Surface Water - Alabama Department of Environmental Management

Methodology
90th percentile were rated as high; scores greater than the 50 th percentile, but less than the
90 th percentile were moderate; scores less than the 50 th percentile were low.
High ranked sub-watersheds also having a high non-rural NPS potential were further
evaluated to determine the probable source location in relation to potential assessment
sites.
Table 9. Range of values used to define “low”, “moderate”, and “high”potential for impairment for
each nonpoint source category.
Category
Impairment Potential
Rural NPS Categories Low Moderate High
% Cropland ≤5.1 >5.1 to≤15.3 >15.3
% Pasture land ≤15.0 >15.0 to≤37.2 >37.2
% Mining ≤3.8 >3.8 to≤11.6 >11.6
% Forestry Activities ≤25.5 >25.5 to≤55.7 >55.7
Animal Units per Acre ≤0.37 >0.37 to≤1.75 >1.75
% Aquaculture (Acres/Acre) ≤0.24 >0.24 to≤0.62 >0.62
Sedimentation rate (tons/acre/yr) ≤7.4 >7.4 to≤22.6 >22.6
Score with 7 categories ≤13 >13 to≤19 >19
Score with 6 categories ≤10 >10 to≤15 >15
Table 10. Range of values used to define “low’, “moderate”, and “high” potential for impairment
from each urban or point source category.
Category
Impairment Potential
Urban NPS Categories Low Moderate High
Urban score 10
% Urban ≤5.0% >5.0% to >15.5%
≤15.5%
Development (highest rating)
Total # permits, CSAs/acre of subwatershed
≤0.0003 >0.0003 to >0.0008
≤0.0008
# CSA/acre of sub-watershed ≤0.0002 >0.0002 to >0.0006
≤0.0006
# of failing septic tanks/ac of subwatershed
≤0.0052 >0.0052 to
≤0.0145
>0.0145
The values derived for the BWC Basin Group may not be applicable to water quality
conditions and activities in other basins of Alabama. These categories and ranges are
intended to be descriptive, but are open to differing interpretations considering alternative
data analysis techniques and are subject to refinement as data availability and analysis
warrants.
The local SWCDs also evaluated the streams for each of the sub-watersheds located in
their respective counties. These evaluations were discussed during public meetings and
were used to rank the sub-watersheds as to their perceived priority for conducting water
quality improvement projects. The 1 st priority was given to the sub-watershed with the
greatest need. A single sub-watershed may have more than one priority if two or more of
the counties containing the sub-watershed gave it a top-five priority ranking. This
information was used to supplement the sub-watershed estimates of NPS impairment
potential.
18