what roles does the forest play in watershed
Answers
Answer:
The Role of Trees and Forests in Healthy Watersheds
- Managing stormwater,
- reducing flooding, and
- improving water quality.
Pennsylvania contains almost 83,000 miles of rivers and streams, ranging from small trickles to large rivers. These waterways are important because they provide water for people, farms, and industries; provide habitat for many kinds of wildlife and fish; and also provide us with great places to fish, swim, and boat.
As our landscape changes, it begins to have an impact on stream health. What we do on or to the land affects both the quantity (volume) and quality (pollutant levels) of the water in our streams and lakes. The land area through which any water moves, or drains, to reach a stream is called a watershed.
As we begin to remove forest canopy and replace it with roads, parking lots, driveways, homes, patios, pools (impervious surfaces) and even grass, we immediately have impact on watersheds and receiving streams (or lakes). With the increased amount of impervious surfaces, water runs off the land, traveling on the surface towards the streams. As this 'storm water runoff' travels to the streams it collects pollutants and increases speed. The changes to the landscape, not only increase the volume of water that goes to the stream, it also shortens the amount of time it takes the water to get to the stream. These increased or peak flows cause water to move quickly to the streams. This leads to flooding, stream bank erosion, widening of streams, sediment deposited in streams, a loss of fish habitat, and decline in water quality. In Pennsylvania there are over 12,200 miles of polluted streams and over 3,000 miles of streams that are impaired by storm water runoff.
Forests filter and regulate the flow of water, in large part due to their leafy canopy that intercepts rainfall, slowing its fall to the ground and the forest floor, which acts like an enormous sponge, typically absorbing up to 18 inches of precipitation (depending on soil composition) before gradually releasing it to natural channels and recharging ground water. In a North Carolina Watershed study (Kays, 1980) the mean soil infiltration rate went from 12.4 in/hr to 4.4 in/hr when a site was converted from forest (duff layer on soils) to suburban turf. Other studies (Bharati et al. 2002) have found similar results when comparing hourly infiltration rates and soil bulk density of forested areas with crops and grazed pasture.
Average interception of rainfall by a forest canopy ranges from 10-40% depending on species, time of year, and precipitation rates per storm event. In urban and suburban settings a single deciduous tree can intercept from 500 to 760 gallons per year; and a mature evergreen can intercept more than 4,000 gallons per year. Even young, small trees help. In a recent Forest Service study a single small tree (callery pear) that was only 9 years old, was able to intercept 58 gallons of storm water from a ½ inch rain event (67% of the rain that fell within the canopy).
The role of trees and forests in managing stormwater and protecting water quality is just beginning to be understood by some engineers, planners and community leaders. One of the most powerful statements that help support this came from the Chesapeake Bay Executive Council in 2006 and reads:
'Forests are the most beneficial land use for protecting water quality, due to their ability to capture, filter, and retain water, as well as air pollution from the air. Forests are also essential to the provision of clean drinking water to over 10 million residents of the watershed and provide valuable ecological services and economic benefits including carbon sequestration, flood control, wildlife habitat, and forest products'.