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The CN Values
generated in the default 'Percent Impervious
Input' for L-THIA are based on CN values
obtained from TR-55. The CN values in the
'Percent Impervious Input' are generated
based on the assumption that the corresponding
pervious surfaces for the impervious cover
values provided by the user is 'Grass' in
'Fair' condition.
The user
has the flexibility in the 'Custom CN Input'
to run L-THIA simulations where the pervious
surface is not grass in fair condition.
This can be
done by generating CN values for the required
pervious cover surfaces. For this the user
needs to use the following formula:
CNn%=CN0%+(n%/100)*(98-CN0%)
The user then
needs to substitute the corresponding CN
of the land use of interest in place of
98 in the above formula.
The table and
the graph on the right show the CN values
for percent impervious cover on the four
hydrologic soil groups with the pervious
cover being grass in fair condition.
Similar graphs
and Custom CN values can be generated for
various other pervious surfaces for which
CN values are given in TR-55. An L-THIA
results can then be simulated using the
'Custom CN Input' option.
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