Monitoring for policy-relevant regional trends over time

The term trend describes the continuing directional change in the value of an indicator, generally upward or generally downward. Many policy questions concern trend across a number of sites, such as lakes in a region, rather than trend at a single site. Power to detect regional trend seldom is discussed, and monitoring designs suitable for detecting such trends rarely are explored. Components of variance and temporal sampling designs play central roles in characterizing trend detection. We present relative numerical values of important components of variance, developed from the Surface Waters component of U.S. EPA's Environmental Monitoring and Assessment Program (EMAP) field data, and use them as a basis for further assumptions of values. We discuss power curves in general and present them in relation to temporal designs, years of field observation, components of variance, and the level of trend detected. Revisit designs give adequate power for moderate trend in 10-15 yr, even when revisits are less frequent than annually.