Forests and Water: A State of the Art Review for Colorado

• Year: 2003
• Title: Forests and Water: A State-of-the-Art Review for Colorado
• Author: Colorado State University
• Description: Colorado State University 2003
• Publications - Doc Type: Other

ies outside of Colorado generally show that forest har-
vest can substantially increase low flows in percentage
terms, but the increase is very small when expressed in
absolute terms (Austin, 1999). In most cases the increase
in low flows is relatively short- lived, as the increase is
dependent on the increase in summer soil moisture and/
or groundwater levels, and a relatively small amount of
regrowth can eliminate the initial post - harvest increase
in soil moisture. Studies in other areas have shown that
harvest- induced increases in low flows are eliminated
within 5 -10 years due to vegetative regrowth, particu-
larly along the stream channels (e.g., Hicks et al., 1991;
Vertessey et al., 1996).
For Colorado, data on the effects of forest harvest on low
flows are only available for snowmelt- dominated catch-
ments. There is general agreement that most summer
rain storms have little effect on summer streamflows,
as the amount of rain is usually small relative to the
available soil moisture storage. Watershed studies in-
dicate that only about 1 -3% of summer precipitation is
converted into runoff (Bates and Henry, 1928; Berndt,
1960; Troendle and King, 1985; Troendle et al., 2001),
indicating that low flows are primarily a function of soil
moisture storage and groundwater.
Studies at the FEF have shown that forest harvest can
increase summer soil moisture, and this effect is gener-
ally larger in wet years and in areas with deeper soils.
However, the observed increases in soil moisture did
not significantly increase monthly streamflows on Fool
Creek in summer or early fall (Figure 2.3) (Troendle and
King, 1985). Troendle and Reuss (1997) found a sig-
nificant increase in base flows at the foot of a harvested
hillslope in West St. Louis Creek in the FEF, but this
increase in base flows could not be statistically detected
at the catchment scale.
Complete harvest of watershed B at Wagon Wheel Gap
increased summer minimum flows by about 10 %. In
absolute terms this converts to just 0.024 cubic feet per
second per square mile (Bates and Henry, 1928). This
increase was attributed to the reduction in summer ET
and resulting increase in soil moisture on the treated
watershed. Austin's (1999) reanalysis of the daily flow
data from Wagon Wheel Gap showed summer low
flows increased only for the first five years after harvest.
However, the persistence of this increase is confounded
by the drier conditions after the treated catchment was
cleared (Troendle and King, 1985).
An analysis of flow - duration curves from the Coon Creek
study indicated a significant increase in flows from the
40t'' to the 90t' percentile, but no detectable change in the
smaller flows (i.e., 11t to 40t" percentile). A reanalysis of
runoff data using flow - duration curves would be a useful
and more sensitive test of the changes in summer low
flows at Fool Creek and possibly Deadhorse Creek.
15
The changes in low flows that might occur in other forest
types in Colorado can only be estimated in accordance
with our understanding of forest hydrology. In general,
the increase in runoff from forest harvest decreases with
decreasing annual precipitation (Figure 2.2). Some stud-
ies have shown a relatively large percentage increase in
low flows, but because most of the increase in runoff
flow comes during moderate and high flows, the abso-
lute increase in low flows is very small (Hibbert and
Gottfried, 1987; Austin, 1997).
Hydrologic theory and studies in other areas also suggest
that the persistence of any increase in low flows will be
much shorter than the recovery period for annual water
yields (e.g., Austin, 1999). The shorter recovery period
for low flows is attributed to rapid recovery of summer
evapotranspiration rates relative to winter interception
rates. As observed for annual water yields, a harvest -
induced increase in low flows will be smaller or non-
existent in dry years, and the recovery of low flows will
be faster in drier areas and areas with shallow soils. It
may be of some scientific interest to further explore the
effects of forest management on low flows over time and
on a larger scale through hydrologic models, but pres-
ent knowledge suggests that water managers should not
expect significant increases in low flows as a result of
forest management.
2.3. Effects of Roads on Runoff
The previous section focused on the effects of forest
harvest on runoff, and this implicitly presumed that the
vegetation was cut without any other disturbance. In
reality, forest management activities usually require the
construction and use of roads, skid trails, and landings.
Many forest roads also provide access for recreation and
homeowners. The effects of roads, trails, and associated
features must be considered when evaluating the effects
of forest management on runoff, although the changes
in erosion and downstream sedimentation are generally
of even greater concern (Chapter 3). The problem is
that most catchment -scale studies combine the effects
of roads and forest management, or study the effects of
roads for only one or two years prior to forest harvest,
and this makes it difficult to clearly separate the effects
of roads on runoff from the effects of forest manage-
ment, particularly at the catchment scale.