BASIN Text Image
Current Theme: Watershed
The Pros and Cons of An Environmental Index

  • Takes complex scientific information and synthesizes it in a way that makes it easily understandable at a glance.
  • Helps to translate a wide variety of environmental indicators into a simple system that can be easily communicated.
  • Can provide easy-to-understand information to citizens in order to help them comply with regulations (like no-burn days for air quality index) or make personal lifestyle choices that will benefit the environment.
  • Can be overly generalized and unscientific, relying on subjective judgements to weigh only a few of the numerous variables.
  • A single index cannot tell the whole story; an index may indicate that a river is unfit for drinking water standards, but the river may be fine for swimming and a healthy habitat for fish and macroinvertebrates.
  • An index is often limited in terms of time and space-- an unusual reading at one location or during a particular time period can skew the index.

Weighing Factors and Q Values

Why isn't pH as important as DO...and how come Fecal Coliform is the most important?

When an index is put together, not all variables are equal. (See How was the NSF index developed?) Who determines what indicator is more important than another? Water quality experts....and naturally experts will disagree based on their own understanding and research. Experts with a background in aquatic habitat will likely have different priorities than those focused on human health concerns. Thus, coming up with one water quality index that everyone will be happy with is virtually impossible.

The EPA's Index of Watershed Indicators has attempted to bring in other unscientific factors, such as "Meeting All Designated Use," (see sidebar) which is heavily weighted in the Index and is based on local and in some cases biased attitudes of how a particular waterway should be used. This approach can still provide a valuable insight into a particular watershed's overall health, but its scientific validity is compromised in the process.

The National Sanitation Foundation's Water Quality Index that BASIN is making use of was created in 1970 and has been controversial since it was first used. Numerous other WQI have been developed, but we have used the NSF WQI as a way of beginning the ongoing dialogue needed for our community to better understand the complex issues of our local water quality. The NSF's experts determined that Fecal Coliform was the most important factor in the index and should be weighted most heavily, and that DO is more important than pH.

The NSF system has also been criticized because several of the Q value curves top out in the high 80s or low 90s, which thereby allows for a maximum top score of about 97. Thus an A only covers a range of 7 points while B and C each cover 20 points each, and a D 25 points.

EPA'S Index of Watershed Indicators
How are indicators values and the overall index calculated?

For each condition indicator, values were selected which, in EPA's professional judgement, represent an appropriate basis to describe the aquatic resources within the watershed as having good quality, fewer problems or more problems. Similarly, for each vulnerability indicator, the Agency selected values that we believe are appropriate to differentiate "lower" from "higher" vulnerability. For most indicators we established a minimum number of observations necessary to assign a "score." In aggregating the 15 Indicators into the overall Index, Indicator #1, Assessed Rivers Meeting All Designated Use, is weighted more heavily than other Indicators because it is a comprehensive assessment and EPA believes considerable weight should be given to the State/Tribal 305(b) assessment process. All other indicators are weighted equally. Where there is insufficient data for a particular indicator we will display that on the map and present it in the Profile. At least 10 of the 15 data layers must be present to calculate the overall index for any given watershed. If Indicator #1 is not available, the values of the other indicators of condition are multiplied by three to derive an Index score.

INVITATION BASIN is a community project actively seeking public participation. We appreciate all feedback and welcome comments, suggestions and contributions. To find out more about how you can be involved, click here. Help BASIN serve your needs, take our "10 questions in 10 seconds" survey.

BASIN is supported by the US EPA, the City of Boulder, the Keep it Clean Partnership, BCWI and BCN

Home | Site Map | Glossary | Bibliography | Contributors
About BASIN | Attribution | Feedback | Search
Last Page Update - Tuesday December 27, 2005