Watershed Primer Part 4 - Land Use Impacts & Watershed Economics
Impacts of Land Use on Watershed Resources
Everything we do on the land surface impacts the quality or quantity of water resources (ground water and or surface water) to some degree. Disturbing a site with grading for construction compacts the soils under the cleared area and reduces the amount of water that can infiltrate as ground water recharge. Constructing swimming pools, buildings, roads, sidewalks, parking areas and other features that are "impervious" (do not allow water to penetrate through them and into the underlying soils) eliminate ground water recharge and convert that volume of water to stormwater runoff. Agricultural lands, depending on the farming practices used, can generate large amount of rainfall runoff and reduce infiltration.
Chemicals intentionally applied to the land (fertilizers and pesticides for example), if applied in quantities greater than what is taken up by the target plants, become suspended in runoff from rainfall events and become "pollutants" that are carried into the streams. Buildup of such chemicals in the soil can cause them to be leached from the soil by rainfall infiltrating through the soils, and then are carried as pollutants into the underlying aquifers.
Substantial research has (and continues to) document the often unnecessary impacts of land uses on watershed resources. Land uses do not need to be discontinued. There are techniques to accomplish all desired land uses in a manner that helps to sustain rather than deteriorate these water resources.
One of the primary indicators of watershed "health" is the percent of impervious cover in the watershed. Based on numerous research efforts, studies and observations, a general categorization of watersheds has been widely applied to watershed management based on percent impervious cover (Schueler 1995). These are summarized in table 3
Table 3: Impervious Cover as an Indicator of Stream Health (Schueler 1995)
|Percent Impervious Cover||0% to 10%||11% to 25%||26% to 100%|
|Channel Stability||Stable||Unstable||Highly Unstable|
|Water Quality||Good to Excellent||Fair to Good||Fair to Poor|
|Stream Biodiverisity||Good to Excellent||Fair to Good||Poor|
|Pollutants of Concern||Sediment and temperature only||Also nutrients and metals||Also bacteria|
Table 4 summarizes several of the impacts of traditional development on streams and watersheds, most of which are created by the addition of impervious cover across the portions of the land surface. Two figures, one comparing hydrographs of conventional stormwater management with innovative management and a second on the potential impacts of development on stream channels, illustrate the changes to the volume and duration of runoff as well as the physical stream channel before and after development. The figure comparing the two hydrographs also illustrates the benefits of using various BMP’s and low impervious techniques to managing stormwater. As the illustration on potential impacts depicts, traditional development within a watershed may raise the elevation of the floodplain limit and reduce summer low flows when compared to predevelopment conditions.