Watershed Impacts from Bioenergy

Figure 2.1. The major watersheds of the United States.  Source: http://media.mgbg.com/wkrg/photos/weather/watershed_US.JPG A local watershed map may be obtained at: http://cfpub.epa.gov/surf/locate/index.cfm

Watershed Level Water Quality and Quantity Impacts

The land area in which all the water drains to the same place is called a watershed. There are major watersheds (or drainage basins), sub-watersheds and sub-sub watersheds (see figure 2.1).

How water moves in a watershed affects the water quality and quantity of the watershed. Stormwater runoff is rainwater or snowmelt that moves over the land and empties into ditches and streams leading to rivers. Ultimately, the bulk of runoff in the United States goes into the oceans. Agricultural farms and fields, developed urban areas and construction sites, green spaces such as parks and forests—all play a role in the health of the watershed.

Plants slow runoff and promote infiltration. Plantings by a waterway or a ditch that leads to a body of water help determine the amount of runoff that will occur at the site. Slope, soil type and vegetation affect the speed of runoff and amount of erosion. Non-vegetated, steep slopes with poorly permeable soils promote extensive runoff carrying sediment. Stream sedimentation negatively affects oxygen levels, light penetration, sites for fish egg laying, function of fish and invertebrates’ gills, fish and macroinvertebrate feeding, and cover and aquatic plant growth. Not only may sediment clog a stream, but it also brings increased nutrients causing increased aquatic plant growth and algae. The Environmental Protection Agency identifies sediment as the number one water pollutant.

The type of vegetation or crops in the watershed affects the amount of runoff. The Natural Resource and Conservation Service has created runoff curve numbers (RCN) for various surface covers on unfrozen ground. A RCN is estimated for various types of cover based on the hydrologic condition – a combination of factors affecting infiltration and runoff: density and canopy of vegetative areas, amount of year-round cover, amount of grass or closed-seeded legumes in rotations, percent of residue cover on land surface [good is > 20 percent cover] and degree of roughness. The RCN provides a guide to the relative amount of runoff from various land covers, ranging from woods (forest) cover to row crops. In each case a greater RCN indicates the potential for a greater runoff quantity. For example, a site with annual crops and no cover crop would have greater runoff than a forest or meadow. Based on the RCN, a perennial grass field, protected from grazing and mowed would have a runoff potential as low as a good woods cover. Perennial crops, native grasses and forest cover slow runoff, reduce the inputs of sediments and chemicals to water systems and increase infiltration in the watershed [1].

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