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

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2 June 3 July 3 Aug. 3 Sept.
Figure 2.3. Mean annual pre- and post treatment hydrographs for Fool Creek, Fraser Experimental Forest.
ET, and this change in ET drives the change in runoff in
accordance with equation 1. However, the snow -domi-
nated areas of Colorado are unusual in that winter inter-
ception is a much larger component of the overall water
balance than for most other forested areas. Studies on the
FEF have shown that completely removing the tree cano-
py increases the amount of snow water equivalent (SWE)
on the ground by approximately 20 -45 %, depending on
aspect. Clearcuts on the Manitou Experimental Forest
increased the SWE by 8 -35% (Gary, 1975).
The decrease in winter interception due to forest harvest
can be as large or larger than the decrease in summer
ET, particularly in wetter years. The exceptionally high
winter interception rates at the FEF can be attributed
to the low relative humidity, high average wind speed,
large surface area of snow in the tree canopy, and the
nearly continuous presence of snow in the tree canopy
in winter, especially on north- facing slopes. A series of
detailed studies have shown that the observed increase
in SWE following forest harvest or thinning is due
almost entirely to the reduction in winter interception
rates rather than the redistribution of snow into open-
ings as claimed in some earlier studies (Troendle and
King, 1987). For a given vegetation type and aspect the
amount of winter interception is nearly a fixed percent-
age of winter snowfall, and the increase in peak SWE is
directly proportional to the amount of the canopy that is
removed. In the subalpine zone the mean percent inter-
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ception loss in winter and spring is approximately 25%
for conifers and II% for aspen (Troendle et al., 2003).
In lower elevation areas the increase in SWE after tim-
ber harvest is generally smaller, and a 22% reduction in
basal area did not significantly increase the SWE at the
Manitou Experimental Forest (Gary, 1975).
The other factor that governs the change in water yield is
the amount of evaporation and transpiration in the grow-
ing season. The general principle is that in drier years
and areas with shallower soils, summer ET savings are
minimized because the combination of soil evaporation
and transpiration from the residual vegetation will use
nearly all of the available water. Under these conditions
the reduction in winter interception will provide nearly
all of the increase in water yield from forest harvest. In
wetter years and on aspects with less winter interception,
proportionally more of the increase in water yield will be
derived from the reduction in summer ET (Troendle and
King, 1985; Troendle and Reuss, 1997).
Plot -scale data for different levels of thinning in lodge -
pole pine indicate that the reduction in winter intercep-
tion on the FEF is approximately two to three times larger
than the reduction in summer soil water depletion (Wilm
and Dunford, 1948). For the Fool Creek Experiment it
has been estimated that, on average, approximately 50%
of the increase in water yield is due to the reduction in
snow interception, and 50% is due to lower ET during