Erosion, physical degradation or chemical degradation may result from land use and land cover impacts on soil. In turn, these factors may affect water quality. The extent to which this occurs is dependent on the soil suitability, defined by the soil's ability to sustain a land use activity while protecting and maintaining water quantity and quality. Clearcutting of trees can lead to large increments of sediment reaching a nearby stream. These sediments can disrupt the natural riverine ecosystems when introduced rapidly and in large quantities. The amount of sediment may be so large as to blanket the river bed, thus disrupting the feeding and breeding habits of many benthic aquatic organisms. Similarly, the change in turbidity induced by suspended sediment may disrupt the feeding and breeding of many surface feeders and also inhibit sunlight from stimulating plant growth. In many rural areas, where septic systems are used in place of a central sewerage system, the suitability of soils to accept septic effluent is critical. If the soils are not suitable for proper septic system function, the water quality may be compromised.
Depositions (addition to surface) and erosion (removal of surface material) combine to redistribute mineral material. Most particles eroded from one site or kind of soil within a landscape are deposited on another soil at a lower elevation within the same landscape (Ruhe, 1969). While erosion and deposition are natural processes, land use and human induced land use change (direct and indirect) have dramatically influenced their rate and spatial distribution (Buol 1994).
Erosion Potential
Different soils have varying erosion potential. Furthermore, when the soils surface is not protected by a vegetative cover, the rate of erosion increases. The two major agents of soil erosion are water and wind. The most prevelant types of soil erosion by water are:
sheet erosion - a combination
of splash and surface water movement downhill
rill erosion - a movement
of surface water to small depressions where it gains depth and velocity
and initiates erosion
gully erosion - when
both U-shaped or V-shaped channels are formed in soils due to increased
depth, volume and velocity of water
Determining Soil Loss from Runoff
Using a simple formula, the universal soil loss equation, an estimate of soil loss to runoff can be calculated. Five factors are considered in this equation: vegetation, soil type, slope inclination and length, practices, and frequency and intensity of rainfall. Soil erosion in tons per acre per year is calculated by:
A = R x K x S x C x P
Where:
A = soil loss, tons per acre per year
R = rainfall erosion index
K = soil erodibility factor
S = slope factor, steepness, and length
C = plant cover factor
P = practice factor
Intensive rainfalls and total annual rainfall are considered in the rainfall erosion index, developed by USDA. This index represents the annual erosive energy of rainfall that affects the soil surface. Vegetation is considered the most important control on soil erosion; foliage intercepts raindrops, reducing the force at which they strike the soil surface (Marsh 1991). Three soil factors are strongly associated with soil erosion potential: texture, compactness and structure. Of these, texture plays the most dominant role. Intermediate textured soil types tend to be most erodible, whereas clay and particles coarser than sand are more resistant to erosion. Slopes influence the rate and amount of runoff, and in turn these influence erosion.
