1. vol,. 32, NO.6
WATER RESOI'RCES BI'LLETIN
AMERICAN WATEN RSSOTJRCES ASSOCIITION DECEMBER 1996
SNOWPACK CIIANGES RESI]LTING FROM TIMBER I{ARVESTI
INTERCEPTI ON, REDISTRIBUTION, AND EVAPORATI ONl
SteuenV. Stegman2
ABSTRACT: Three processes were examined as causing snowpack
changes in forest clearings. TVo ofthe three contribute to increases
and one counteracts by reducing snowpack. The two that increase
snowpack are redistribution and decreased loss to interception.
Snow evaporation from a clearing counteracts snowpack increases.
Research has indicated that as vegetation density increases, so too
does the loss to interception. As snow in the canopy reaches the
limit that the canopy can hold (the threshold amount) evaporation
increases. Aemdynamics of the forest canopy were studied as well.
As timber is cut, wind pattems are disturbed, creating dismptions
in the wind velocity gradient depositing snow in openings. This
redistribution leads to an increased snow water equivalent and
augments runoff. Snow evaporation was shown to increase propor-
tionally with opening size. Evaporation offsets the water yield
gains derived from forest cut. It was found that this offset is inclu-
sive to the measurements of water yield changes in experimental
forests. An optimal size of harvest block may be five tree heights in
width as suggested by numerous studies.
(KEY TERMS: interception; forest hydrology; snow evaporation;
snow and ice hydrrlogy; streamflow; water yield increase; water-
shed management/wildland hydrolory.)
INTRODUCTION
As water demand in the western United States
increases with a growing population along with a
greater demand for timber, understanding the hydro-
logic effects of timber removal becomes increasingly
important. Without recognition of the effects of timber
harvest on water supplies, continued degradation of
water can only continue. The effects of harvesting are
widespread, having implications on water quality, soil
erosion, sedimentation of stream channels, and (as
will be addressed in this paper) water yield from the
snowpack.
Numerous studies have found that forest clearcut-
ting and thinning leads to an increase in water yield
from snowmelt (Alexander et al., LB85; Berndt, 1965;
Gary and TYoendle, L982; Haupt, 1979; Troendle,
1983, 1986; Tloendle and King, 1987). Three factors
are recognized as being related to this water yield
increase. The frrst two contribute to the snowpack by
(1) decreased loss to interception and (2) disruption of
wind velocity gradients and redistribution. The third
process is counteractive to the gains mentioned; it is
the increased loss to snow evaporation from in situ
snowpacks in forest clearings.
The significance of snowpack evaporation is the
subject of much debate. Tvo schools of thought have
evolved, one believing in its significance, the other
dismissing it as negligible. Research has indicated the
significance of evaporated snow as a loss of water
yield (Bernier, 1990, Bernier and Swanson, 1993).
Other studies have acknowledged the presence of
snow evaporation but dismissed it as negligible (West,
1962). For the purpose of this paper it is assumed
that snow evaporation is significant. Assuming signif-
icance, the processes of increased snow evaporation
and of increased snow deposition counter each other.
Thus, an equilibrium size of cut can theoretically be
determined.
Understanding that timber harvest is a necessary
ingredient for construction and economic growth, how
can we best achieve harvest while limiting the possi-
ble degradation to water supplies? Assuming that
snow evaporation does have a significant effect on the
water equivalence of a snowpack, would there be an
optimum size of cut to increase snow accumulation
while limiting the loss to direct evaporation? These
rPaper No. 951?9 of the Water Resources Bulletin, Discussions Ere open until rlune 1, 1997. (Recipient of the 1995 AWRA/UCOWR
Student Paper Competition Award, Undergraduate Division)
zGraduate Student for M.A., Geography, University of Montana, 404 Woodford St., Missoula, Montana 59801.
1353 WATER RESOURCES BULLETIN

2. are the questions which will be addressed in this
paper.
LITERAIURE REVIEW
The firstipublished study relating forest harvest to
water yield change was done at Wagon Wheel Gap in
Colorado in 1909. Subsequent experiments on forest
harvest and water yield have been conducted world-
wide to understand the varying effects of cutting in
different vegetation. A review on 39 studies ofexperi-
mental watersheds yielded three general observations
(Hibbert, 1967).
