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

Methods for Eetimating Water Yields for the Forest Plan Revision
Carl Chambers,Forest Hydrologist
This paper outlines the methods used for estimating water yield increases diie
to vegetation management for the Forest Plan revision. The methods are
primarily based on thoae found in Chapter Three of "An Approach to Water
Resources Bvaluation of Non-point Silvicultural Sources (A Procedural
Handbook)", known as WRENSS (FsPA, 1980). However, WRENSS was designed as a
site specific procedure, and some assumptions were required to generalize it
for use in (non-site specific) Forest Plan water yield modeling. Suggestions
on modeling methods and assumptions were provided by Chuck Troendle, of the
Rocky Mountain Forest and Range Experiment Station (RMS), and Jim Maxwell, R-2
Regional Hydrologist, and their comments have been incorporated into this
document. Complete results of the water yield analysis are availble in the
planning record.
There are several computer models of the WRENSS procedure. The computer model
used for this effort was one developed by Robert Swanston, formerly of the
Canadian Forestry Service (the Canadian model). While there is a version
developed by the RMS which provides some modifications that reflect the
advancing state of the science, data entry and editing is much more difficult
if the user is not intimately familiar with the software. The Canadian model
has an interface which facilitates ease of use.
Some modifications to the original WRSNSS procedure have also been made to the
Canadian Model. Inputs for wind speed, numbers of days with no snow, and snow
scour are all designed to modify the original procedure with regard to snow
loss and redistribution. However, documentation provided with the model
indicates that if Wind Speed is set to "zero", number of days with no snow is
set to °zero", lapse rate is set to "one", and snow scouring is set to "yes",
the model will provide the same results as the original WRENSS procedure.
Chuck Troendle pointed out that while there is nothing wrong conceptually with
adjusting snow scour factors, the WRENSS procedure inherently includes snow
scour conditions that were present for the data sets used to develop the model,
and that before these variables are added to the model, the original nomographs
or their digital versions would need to be modified to zero out the effects of
the assumed conditions. He recommended setting the model to duplicate the
original procedure. We followed his advice in our analysis.
FVS (Forest Vegetation Simulation) is the model used to provide tree growth and
yield information for FORPLAN. It is the standard model used nationally by the
Forest Service. Output from FVS was used to provide the vegetation information
needed for input into WRENSS, including maximum basal area, pre- and
post-treatment basal area, and tree height. Using FvS results ensured that we
were working from the same basic set of data for water yield analysis that was
used for other resource analyses that were done for the Forest Plan.
To simplify modeling, FVS simulation was provided for a number of vegetation
regimes. Each regime was composed of a tree species, size and location. Eight
regimes were modeled for water yield. They were, lodgepole pine large east,
lodgepole pine medium east, lodgepole pine large west, lodgepole pine medium
west, ponderosa pine medium east, spruce-fir medium to large east, spruce-fir