It is possible to identify all of the subwatersheds of a river. The use of these smaller units to assess water quality increases the likelihood of identifying most of the sources of NPS pollution and allows easier determination of the dominant contributors to water pollution. Solving the problems associated with NPS pollution presents serious challenges. NPS pollution does not originate from a single, easily identifiable source but rather it originates from diverse sources. An understanding of how water quality is affected by land use and the type of cover present in a given area is vitally important.
Most people manage land in small units such as a field, a stand of trees, a pasture, or a home site.
The activities that take place in these small areas usually have only minimal impact on local water quality. However, when the effects of management activities on each small area within a watershed are added together, the impact can be substantial. This added-together effect is referred to as cumulative. Addressing cumulative impacts is generally much more complicated than addressing the problems of a single water pollution source. Making management decisions to avoid problems associated with cumulative impacts is extremely difficult.