. Reduced forest cover increases water yield.
. New growth or planting in sparsely vegetated
areas decreases water yield.
. Response to harvest or planting is highly vari-
able.
After 1967, an additional 55 experiments brought
the total to 94. In a review of these experiments,
Bosch and Hewlett (1982) showed a 40 mm increase
in water yield with a 10 percent decrease in conifer-
ous forest density. In the same review, it was noted
that a 10 percent reduction in deciduous forest densi-
ty increased water yield by only 25mm, and even less
for brush or grass. Examination of this data indicates
a wide degree of variability and is indicated by pub-
lished correlation coefficients of stand density vs.
water yield increases of .650, .506, and .340 for conif-
erous, deciduous, and brush, respectively. The review
by Bosch and Hewlett (1982) included few studies in
snow dominated climates, but it shows the impor-
tance of vegetation type on the degree of water yield
change.
Thirty-three of the 94 experiments reviewed by
Bosch and Hewlett (1982) reported small or insignifi-
cant changes to water yield, as other studies showed
increases greater than twenty percent in the first
year. Of the 33 that showed little or insignificant
changes, two studies (Harr, 1976, 1980) were attribut-
ed to a possible reduction in fog interception and drip.
This phenomenon was suggested again by Harr (1982,
1983) in local areas, particularly riparian, where
phreatophytic vegetation became established after
harvest, compounding the loss of water to decreased
fog drip. Once again, the degree of effect varies dra-
matically with location.
Experiments at the Fraser Experimental Forest in
Colorado conducted from the 1940s through the 1970s
found that water yield increases as a function of for-
est harvest. Numerous harvest block designs and pre-
scriptions were performed to determine the optimum
WATER RESOURCES BULLETIN
shape and size to increase water yields. An increase
in water yield as much as 40 percent was observed as
a result ofharvest (TYoendle and King, 1987).
Alexander et al. (1985) expressed water yield
increases as a function of mean annual precipitation
as well. Water yield increases caused by timber
removal or decreases due to planting have been the
greatest in high rainfall areas. It was also pointed out
that the effects of forest manipulation occur for a
shorter time period in wetter areas due to the rapid
regrowth of vegetation. These differences in climatic
regimes illustrate the effects of annual differences in
precipitation, as water yield changes are more dra-
matic during wet years than during dry years.
A number of studies have looked specifically at
snow evaporation and have used experimental har-
vest blocks. Bernier and Swanson (1993) measured
evaporation at the center of openings using evapora-
tion pans embedded in the snowpack. While there is
ongoing debate as to the relative effects ofsnow evap-
oration, data presented from the studies in Alberta do
indicate a significant loss to evaporation after
clearcutting (Bernier and Swanson, 1993). Variables
contributing to the amount of evaporation are: rela-
tive humidity, wind speed, opening size, aspect, and
orientation to prevailing winds (Bernier, 1990; Gold-
ing, 1978).
PROCESSES
The processes examined in this paper are the evap-
oration of intercepted snow, changes in the wind
velocity gradient, and snowpack evaporation. The
first two processes may increase the snowpack and
were singled out because they are key to snowpack
changes resulting from timber harvests (TYoendle and
King, 1987). Snow evaporation was examined as
important in offsetting the gains in snowpack levels.
Intercepti.on
Different vegetation types differ in morpholory and
their ability to catch snowfall causing variation in
interception losses. Deciduous forests will intercept
less than coniferous forests during winter due to the
absence of leaves. Even in a coniferous forest, various
types of pine, spruce, and fir have different crown
structures. Some have evolved around high stand
densities and intercept more snow than others such
as ponderosa pine on south facing slopes with greater
distances between stems. In addition, variations in
needle configurations, branch angle, and tree height
Stegman
1354

3. Snowpack Changes Resulting from Timber Harwest: Interception, Redistribution, and Evaporation
and width will alter the amount of intercepted snow.
Numerous studies have observed lodgepole pine
forests and where possible, this review will be limited
to these studies for comparative purposes.
