BMPs. Correlation of water quality and land treatment changes by itself is not sufficient to infer causal
relationships. Other factors not related to BMPs may be causing water quality changes, such as
changes in animal numbers, cropping patterns, land uses, known pollutant sources, or amount of
impervious land surface; season; stream discharge; precipitation; ground water table depth; salinity; or
other climatic or hydrologic variables. Factoring explanatory variables into trend analyses yields water
quality trends closer to those that would have been measured had no changes in climatic or other
explanatory variables occurred over time. Accounting for variability in water quality due to known
causes also decreases variation in adjusted water quality data, facilitating documentation of statistically
significant trends. Explanatory variables should be monitored at the same frequency as the principle
water quality variables.
A good experimental design for water quality and land treatment monitoring is essential in order
to provide clear documentation of the relationship between land treatment and water quality changes.
The paired watershed monitoring design can best demonstrate the relationship between land treatment
and water quality in the shortest period of time.
To determine if the trends in water quality match the mechanistic prediction of trends, pre- and
post-BMP implementation monitoring and data analysis must combine water quality, land treatment,
and land use data on suitable spatial and temporal scales. Incorporation of explanatory variables
facilitates isolation of water quality changes that result from land treatment.
2.2.3 CASE STUDY
RCWP examples are available in "Evaluation of the Experimental Rural Clean Water Program,"
shown at the end of this chapter.
Clausen, J.C. and J. Spooner. 1993. Paired Watershed Study Design. Office of Water, US
Environmental Protection Agency, Washington, DC. EPA 841-F-93-009. 8 p.
Coffey, S. W., Spooner, J., and Smole, M. D. 1995 (Feb). "The Nonpoint source manager's Guide
to Water Quality and Lane Treatment Monitoring," North Carolina Cooperative Extension Service,
North Carolina State University, Raleigh, NC, under a grant from the Assessment and Watershed
Protection Division, US Environmental Protection Agency, Washington, DC.
Gale, J. A., Line, D. E., Osmond, D. L., Coffey, S. W., Spooner, J., Arnold, J. A., Hoban, T. J., and
Wimberley, R. C. 1993. "Evaluation of the Experimental Rural Clean Water Program," North