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Summer/Fall 2008 Vol. 8 No. 2

Table of Contents

Through the Woods

Stream running through Cottonwood Canyon, Sedona, Ariz., ©Stockbyte

Forest Management Important for
Fresh Water Supplies

Upon first guess, most people would think lumber is the most important byproduct of forests, but another output often overlooked is fresh water. Forests process nearly two-thirds of the fresh water supply in the United States by cycling precipitation through the soil, and ultimately delivering water through streams and rivers to larger bodies of water. As demand for water increases, supply managers question whether different approaches to land use in forests can help meet downstream demands.

Research has already produced a solid foundation of knowledge about how water is connected to and moves through forests and how forest structure and composition can alter water quantity and quality. A new report from the National Research Council says, however, that future research should move beyond comprehension to predicting how future changes in landscape will impact forest hydrology.

For example, some effects of climate change on forests and water are already evident -- such as changes in the snowmelt and increases in wildfires -- and future climate changes are likely to have major effects on forest hydrology. But research should explore the direct and indirect effects of climate change on water yield and quality, as well as the consequences of wildfires and disease. Furthermore, studies should examine how fire and insect outbreaks vary over time and spatial scales and how these affect water quantity, quality, and flooding.

In addition to naturally occurring phenomena, manmade activities such as timber harvesting, chemical applications, and roads also have an impact. Partial or complete removal of the forest reduces the canopy and, in turn, the amount of water the soil absorbs. Replacing these trees with new ones also reduces the water yield, as young trees need ample water to grow to maturity.

Roads and trails that have impervious surfaces affect water timing and quality, but the magnitude of the effect depends upon road design, slope steepness, soils, and the configuration of the road system relative to the stream and river drainage networks. Chemicals such as fertilizers and fire retardants affect water quality, acidify forest soils, and deplete soil nutrients. To better understand implications of human actions, the report recommends scientists develop a next generation of hydrologic models and use remote sensing.

Most importantly, to ensure progress, watershed councils and citizen groups should work with agencies to better protect and sustain water resources.   -- Jennifer Walsh

Hydrologic Effects of a Changing Forest Landscape. Committee on Hydrologic Impacts of Forest Management, Water Science and Technology Board, Division on Earth and Life Studies (2008, approx. 194 pp.; ISBN 0-309-12108-6; available from the National Academies Press, tel. 1-800-624-6242; $39.00 plus $4.50 shipping for single copies).

The committee was chaired by Paul K. Barten, associate professor of forest resources at the University of Massachusetts, Amherst. The study was funded by the U.S. Forest Service and the Bureau of Reclamation.

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Copyright 2008 by the National Academy of Sciences