There are many variables affecting the actual
amounts of interception loss. Empirical models to
estimate interception loss include variables to account
for saturation vapor pressure, snow depth in the
canopy, thieshold value ofsnow in the canopy, aerody-
namic resistance, and roughness length. Most empiri-
cal models have been derived at specific locations.
Therefore most, if not all, do not account for differ-
ences in solar radiation and sun angle based on lati-
tude, slope aspect, or time of year.
The most important factors affecting sublimation of
intercepted snow are relative humidity, aerodynamic
resistance, and the mass of intercepted snow (Lund-
berg and Halldin, 1994). Lundberg and Halldin (1994)
estimated evaporation from a partly snow-covered
canopy using the equation:
whenc=s E^=E'l!!!! (1)
a+ (S/ C)y
WhenC>S EA=E,
where.Ea is actual interception evaporation flux from
a partly snow covered wet canopy (kg m-2 s-1); E1 is
evaporation flux in a canopy when the intercepted
amount exceeds its threshold/saturation value (kg m-2
s-1);A is the change in saturation vapor pressure with
temperature (Pa 'C-1); y is the psychometric constant
(Pa 'C-1); C is the equivalent depth of snow held on
canopy (mm); and S is the threshold value of snow or
maximum depth of rain retained by a canopy (mm).
Aerodynamic resistance was determined with the
equation:
where ro is the aerodynamic resistance to transport of
vapor (s m-1); z is the reference height above surface
(m); d is the zero plane displacement height (m); zo
is the roughness length (m); & is the von Karman's
constant (.41) dimensionless; and u(z) is the wind
speed at height z (m s'1;.
Results from Lundberg and Halldin (1994) showed
Iosses of 0.3-3.3 mm / 24 hours for 50 of the 250 win-
ter days over two years. As the amount of snow
increases to the point at which the vegetation can
hold no more (the threshold value) evaporation of the
intercepted snow increases. A conclusion ofthe study
showed a positive linear relationship between evapo-
ration and the percent of the threshold value in the
canopy. It is important to realize that different vege-
tation have different characteristics and different
threshold values of snow accumulation, indicating the
importance of vegetation on the amount of intercep-
tion.
The results of the study are diffrcult to extrapolate
over a large area or to apply to different geographic
locations. The area of their study was located at 10
meters above mean sea level at 65'37'N. At this lati-
tude the low sun angles would have less of an affect
than at the central Rocky Mountain location, between
35 and 45'N. Latitude will impact the degree of water
yield increase between block sizes, with high latitude
sites receiving less insolation with larger clearcuts.
Sun angles and day length will also affect the type of
vegetation that is predominant to that area, causing
differences in interception.
Another model for interception loss is the intercep-
tion loss equation presented by Merriam (1960) for
use with rain and snowfall data. A good review can be
found in Satterlund and Haupt (1967). The general
equation for snow catch is used by Satterlund and
Haupt (1967) and is as follows:
1"=S/1+e'k(P-P) (4)
where 1, is interception storage; & is the constant of
rate ofinterception storage; S is the interception stor-
age capacity of the vegetation; e is the base of the nat-
ural logarithm;P is storm precipitation in inehes; and
P, is the amount of snowfall accumulation at the time
of most rapid storage
It is clear from this equation that interception loss
increases with increased vegetation surface area,
evaporation rate, and storm duration. The importance
ofvegetation is seen in the interception storage capac-
ity which decreases as a function of precipitation.
Satterlund and Haupt (1967) used the snow catch
equation by plotting interception storage in inches of
water equivalence to snowfall inches of water equiva-
lence in douglas-fir and white pine (Figure 1). The
results of this study show a sigrnoidal growth curve
with little interception at low snowfall levels, followed
by rapid increases in interception at or about 0.15
inches of water equivalence. The interception leveled
at about 0.25 inches of snow water equivalence (SWE)
which was probably the threshold level for the vegeta-
tion studied.
Schmidt & Gluns (1991) built on the results of Sat-
terlund and Haupt (1967) by applying the sigmoidal
growth curve to data collected near Fraser, Colorado,
and Nelson, British Columbia. They found a similar
relationship between snowfall and interception in
(2)
(3)
1 355 WATER RESOURCES BULLETIN

4. Stegman
Engelmann spruce, subalpine fir, and lodgepole pine.
