![[logo] US EPA](http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Spatial Coverage
The spatial array of sampling sites in a given watershed or geographic region will determine to what extent we can extrapolate biological condition and water quality to areas beyond the exact sites. Two primary guidelines can be identified for extrapolating biological assessment data to whole watersheds. First, the structure of aquatic assemblages in aquatic systems naturally and predictably changes with an increase in size of the ecosystem (Vannote et al. 1980). Thresholds in this continuum of change can be established through an analysis of regional databases. The biological condition at any particular site should only be used to represent areas of similar physical dimensions and flow characteristics (for streams and rivers). Likewise, lake or wetland size will influence the number of sites needed to adequately characterize these waterbody ecotypes. For instance, in small lakes, one site will generally be sufficient. In large lakes with multiple watersheds or in reservoirs with various zones (inflow, midsection, outflow, backwaters), a site representative of each basin or zone may be needed. Second, the change in land use patterns along a stream gradient or lake shoreline should be considered. Changes from agricultural land use to urban centers, forested parkland, etc., would warrant different representative sampling sites. A waterbody with multiple dischargers may also require numerous sampling sites to characterize not only the overall biological condition of the waterbody but the impact of individual sources as well. The spatial density of sites within a watershed and the placement of those sites with respect to watershed area is another important consideration.
Available resources and the desired outcome of the sampling design are key determinants in achieving adequate coverage. Multiple designs are more likely to achieve the multiple objectives of State and Tribal water quality programs and will address needs beyond the determination of status. For instance, most States sample only a tiny percentage of their channel lengths or lake areas. To obtain unbiased estimates of condition in these situations, a probabilistic sampling design is advantageous for statistical inference of status and trends to all waters from a relatively small sample. Sites or streams that represent the resource population of interest are selected randomly. Thus, we can extrapolate from the survey results to the entire population of waterbodies. For instance, streams are identified by resource type, i.e., intermittent, perennial, etc., and further stratified by size to obtain a framework for randomizing the streams to be sampled in the resource population of interest. This design is cost effective in that the entire resource does not have to be sampled – only a representative number of streams. This sampling design was developed by USEPA's Environmental Monitoring and Assessment Program (EMAP) and has been used to assess the ecological status of waters on basin-wide, statewide, regional, and national scales (Paulsen and Linthurst 1994; Hughes et al. 2000; MAIA USEPA 2006). The statistical-survey design is required for State/Tribal Water Monitoring and Assessment Programs (USEPA 2003, FR 73[138]:41069-41076). Rotation of sampling among watersheds has also been found to be an effective approach for statewide monitoring over a specified period, which organizes the collection of ambient data in a timely fashion. However, where temporal and climatic variations dictate, this design may require modification.
Some Frequently Asked Questions
Question: How do we know what particular spatial design is most appropriate?
Answer: Spatial designs can take several forms, depending on the question (assessment or causal). In the Big Darby Creek example (Figure 10-1), a geometric and intensive survey design was used at the 11-digit HUC scale. The Fish IBI and Mussel Species Richness were reported at this scale. This design is important in the implementation of Tiered Aquatic Life Uses (TALUs). In the Big Darby, out of 43 tributaries that were originally designated Exceptional Warmwater Habitat (EWH) based on a default tributary membership (1978), only 16 were confirmed as EWH based on later monitoring using numerical biocriteria and TALUs in the Ohio WQS.
Question: What design is best for my 305(b) assessments?
Answer: If an intensive survey design that comprehensively covers a watershed is not possible, a probabilistic sampling design is most appropriate to provide a scientifically sound means of assessing attainment of uses. Some states apply a probabilistic sampling design to their rotating basins, and, thus, sample their state waters over a period of years.
Question: What spatial design should I use to address 303(d) listings?
Answer: A targeted design that is specifically tailored to assess the extent and magnitude of the pollutant in the waterbody is recommended. Depending on the configuration of the watershed (e.g., 8-digit HUC, 11-digit HUC) the number of sites will vary from a few (<5) to substantially more (>10). In any case, the regional reference condition established, independent of the TMDL, should serve as the benchmark.
Example of Spatial Design in using a Geometric Watershed Design, i.e., Big Darby Creek, Ohio. Implications of having a sufficient density and coverage of sites to the designation of aquatic life uses is discussed in the Ohio case study in USEPA (2005).
References
Hughes, R.M., S.G. Paulsen, and J.L. Stoddard. 2000. EMAP-surface waters: a national, multiassemblage, probability survey of ecological integrity. Hydrobiologia 423:429-443.
Paulsen, S.G. and R.A. Linthurst. 1994. Biological monitoring in the environmental monitoring and assessment program. Pages 297-322 in S.L. Loeb and A. Spacie (editors). Biological Monitoring of Aquatic Systems. Lewis Publishers, Boca Raton, FL.
USEPA. 2006. Mid-Atlantic Integrated Assessment (MAIA) State of the Flowing Waters Report. EPA-620-R-06-001. U.S. Environmental Protection Agency, Office of Research and Development, Washington, D.C.
USEPA. 2005. Use of Biological Information to Better Define Designated Aquatic Life Uses in State and Tribal Water Quality Standards: Tiered Aquatic Life Uses. EPA-822-R-05-001. Office of Water, Washington, D.C.
USEPA. 2003. Elements of a State Water Monitoring and Assessment Program. EPA-841-B-03-003. U.S. Environmental Protection Agency, Office of Water, Washington, D.C.
Vannote, R.L., G.W. Minshall, K.W. Cummins, J.R. Sedell, and C.E. Cushing. 1980. The River Continuum Concept. Can. J. Fish. Aquat. Sci. 37:130-137.
For additional information on this topics, please visit