Friday, July 27, 2012

Your Backyard Woods - the water cycle

Trees and other vegetation play a key role in the cycling of water from ocean to land and back to ocean. Rainfall entering the soil fills the soil spaces at the surface. Gravity pulls the water deeper into the soil. Most of the water remains in spaces attached to soil particles and humus. Trees and other vegetation remove most of the stored water, creating space in the soil for more rainfall. The water rises as sap through the trunk and branches to the leaves, where most of the water is transferred to the air. For the Continental United States on average, vegetation (mostly trees) returns about 60 percent of the water entering the soil back to the air. If you add evaporation from leaves and other surfaces, a total of about 70 percent of the precipitation that falls on your backyard woods is returned to the air. Only 30 percent of the rain or snow arrives at a stream.

Removing trees from your woods increases the amount of water in the soil, and the amount of water moving to a stream. Less trees results in less water removed from the soil. More than 50 percent of the trees need to be removed from your woods before an increase can be measured. Tree removals can be caused by fires, storms, insect and disease outbreaks, or tree harvests. Most of the streamflow increase occurs during the growing season when streamflows are normally low. The path water takes through your backyard woods remains the same. Water still enters the soil because the roots and litter layer are still present. As new trees grow back, your backyard woods gradually returns to its normal water removal rates.

Changing a woodland to a crop field, pasture, residential, or urban use has a different outcome on the amount of water reaching a stream. Changing the use of the land removes the trees, litter layer, compacts the soil, and in some cases covers the soil with surfaces that prevent water from entering it. Water moves over the surface and more water reaches the stream at a faster rate. Greater streamflow throughout the year increases streambank erosion. Streambank erosion reduces fish habitat and decreases water quality. Increased streamflow continues until the land use is changed back to woodland.

Water Management Practices
A few simple practices can help your backyard woods produce clean water.
• Keep the woods you have and plant more trees in old fields and other open land.
• Plan road and trail locations to reduce the area they cover. Avoid wet soils, and keep them away from streams if possible.
• If you must drive off a road and trail into your woods, do it when the ground is frozen or during dry periods.
• Plan stream crossings to eliminate soil movement into the stream or channel.
• Cover roads and trails with woodchips or gravel, especially on steep sections and on approaches to stream crossings to reduce soil movement.
• Be sure that water flowing off roads and trails enters your woods rather than going into a stream or a ditch that flows into a stream.
• Keep livestock out of your woods, confine them to trails and control the surface runoff, or manage the trees and grass as a system that maintains water movement into and through the soil.
• Maintain a minimum 35 to100-foot wide woody riparian area next to any water on your property.

In the Forest
The greatest threat to water quality in the United States is nonpoint source pollution. Nonpoint source pollution includes soil, nutrients, animal waste, pesticides, and other substances from many places over the landscape. Soil is the principal nonpoint pollutant from forests. Nationwide, only 3 to 9 percent of the total nonpoint pollution comes from forest management practices such as road construction, timber harvesting, planting site preparation, and fireline clearing. Even though forest land is not a major pollution source, pollution from forest land practices should be controlled because forested areas have high quality water and small changes in this quality can have an impact. Best Management Practices (BMPs) have been developed and adopted by the Forest Service, State forestry agencies, and the forest industry. These BMP’s are very similar to the practices recommended for your backyard woods.

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