Data fit the sigmoidal growth curve well with the
curve leveling off at around 7 mm (0.27 in.) SWE for
the spruce, pine, and fir. These threshold values com-
pare well with Satterlund and Haupt (1967) and sug-
gest limits to snow catch by coniferous branches.
20 30
Cumulative snowfall (inches w.e.)
Figure 1. Plot of Snow Catch by Douglas-Fir and Western
{hite Pine from Satterlund and Haupt (1967).
Other factors contribute to the amount of snow
stored in the canopy. Schmidt and Gluns (1991) mea-
sured large deviations from the $owth curve in a few
storms, suggesting influence of other factors. Snowfall
with higher specifrc gravity during higher wind speed
resulted in trends below the curve and suggest less
interception due to redistribution. For these reasons
the average curve derived by Schmidt and Gluns
(1991) excluded storms with wind speeds > 0.5 ms-l
and density less than 0.1.
Snaw Redistri.bution
In a full canopy of lodgepole pine (or any vegeta-
tion, for that matter), winds above the boundary layer
increase with height above the canopy. After forest
clearing, the edge of the harvest block creates turbu-
lent eddies of air at the upwind edge of the cut area.
The wind velocity gradient above the canopy is dis-
rupted, resulting in deceleration and subsequent
snow deposition.
Numerous snow accumulation studies (Gary and
TYoendle, 1982; Alexander et al., 1985; Gary and
Watkins, 1985; among others) have shown the rela-
tionship between harvest level and increased snow
WATER RESOURCES BULLETIN 1356
storage. Alexander et al. (1985) showed similar
results at the Fraser Experimental Forest (Figure 2).
They measured snowpack levels and tree inventories
in 1938 and then logged 25 acre plots of mature lodge-
pole pine. Different levels of harvest were completed
leaving 6000, 4000, 2000, and 0 board feet per acre
remaining. The highest water yield increases were
achieved with a complete cut leaving no trees. It was
noted from the data at Fraser that there was less dif-
ference in snow storage between a forest cut leaving
2000 board feet per acre and one leaving none. This
was attributed to increased redistribution of the
snowpack into nearby forested areas adjacent to the
harvest block. This may explain the fact that stream-
flow did show a greater percentage increase than
snow storage.
Reserve Stand (fum)
Figure 2. Plot of Streamflow and Snow Storage Increases
in Different Stand Densities Indicating the Significance
of Snow Distribution (from Alexander et al., lg85).
Spruce and fir were cut in a similar fashion. How-
ever the results differed from those of the lodgepole
pine forest, indicating the effects of vegetation types
on the change in water yield. Less change in water
yield was noticed, but other factors could have con-
tributed to this since it was in a different basin alto-
gether.
McCaughey et al. (1995) measured snow water
equivalent increases of 19 to 85 percent in forest
clearings in the northern Rocky Mountains of central
Montana. Average increase was 38 percent which cor-
relates well with Ttoendle and King (1987) who mea-
sured a 34.3 percent increase with 100 percent of the
basal area removed, and Gary and Watkins (1985)
with a 30 percent increase after thinning.
Troendle & King (1987) performed circular
clearcuts on the south facing slopes of the north fork
of the Deadhorse creek watershed, Colorado. The
effects of timber removal on water yield were 2 inches
annually at the lower edge.of the slope, or a 22 per-
cent increase. However snow accumulation showed
0.l6
0.14
-B o.tz
I
3 0.1
ou
E o.os
3 o.oo
aoo
g o.o4
o.u2
40
E30
E
Ed20
c
Ero
s
0

5. Snowpack Changes Resulting from Timber Harvest: Interrception, Redistribution, and Evaporation
little or no change. This was attributed to the south
facing slope's increased snow ablation caused by the
increased exposure.
Gary (1974) suggests that shape and orientation of
harvest blocks have an effect on snow deposition.
