EIS Arapahoe & Roosevelt National Forest, Pawnee National Grassland

• File Number: 8461.250
• Description: Water Issues
• State: CO
• Basin: South Platte
• Water Division: 1
• Author: USDA, Forest Service, Rocky Mountain Region
• Title: EIS Arapahoe & Roosevelt National Forest, Pawnee National Grassland
• Water Supply Pro - Doc Type: EIS

The model is first zun with unadjusted precipitation to estimate pre-treatment
water yield, then the precipitation ia increased and the model is run again to (
estimate post-treatment water yield. "Windward Width" ia aet to zero £or both ?
runs to effectively turn off the models adjustment of precipitation. Water
yield increase for the regime is then the difference between pre-treatment and
post-treatment water yield.
Lapse - Lapse allowe the user to enter a multiplier which corrects for the
effecta of elevation on precipitation. It allows the use of local
precipitation guage data in a watershed. Because local data was not available
for an entire Forest and for the reasons documented above with regard to
duplicating the original results of the WRENSS procedure, this parameter was
set to "1", no correction.
Elevation - The model does not allow entry of an elevation for the Rocky Mtn.
Region (WRENNS Region 4).
Tree Height - While we entered the average tree height provided by FVS for the
first five decades, it is irrelevant if the adjustment to precipitation is made
as discussed above.
'Iype - The oaly choices for the Rocky Mountain region are Lodgepole pine,
Spruce-Fir, Ponderosa Pine, or deciduous. Although there is a small component
of Douglas Fir in this Forest's suited and scheduled lands, it is so small that
the forester's lumped it with Ponderosa Pine for growth and yield modeling. We
did the same for water yield modeling.
Wind Speed - As suggested above, wind speed was set to zero.
Days of no snow - As suggested above, days of no snow was set to zero.
Guage - Guage was set to none. This variable allows the user to set the type
of wind shield used on a gage. It is not applicable if precipitation is not
based on site specific data.
Exposure - Exposure was set to zero. This, in conjunction with "Guage", allows
the user to adjust precipitation. It is not applicable for this effort.
Unimpacted vs. Impacted - The Canadian model of the WRENNS procedure estimates
water yield rather than the change in water yield. In order to estimate water
yield increases, pre-treatment stand conditions were entered into the
UNIMPACTED column, and post-treatment stand conditions were entered into the
IMPACTED column. The model was then run first with unadjusted precipitation
and again with adjusted precipitation. The change in water yield was the
difference between water yield for the UNIMPACTED, unadjusted precipitation
units and the IMPACTED, adjusted precipitation units.
Max BA - The maximum BA predicted from FVS was entezed. For stands treated
with shelterwood harvest, maximum basal area was found on the unharvested
simulation (several entries over the rotation may prevent the stand from
reaching maximum basal area).
SA - As discussed above, the minor changes in pre-treatment basal area
predicted by FVS for Che first five dacades did not produce differences in `_