Tool 1: Land Use Planning
Since impervious cover has such a strong influence on watershed quality, a watershed manager must critically analyze the degree and location of future development (and impervious cover) that is expected in a watershed. Consequently, land use planning ranks as perhaps the single most important watershed protection tool. When preparing a watershed plan, a watershed manager needs to:
- Predict the impacts of future land use change on water resources.
- Obtain consensus on the most important water resource goals in the watershed.
- Develop a future land use plan that can help meet these goals.
- Select the most acceptable and effective land use planning technique to reduce or shift future impervious cover.
- Select the most appropriate combination of other watershed protection tools to apply to individual subwatersheds.
- Devise an ongoing management structure to adopt and implement the watershed plan.
Land use planning is best conducted at the subwatershed scale, where it is recognized that stream quality is related to land use and consequently impervious cover . One of the goals of land use planning is to shift development toward subwatersheds that can support a particular type of land use and/or density. The basic goal of the watershed plan is to apply land use planning techniques to redirect development, preserve sensitive areas, and maintain or reduce the impervious cover within a given watershed.
Understanding the concept of impervious cover is essential to understanding one of the biggest ways that urbanization impacts our streams. Impervious cover is defined as the sum total of all hard surfaces within a watershed including rooftops, parking lots, streets, sidewalks, driveways, and surfaces that are impermeable to infiltration of rainfall into underlying soils/groundwater. Impervious cover changes the natural landscape and is a major influence on aquatic resources because instead of allowing precipitation to permeate the ground, it runs off. Generally the more impervious surface there is in a watershed, the less the amount of groundwater recharge, plus the more runoff and related erosion in streambeds from greater flow. Further, studies show a direct correlation between the percentage of impervious cover in a watershed and the level of degradation to aquatic organisms. Streams degraded by high percentages of impervious surface in their watersheds are often prone to larger and more frequent floods (which cause property damage as well as ecological harm) and lower base flows (which degrade or eliminate fish and other stream life, as well as reduce the aesthetics of the stream). Impervious surfaces also raise the temperature of runoff, which reduces dissolved oxygen in the stream, harms some gamefish populations, and promotes excess algal growth.
Classification of subwatersheds by the amount of impervious cover is one step toward determining land use planning goals. Although presence of vegetated streamside buffer zones or wetlands can help counteract impervious cover impacts, a watershed exceeding 10% impervious cover will generally not be able to support a high quality stream system. In this particular classification system, subwatersheds with impervious cover of less than 10% are classified as sensitive. A subwatershed with 10 - 25% impervious cover is classified as a degraded or impacted system. Any stream's watershed having greater than 25% impervious is classified as a non-supporting stream with characteristics such as eroding banks, poor biological diversity, and high bacterial levels.