Strip cuts perpendicular to prevailing winds are ideal
for capturing blowing snow, but create a deficit down-
wind. He performed cuts of one tree height (1H) wide
and frve tree heights (5H) long in lodgepole pine 2
miles southeast of Fox Park, Wyoming. Cuts were
aligned perpendicular to prevailing winds. Again, the
results showed an increase in snowpack with harvest
yielding an increased SWE. After two years of obser-
vation, it was indicated that the upwind forest had a
SWE of 13.1 inches. In the opening water equivalence
increased Lo 16.2 inches (up 24 percent), while the
measure decreased to t2.2 inches in the downwind
forest (down 7 percent).
This downwind defrcit offsets the increased water
equivalent in the opening. Snow was redistributed in
the forest but there was no net gain in water yield
over a larger area. Melt rates were measured in the
opening at twice the rate of the forest. Measurements
in the middle of snow melt season showed water
equivalent as 12 percent less in the opening than in
forest. Gary (1974) attributed this to the possibility of
increased solar exposure in the sunnier north end,
although wind redistribution was not ruled out.
Snowpock Euaporation
As has been indicated from the results of Lundberg
and Halldin (1994), increased surface area of snow-
cover due to interception increases snow evaporation.
What then are the effects in an open field? There are
two key differences between openings in vegetation
and full forest: (1) in full forest, surface area is
increased allowing for increased interception of snow;
and (2) once a snowpack forms in an opening, only the
upper layers are available for sublimation since they
are in direct contact with air masses.
It has been shown that as much as 115.8 mm of
precipitation are lost from December to March in the
foothills outside Alberta (Bernier and Swanson,
1993). However, this study was done in a douglas fir
forest with different characteristics than lodgepole
pine. Measurements of evaporation were conducted
and compared to the calculated values using the aero-
dynamic formula. Although this approach is far from
ideal it was used by Bernier and Swanson (1993)
because of its simplicity, requiring only site measured
meteorological data and real or estimated snow tem-
peratures. The evaporation rate was computed as:
p
- PDnk" - eo) 0.6 (5)
z-
P
where E is the evaporation rate (g cm-2 s-1); p is the
density of air (g cm-3); DB is the bulk vapor exchange
coeffrcient (cm s'1); e" is the saturation vapor pressure
at the snowpack (kPa); eo is the vapor pressure at
height za (kPa); and P is the atmospheric pressure
(kPa).
Bulk vapor exchange coeffrcient was computed as:
o, _ k2t!.oiCl-b&,)z (6)
" ln2zo / zs
where & is von Karman's constant (0.4); uo is wind
speed at height zo(cm s-1); b is the empirical constant
(estimated at 5 by Dyer, t974);.R6 is the bulk
Richardson number; zo is the measurement height
(cm); and z6 is the roughness length (cm).
Results of the measured evaporation rates in the
study are indicated in Figure 3. Excessive melt on the
last day of March caused the exclusion of that data
since snowmelt may have been mistaken as snow
evaporation (Bernier and Swanson, 1993). A positive
association was determined between individual evap-
oration measurements and opening size with an R2
value of 0.983. It is important to note that when indi-
vidual daily values are used, the R2 value is 0.55 indi-
cating a high degtee of variability.
1.2
1
a
5 o-8
€ 0.6
> 04
0.2
0
510)52025
Opening size (H)
30 35 40
R2 = 0.9831
Figure 3. Scatterplot of the Mean Evaporation of
Each Opening Size and Opening Size in H
(data from Bernier and Swanson, 1993).
Measurements are compared to calculated amounts
of evaporation in Figure 4. In this plot, each day's
measurements are compared to the calculated
amounts. The computed values appear to have a high-
er variation than the measured values. This may be
1 357 WATER RESOURCES BULLETIN

6. Stegman
due to the aerodSmamic formula's excessive treatment
of one or more variables.
It was suggested by Bernier and Swanson (19g3)
that openings of lH to 6H were ideal in that they pro-
vided protection from solar radiation. Environmental
confitions during this study were not ideal for evapo-
ration. Although relative humidity was low, air tem-
peratures nnre above 0'C, and wind speeds were low.
These factors contribute to reduce the vapor pressure
gradient. With temperatures slightly lower than 0'C,
and wind speeds of 60 km hr-l there is a potential to
evalrcrate 87mm of precipitation in 100 days (Bernier
and Swanson, 1993).
