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:
- tributary confluence
- point source discharges
- physical break points (waterfalls, channelization, bridge crossings
and culverts)
- land use
- availability and homogeneity of data
The most important consideration for assessment unit delineation
is that assessment information in the database apply to the assessment unit
in its entirety.
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: 