MONITORING FOR POLICY-RELEVANT REGIONAL TRENDS OVER TIME

The term trend describes the continuing directional change in the valu e 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 t rend seldom is discussed, and monitoring designs suitable for detectin g 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 vari ance, developed from the Surface Waters component of U.S. EPA's Enviro nmental Monitoring and Assessment Program (EMAP) field data, and use t hem as a basis for further assumptions of values. We discuss power cur ves 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.