This projection seems somewhat excessive. Assum-
ing that temperatures were slightly below 0'C at 6b'N
latitude for 100 days may be too conservative. Tem-
peratures at such a high latitude continental climate
are ofLen much colder for daily highs during winter.
This would decrease the estimation of potential evap-
oration over 100 days.
2'7 Feb.
Date
Figure 4. Plot of Measured and Computed Snow Evaporation.
The higher variability ia the computed values indicates
excessive trcatment of one or more variable in the
aerodynamic formula (data fmm Bernier and Swanson, 19g3).
A study in Alberta may be inappropriate to apply
to Rocky Mountain forests of Colorado at 40'N with a
25'higher sun angle during winter than at 6b'N in
Alberta. However, it is important to understand the
effects in general. At 40'N, a lH opening might be
sheltered less from solar insolation with the end
result being more loss to evaporation. The measured
evaporation losses as a result of cutting may offset
the proposed increase in water yield.
It is also important to note that studies done in
Fraser examined the end result of any differences in
water yield directly. This examination measured
streamflow through V-notch weirs and trapezoidal
WATER RESOURCES BULLETIN 1358
weirs (Alexander et al., 1985). Snow evaporation is
inclusive to the empirical measurements of stream-
flow. Therefore, if snowpack evaporation is considered
significant, it will have already been included in any
formula derived from empirical data.
Although the snow water equivalent increased as a
result of aerodynamic changes increasing deposition,
the increases were only observed within the water-
shed. Gary (1974) showed that less deposition
occurred in the downwind forest. Therefore, observa-
tion of a watershed cannot be assumed true over a
larger scale area due to the heterogeneity between
separate catchments.
Humidity has a large affect on the evaporation of
intercepted snow (Bernier, 1gg0; Bernier and Swan-
son, 1993; Lundberg and Halldin, 1gg4). As the rela-
tive humidity above the snow mass increases, the
ability of snow to sublimate to a vapor form is
reduced. This is closely related to wind velocity and
its gradient, since wind will be able to replace satu-
rated air masses with unsaturated air, thereby lower-
ing the relative humidity and increasing the loss to
sublimation by allowing it to continue (Figure b). The
data show that as relative humidity increases, evapo-
ration rates decrease.
These data were collected by Bernier (1g90) in
mature lodgepole pine with 20m tree heights in Alber-
ta, Canada. Figure 5 clearly shows an evaporation in
a full canopy (2500 stems per hectare) of one third
that of an open field. Numerous variables exist that
would make it difficult to extrapolate over larger
areas. It does however, indicate the relationship
between density of vegetation and snow evaporation
Iosses.
14 '76 87 90
Retative humidiry (%)
Figure 5. Plot of Snow Evaporation as a Function of Relative
Humidity in Various Stand Densities Data
(fromBernier and Swanson, 1998).
2.5
E2
;
E r.s
,=
E
05
0.6
E 0.5
E o.+
€
B 0.3
>
Z 0.2
0.1
0

7. Snowpack Changes Resulting from Timber Harvest: Interception, Redistribution, and Evaporation
The studies of Bernier and Swanson (1993) were
done in February, when sun angles would have been
low. The effects, then, of forest cut in more temperate
regions (30-40") would probably be increased. At these
lower latitudes, sun angle would be higher and days
would be longer in winter months. These factors
would increase the solar radiative flux to the snow-
pack. The Iesult may be increased water loss from
increased time of insolation received by the snowpack,
with other variables held constant.
CONCLUSION
Review of the effects of timber harvest on water
yield has brought out many key concepts in hydrolory.
While the importance of some of the processes is
arguable, geographic location plays a part. In some
areas, evaporation of snowpack may be valid and sig-
nificant, while in other areas it may not. One recur-
ring observation is that with a decrease in vegetation
density, water yield increases.
While an increase in water yield and snow water
equivalent were found to be a result of less vegeta-
tion, the counteractive effect of evaporation of snow
must be considered. With an increase in opening size,
more of the opening is subject to solar insolation at a
given angle. As the time of exposure and intensity of
insolation increases, so does the duration of snow
evaporation.
