How does the NAWQA design and sampling-site selection differ from a probabilistic-monitoring design?
NAWQA uses a “targeted” monitoring design in which sites are not selected randomly within a grid, but because the represent certain human activities, land uses, environmental settings, or hydrologic conditions during different seasons or times of year (see question above). Such monitoring is useful to answer questions related to water-quality conditions and the natural and human factors that cause those conditions. Probabilistic monitoring involves random selection of sites across a certain geographic area. Such monitoring is useful for getting an unbiased, broad geographic snapshot of “whether or not there is a problem” and “how big the problem is.” Many probabilistic monitoring programs implemented by the States and the U.S. Environmental Protection Agency (EPA) provide quantitative, statistically valid estimates of, for example, the number of impaired stream miles with a region or State (http://www.epa.gov/emap/) . Probabilistic monitoring and more “targeted” monitoring (such as by
