# Linking CWA Section 305(b) and 303(d) - A Statistical Perspective

__N. Scott Urquhart__ and __F. Jay Breidt__

Colorado State University

Fort Collins, CO 80523-1877

Section 305(d) of the Clean Water Act (CWA) requires states to make biannual reports on the conditions of all waters in each state's jurisdiction. EPA's Office of Water encourages states to use probability-based surveys to fulfill requirements of that section of the CWA; a number of states and a few tribes are moving that direction. Once the scale of "impaired" waters has been estimated from a probability-based survey, the next question is, "Where are the impaired waters?" The statistical approaches of small area estimation address a similar problem in other areas of endeavor. This pair of talks will address small area estimation for the CWA from programmatic and technical perspectives. Both of the following talks are planned for environmental scientists and managers, not statisticians.

Part 1 (Urquhart): EPA has funded the Spatial-Temporal Aquatic Resources Modeling and Analysis Program (STARMAP) at Colorado State University. It is charged with developing the statistical approaches that would be suitable for small area estimation. This talk will focus on STARMAP, the general nature of spatial models, potential covariates, and associated limitations.

Part 2 (Breidt): Small area estimation has been used successfully to extend survey results to areas that lack sufficient data for defensible estimates from the primary survey results. This talk will focus in the approaches for small area estimation, some successful examples, and limitations.

**Keywords**: surface water, statistics, surveys, small area estimation