It is important to consider that the degree of pro-
tection from radiation is a function of latitude, orien-
tation to prevailing winds and aspect. For these
reasons the optimum size of clearing will vary with
location and direction. At high latitudes it may be
possible to create large openings that would not be
subject to the same degree of evaporation as at lower
latitudes. In addition, north facing slopes will limit
the loss to direct evaporation, although local and
regional wind patterns may alter the optimum aspect
for a clearing.
The balance between losses and gains is subject to
environmental factors. As indicated previously (Gary,
1974), the result of forest cut is the redistribution of
snow. Results are not an overall increase in water
yield as deficits downwind offset gains upwind. Wind
carries snow further than the gravitational bound-
aries of watersheds. For this reason, the results of
these studies cannot be taken as universal but rather
observations at specific loeations.
The initial question of this paper asked if there
was a balance between increases resulting from har-
vest and the decreases due to evaporation. Snow
evaporation is inclusive to water yield measurements
and therefore such measurements can suggest the
optimum size of opening. It has been suggested by
Bernier and Swanson (1993) that openings 1H to 6H
still offer some protection from radiation. Alexander
et al. (1985) and Tloendle and Leaf (1980) show the
highest snow retention coefficient for 5H openings,
although these studies were concerned more with
wind scour than evaporation. Golding and Swanson
(1978) found the highest snow water equivalent in 2H
openings, but the values were quite similar to those of
the 5H openings. These studies have yielded varying
optimal sizes but the overlapping of these sizes seems
to center around the 5H. It is possible that the 5H
opening size provides significant increases in water
yield while still offering protection from sublimation
and scour.
Snowpack change is but one of a number of effects
resulting from timber harvest. The overall result
appears to be an increase in net water yield, although
this is not true in all situations. Proper management
may be able to increase water yields, particularly in
the western region of the United States where water
is in shorter supply. However, care must be taken not
to allow increases beyond a level that local streams
can accommodate. This may be of great benefit in
easing the strain on western water resources which
arises from the ever growing population in the
western United States.
It is important that future studies recognize the
hydrologic effects on a larger than watershed level.
By observing one watershed, upwind and downwind
effects are excluded. This may be necessary to under-
stand one process in detail, but additional variables
must be accounted for when considering a larger area.
Although experiments on larger watersheds may be
costly, and in some cases physically impossible, they
would give a better understanding of the processes
involved. Linking the cumulative effects of smaller
watersheds within a larger one would relate individu-
al processes to the water yield of higher order streams
and available water supply for consumptive use and
instream flows.
LTIERATIIRE CITED
Alexander, R. R., C. A. Tloendle, M. R. Kaufrnann, G. L. Shepperd,
G. L. Crouch, and R. K. Watkins, 1985. Fraser Experimental
Forest Colorado: Research Program and Published Research
1937-1985. U.S. Department of Agric. For. Serv., Gen. Tech.
Report RM-118.
Berndt, H. W., 1965. Snow Accumulation and Disappearance in
lndgepole Pine Clearcut Blocks in Wyoming. Journal of Forestry
63:88-91.
Bernier, Pierre Y., 1990. Wind Speed and Snow Evaporation in a
Stand ofJuvenile [.odgepole Pine in Alberta. Canadian Journal
of Forest Research 20:309-314.
1359 WATER RESOURCES BULLETIN

8. Bernier, P. Y. and B. H. Swanson, 1993. The lnfluence of Opening
Size on Snow Evaporation in the Forests of the Alberta
Foothills. Canadian Journal of Forest Research 23:234-239.
Bosch, J. M. and J. D. Hewlett, 1982. A Review of Catchment
Experiments to Determine the Effect of Vegetation Changes
on Water $eld and Evapotranspiration. Journal of Hydrology
55:3-23.
Dyer, A. J., 1974. A Review of Flux-Profrle Relationships. Bound-
ary-Layer Meteorolog. 7 :363 -37 2.
Gary, Howard t, Lgll. Snow Accumulation and Snowmelt as Influ-
enced by a Small Clearing in a Lodgepole Pine Forest. ilater
Resources Research 10 :348-353.
