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

the growing season (C. Troendle, Matcom Corp., pers.
comm., 2000).
Plot- and catchment -scale studies on the FEF indicate
that the pattern of forest harvest has a surprisingly small
effect on the magnitude of the resulting water yield
increase. In the Deadhorse North experiment 36% of
the basal area was removed by patch cutting, and the
increase in annual water yield was proportional to the
percent of basal area removed. Removing 40% of the
basal area in the first step of a shelterwood cut resulted
in a 16% increase in peak SWE, and the observed in-
crease in streamflow, while too small to be statistically
significant, was consistent with the observed increase in
SWE (Troendle and King, 1987). These results suggest
that the reductions in winter interception were directly
transformed to streamflow, and the additional water
generated in a spatially - distributed manner across the
hillslopes was able to reach the stream channel.
In drier areas and areas where summer ET is the pri-
mary source of "saved" water, the pattern and location
of forest harvest can have a much greater effect on the
observed change in annual water yields. Baker (1986)
found that watersheds with longer and flatter hillslopes
generated smaller increases in runoff after forest thin-
ning than steeper and narrower watersheds. Other studies
have suggested that harvest- induced increases in runoff
can be reduced because the remaining trees can scavenge
the "saved" water from upslope areas before it is able to
reach the stream channel (e.g., Ziemer, 1968; MacDon-
ald, 1987). In general, the pattern of forest harvest and
the location of the harvest relative to the stream channel
are more likely to affect the magnitude of a water yield
increase in drier, lower - elevation areas and possibly in
drier years in sub - alpine areas. In the higher - elevation,
wetter sub - alpine zone most of the interception savings
will probably be transformed into runoff, regardless of
the pattern of harvest or proximity to a stream channel.
yields on north- facing slopes than south - facing slopes
(Troendle et al., 1994). In contrast, aspect has relatively
little effect on the magnitude of the reduction in summer
ET after forest harvest, as the amount of ET is limited
primarily by the amount of water rather than the amount
of incoming energy (Troendle et al., 1994).
Aspect can affect the magnitude and timing of potential
water yield increases in higher- elevation forests by af-
fecting both the amount of sublimation and the rate of
snowmelt. In the sub - alpine zone, north - facing slopes
typically have denser vegetation than south - facing
slopes. This denser vegetation has higher interception
rates, as the greater leaf area and higher winter branch
turgor capture and hold more snow. Recent modeling
studies assume a winter -spring interception loss rate of
32% for north- facing slopes, 26% for east- and west -fac-
ing slopes, and only 17% on south - facing slopes (Tro-
endle et al., 2003). The difference in winter interception
rates results in a greater potential to increase water
In the case of Deadhorse Creek, openings on south -fac-
ing slopes had a smaller increase in SWE than expected .
from interception studies on other aspects (Troendle and
King, 1987). This suggests that the orientation of for-
est openings relative to the sun can affect the increase
in SWE resulting from forest harvest, and this effect
is probably due to differences in incoming shortwave,
reflected shortwave, and longwave reradiation from the
surrounding leave trees. In general, forest harvest will
increase melt rates by increasing the amount of incom-
ing solar radiation, increasing the turbulent transfer of
sensible heat to the snowpack surface, and increasing the
transfer of latent heat by condensation and freezing. All
of these factors will vary with aspect, slope, prevailing
wind direction, and wind speed. These differences mean
that the increase in SWE, the increase in water yield, and
the timing of an increase in water yield will vary with
aspect (Baker, 1986).
The size of the openings created by forest harvest and
the amount of roughness also can affect the magnitude
of the increase in SWE after forest harvest. Larger open-
ings are more subject to wind scour, but this effect can be
ameliorated if there is sufficient slash or other sources of
roughness to capture and hold the snow within the open-
ing (Troendle and Meiman, 1984). Because the annual
runoff in snow - dominated areas is closely related to the
maximum SWE, the ability of forest openings to retain
snow can directly affect the magnitude of the increase in
water yield after forest harvest.
The water balance equation and the interplay between
evaporation, interception, and transpiration can explain
the observed interannual variability in water yield in-
creases in response to forest harvest. In dry years a great-
er proportion of the precipitation is needed to recharge
soil moisture, and there may also be slightly greater
soil moisture depletion due to the increase in potential
evapotranspiration. In wet years there will be more soil
moisture carryover, and less snowmelt or winter precipi-
tation is needed for soil moisture recharge. Hence more
of the interception and transpiration savings can be con-
verted into runoff. The net result is that the increases in
runoff following forest harvest are substantially greater
in wet years than in dry years (Baker, 1986; Troendle and
Kaufmann, 1987).
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