With this approach, a second site (a baseline reference site) in a very similar watershed is monitored. Changes in the BMP site are compared to this baseline. The monitoring approach is greatly strengthened if data are collected at both sites before AND after BMP implementation (referred to as BACI, or Before After Control Implementation design).
The reference site is similar enough to the sample location site that it will provide a baseline for comparison. This should be evaluated before implementation of the BMP.
Data that are collected at the same time can be compared so water-year differences and other seasonal and annual variation is reduced.
It can be difficult to find a good reference site, particularly if seeking an “unimpaired” or “natural” site.
Be careful about comparing sites that are not sufficiently similar. At a minimum, streams of a similar order, flow, geology, stream type, elevation, and land use should be compared. The size, shape, geology, climate, vegetation, and land use of a watershed are all factors that can affect the response of the system.
When to use this approach:
Use this when looking at long-term indicators (biological indicators).
One of the challenges of this monitoring approach is finding adequate reference sites. If this approach is used to directly compare chemical data, the conditions under which the monitoring takes place should be considered (for example, baseflow compared to storm event runoff).
It may be necessary to monitor upstream and downstream monitoring to assure that all potential impacts are captured.