest.

During a later forty-week experiment, Harr (1982) found that the Net precipitation on the ancient forest was 1,739 mm (68 in.), 29 percent more than in eleven-year old clearcuts. On an annual basis, the net precipitation in old-growth was estimated at 2,494 mm, 25 percent higher than in the clearcut watersheds, with the greatest difference (202 mm, or 44 percent) occurring in late spring and summer when water demand is highest. Fog drip contributed an estimated 35 percent of annual precipitation under the old-growth canopy. (Norse, 1990, p. 147)."Granted with permission from Ancient Forests of the Pacific Northwest, E.A. Norse, The Wilderness Society, 1990. Published by Island Press, Washington, DC and Covelo, CA. For more information, contact Island Press directly at 1-800-828-1302, info@islandpress.org (E-mail), or www.islandpress.org (website)."

Although it is not what might be expected, one of the consequences of cutting trees in an area is a decrease in water supply and stream flow.

Precipitation Activity
1. On a simple road map of the Olympic Peninsula, find the four locations in Washington for which you have precipitation data: Forks, Hoh Rainforest, Port Angeles, and Sequim.

2. Look at a topographical map of the same area.

3. Examine the precipitation data charts for the four locations:

Forks, Wa.
Hoh Ranger Station
Port Angeles, Wa.
Sequim, Wa..

4. Do you see a pattern in the precipitation data? What might be the relationship between the topographic information and the precipitation data? How might the topography of this area impact the seasonal differences in precipitation?

Hint: Dry weather comes with the summer months and is associated with high-pressure weather systems flowing across the peninsula from the northwest. Rain returns to the Olympic forests in October, when weather systems begin to flow from the southwest.

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