Guide to Estimating Irrigation Water Needs of Landscape Plantings

considerations apply for individual plants, however,
and they are discussed for shrubs and trees separately.
Shrubs
ks: Species factor values are found in the
WUCOLS list.
kd:
For most shrubs, an average density factor
of 1.0 will be appropriate. For very large
shrubs, an upward adjustment to 1.1 may be
warranted.
kmc:
In most cases, the microclimate factor would
be assigned as discussed in Chapter 2.
Trees
ks:
Species factor values are found in the
WUCOLS list.
kd: For small trees (< 15 feet tall), an average
density factor of 1.0 would be appropriate.
For larger trees, an upward adjustment to 1.1
or 1.2 accounts for the increase in leaf area
found in many canopies.
kmc:
In most cases, the microclimate factor would
be assigned as discussed in Chapter 2. For
large trees, however, an upward adjustment
to 1.2 or 1.3 to account for wind flow through
the canopy may be appropriate.
Example: The urban forester for the city of Modesto
is interested in estimating water needs for a large
Modesto ash tree located in a downtown city plaza
for the month of July.
Water needs for individual trees or shcubs can be estimated using
the landscape coefficient method. Species, density and microclimate
factors all need to be considered.
First, the forester needs to assign values for each of
the landscape coefficient factors. In the WUCOLS
list Fraxinus velutina ‘Modesto’ is classified as
“moderate” with a ks value of 0.4. Since this is a
large, dense tree, the forester uses a density factor
value of 1.1. The microclimate in the plaza warrants
a “high” microclimate factor value. In addition,
the forester wants to adjust for wind flow
through the canopy since no trees or buildings are
nearby to attenuate the wind. The forester selects a
kmc value of 1.5. Using these values, a calculation
of the landscape coefficient can be made.
KL = ks x kd x kmc
KL = 0.4 x 1.1 x 1.5 = 0.66
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