Gary, H. L. and C. A. Tloendle, 1982. Snow Accumulation and Melt
Under Various Stand Densities in lodgepole Pine in Wyoming
and Colorado. U.S. Department of Agric. For. Serv., Rocky
Mountain Forest and Bange Experirnent Station, Research Note
RM417.
Gary, H. L. and B. K, lYatkins, 1985. Snowpack Accumulation
Before and After Thinning a Dog-Hair Stand of todgepole Pine.
U.S. Department of Agric., For. Serv., Rocky Mountain Forest
and Range Experiment Station, Besearch Note RM-450.
Golding, D. L. and R. H. Swanson, 1978. Snow Accumulation and
Melt in Small Forest Openings in Alberta. Canadian Journal of
Forest Research 8:380-388.
Golding, D. L., 1978. Calculated Snowpack Evaporation During
Chinooks Along the Eastern Slopes of the Rocky Mountains in
Alberta. Journal of Applied Meteomlogy L7 :L647 -L65L.
Haupt, H. F., 1979. Effects of fimber Cutting and Revegetation on
Snow Accumulation and Melt in North Idaho. U.S. Department
of Agric., For. Serv., Research Paper INT-224.
Harr, R. D., 1976. Forest Practices and Streamflow in Western Ore-
gon. U.S. Department of Agric., For. Serv., General Techaical
Report PNW-49, 18 pp.
Harr, R. D., 1980. Streamflow After Patch Logging in Small
Drainages Within the Bull Run Municipal Watershed, Oregon.
U.S. Department of Agric., For. Serv., Research Paper PN'1il-249,
22pp.
Harr, R. D., 1982. Fog Drip in the Bull Run Municipal Watershed,
Oregon. Water Resources Bulletin 18(5):785-789.
Harr, B. D., 1983. Potential for Augmenting Water Yield Through
Forest Practices ia Western Washington and Western Oregon.
Water Resources Bulletin 19(3):3{13-393.
Hibbert, A. R.,1967. Forest Tleatment Effects on Water Yield. Int.
Symp. of Forest Hydrolory, Oxford, United Kingdom.
Lundberg A. and S. Halldin, 1994. Evaporation of Intercepted
Snow: Analysis of Governing tr'actors. Water Resources Research
30(9):2587-2598.
Male, D. H. and B. J. Granger, 1981. Snow Surface Energy
Exchange. Water Besouroes Research L7 :6O9-627.
Merrianl R. A., 1960. A Note on the Interception Lnss Equation.
Journal of Geophysical Research 65: 3850 -385 1.
McCaughey, W., H. Hansen, and P. Pharnes, 1995. Preliminary
Information on Snow Interception, Accumulation, and Melt in
Northern Rocky Mountain [odgepole Pine Forests. Proceedings
of the Western Snow Conference, pp. 49-55.
Schmidt, R. A. and D. R. Gluns, 1991, Snowfall Interception on
Branches ofThree Conifer Species. Canadian Journal ofForest
Research 2L:1262-1269.
Satterlund, D. R, and H. F. Haupt, 1967. Snow Catch by Conifer
Crowns. Water Resourtes Research 3(4):1035-1039.
Iloendle, C. A. and C. F. teaf, 1980. Hydmlogy. Chapter III. An
Approach to Water Resources Evaluation of Non-Point Silvicul'
tural Sources. EPA 60018-80-012, Environmental Researth Lab-
oratory, Athens, Georgia, 173 pp.
l}oendle, C. A., 1983. The Potential for Water Yield Augmentation
from Forest Management in the Roclry Mountain Region. Water
Resour.ces Bulletin 19(3):359-373.
WATER RESOURCES BULLETIN
Ihoendle, C. A., 1986. The Effect of Partial Cutting and Thinning
on the Water Balance of the Subalpine forest. Paper presented
at Future Forests of the Mountain West: A Stand Culture Sym-
posium, Missoula, Montana.
Troendle, C. A. and R. M. King, 1987. The Effects of Partial
Clearcutting on Streamflow at Deadhorse Creek. Journal of
Hydmlog 90:145-157.
West, A. J., L962. Snow Evaporation from a Forested Watershed in
the Central Sierra Nevada. Society ofAmerican Foresters, Jour-
nal of Forestry 60:481-484.
Stegman
1360