Assessment Unit Delineation

Currently many states delineate assessment units as watershed oriented collections of stream reaches (the portion of a stream between two tributary intersections). Organizing assessment units by watershed is a useful method for analyzing impairments of water quality, but it leads to spatial indeterminacy in water quality databases (see Spatial Indeterminacy).

Spatial indeterminacy can be eliminated by splitting an existing assessment unit into smaller units that have homogeneous assessments (i.e. all miles are Fully Supporting, or Not Supporting etc., Figure 3)  Several other considerations may be used to divide up existing assessment units.  These include: 

How many assessment units should be created?

The number of assessment units that should be created, and how large they should be, needs to be decided on a case by case basis.  It is unnecessary and probably undesirable to delineate every small tributary, or to split existing assessment units that already have homogeneous assessments.  Only create as many assessment units as are necessary to accurately map and display assessment information.

Naming Assessment Units:

There are no required conventions for setting up unique IDs for the Assessment Units, and they can be up to 50 characters long. It is recommended that the Assessment Unit-ID begin with the two-letter abbreviation for your state. It is also helpful to keep your ID scheme simple, so that it is easy to add new ID's to your system as necessary.

Next: Summary

Figure 3: Watershed Assessment Unit "WBG" (left image) contains 8 sampling stations which are used in water quality assessments by the State of Tennessee.  Based on these sampling stations the State divided the watershed into 9 smaller Assessment Units (right image). 

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