Environmental Index for Assessing Spatial Bias in Watershed Sampling Networks

Bias in the design of stream-sampling networks can be a major cause of inaccurate characterization of ambient water quality. At the state level, sampling bias can impact a state’s ability to produce an accurate assessment of the water quality of all state waters. At the federal level, this bias has hindered attempts by the U.S. Environmental Protection Agency (U.S. EPA) to produce nationwide assessments of water quality. Three types of bias commonly occur in water-quality assessments: design, analytical, and statistical. This paper focuses on “design bias,” especially spatial-design bias, in stream monitoring networks. A geographic information system method is described to develop an environmental index to help recognize spatial design bias, and to prioritize areas (subwatersheds) for sampling. The environmental index, which is developed using data that are generally available from federal and state agencies, provides a means of differentiating the component parts of a watershed, its subwatersheds, in terms of two sets of features: natural landscape features and anthropogenic features or “stressors.” Together, these features largely determine the variability of the quantity and quality of water discharged from watersheds. Subwatersheds with higher environmental-index values are expected to have more variable water quality over the course of a year than subwatersheds with lower index values. They are also expected to exert a greater influence on basinwide water quality. These properties of the index make it useful for (1) identifying the possible presence of spatial bias in existing watershed-sampling networks, (2) making sampling design decisions, and (3) aggregating data from subwatersheds into basinwide measures of water quality. Development and use of the index is demonstrated in a watershed in southern New Hampshire.