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KI·lllnlnh.()lz Sllowd.J~ifts: Hydrologic
Ilnplic(lt:i()llS (It (1 C())O)·(ldo rr)·eeiille Site
Neil H. 8erg1
Abstract--The water equivalent of snow contained in (irifts downwind
of kJUlT1ITlholz vl~getation at treeline on Niwot l,id~le in the Colorado Front
Range Is estimated at 8,060 m:3annually (1830 m3 1<m-2 Iand area), or about
1% of the sumlTH~r streamflow from a 191-ha alpine basin adjacentto Niwot
Rld~le. Most of this water would be lost to sublimation If the vl~getatlon did
not capture the snow. Much of the watl~r becomes available aftl~r the date
of pe?ll< discharge and contributes to late··season water supply and soil
watet recharge.
-------------------------,---_.
Blowing and drifting snow are important factors in alpine
environments. The upper reaches of the subalpine z.one become a deposition site for snow redistributed from the alpine.
At the forest-alpine tundra ecotone, low-lying "krummholz."
vegetation is the first impediment to snow-bearing winds.
When snow is redistribute.d into drifts to t.he lee of krummholz
tree islands, evaporation of wi.nd-driven snow is reduced. As
snow drifts me.lt, the.y become water sources to natural ecosystems and, particularly in their late .. season residence after the
primary snow cover has melted, for human use.
Niwot Ridge, an east-west trending spur on the east slope
of the Colorado Front Range, 35 km northwest of Boulder,
Colorado (4003'20" N, 105035' W), ranges in elevation from
3450 to 3800 m msl. At increasing elevations on Niwot Ridge,
the closed crown forest gives way to stunted growth forms near
the upper limit of tree growth as environmental conditions
increase in severity. Severely deformed "krummholz" exist as
isolated clumps of one or more trees. "Flag" krummholz grow
where environmental conditions allow one or several vertical
branches to survive above the protection of the winter snowpack; below the pack, growth is dense and mat-like. "J\.fat"
krummholz exist at slightly higher elevations under extreme
conditions that do not allow survival of the vertical leaders
(Daly 1984). On Niwot: RidgePicea engeimannii (Engelmann
spruce), with admixture·s of Abies /asiocarpa (subalpine fir)
and Pinus J7eJoilis (limber pine), are the primary mat krum ..
mholz species. Krul1lmholz. occur over an elevation range. of
approximately 200 m and form a discontinuous catchment
zone for blowing snow spanning the length of the Front Range
(Ives and Hansen-Bristow 1983). This paper investigates the
hydrologic. role of krummholz snowdrifts and estimates the
water equivalence and timing of release of snowmelt water
from krummholz drifts on Niwot Ridge.
Methods
To e.stimate the water equivalent of krummholz snow
drifts, four steps were followed: (1) mea.sure krummholz. and
drift geometries; (2) relate krummholz geometry to drift shape
to determine individual snowdrift volumes; (3) quantify krummholz occurrence on Niwot Ridge; and (4) determine water
equivalent.
J\.iat krummholz tend to be wedge-shaped with their apex
height at the downwind edge (fig. 1). Width and height
measurements were made at the apex of 171 randomlyselected mat krummholz during February and J\.iarch, 1975,
near the ce.ntral portion of the Ridge. The length of the
associated lee drifts and snow depth at the apex were also
recorded. Techniques have been established to estimate drift
volume, V. Tabler (1975:95) developed a snow retention
mode.! for sagebrush (Artemisia spp.) in which the drift
forming behind an isolated plant approximates the shape of a
half.. ~one, with length 10H (H:= plant height), so that V =
5.2H.
The areal frequency of mat krummholz. was determined by
relating aerial photograph observations to a vegetation map of
the Niwot Ridge forest-alpine tundra ecotone. HansenBristmv's (1981) vegetation map (1:10,000 scale) shows mat
krummholz. in a discontinuo,!!s band approxi.mately 350 m wide
over an area about 4.4 km L , along the eastern two-thirds of
Niwot Ridge. Si.nce i.ndividual plants were not designated on
this map, color aeri.al photographs ( circa 1:15,000 scale) taken
October 2, 1974 were analyz.ed with a zoom stereoscope
capable of 10 X magnification. Individual trees were identified
1Supervisory Hydrologist, Pacific Southwest Forest and Range
Experiment Station, Forest Service, U.S. Department of Agriculture, P.O.
Box 245, Berkeley, Calif. 94701
141
Figure 1.--Schematlc representation of a mat krummholz and lee
snowdrift.
on the photograph and compared to their location on the map
to determine growth form. Shadow length helped identify
flagged trees.
Determination of snow water e.quivalent requires knowledge of the density of the drifts. Koerner (1969) found density
in krummholz lee drifts on Niwot Ridge to vary generally
between 0.40 g cm-3 and 0.47 g cm-3 and inc.rease through time
into the ablation period. These measurements, based on
approximate.!y 100 sampling points, are the basis for use of 0.45
g cm- 3 as the value for snow density at the maximum accumulation date. Water equivale.nt for individual snow drifts follows
as the. product of krununholz frequency, drift volume, and
density. Total snow drift water equivalent for the basin is
calculated by applying a probability density function developed from the field measurements of apex snow depth (taken
to equal H) to the total krummholz population on the Ridge
by means of the "half-cone" volume equation.
island lengths (table 1), averaging 3.75 m, were similar to those
mapped by Koerner (1969). Mean tree width-to-height ratio
was 2.4, and lee drift lengths averaged 8 times ma~mum drift
depth (table 1). Relative variation of tree length was greater
than for the other measured properties. The coefficient of
variation ranged from 0.40 to 0.47 for the other four variables,
but was 0.65 for tree length. The distribution of snow depths
at the tree apex is not significantly different from normal
(alpha = 0.05) (fig. 2).
Four thousand, six hundre.d and ninety mat krummholz
(approximately 670 km- 2) were estimated as occurring within
the Niwot Ridge forest-alpine tundra ecotone. The volume of
snow in the associated lee drifts was estimated at 17900 m3,
with a snow water equivalent of 8,060 m3, or 1,830 m3 km- 2 (at
snow density = 0.45 g cm-3). To put this value into perspective,
in 1974, a slightly above-average winter precipitation year on
Niwot Ridge, peak daily discharge from a 191-ha alpine basin
immediately south of the Ridge was 35,200 m3 (Carroll 1974).
Drift snow water equivalent therefore approximated onequarter of the peak annual single-day streamflow, or about
0.8% of the mid-June through late-October streamflow.
II
--
12
~
Results and Discussion
Measured tree heights (table 1) were within the ranges (0.5
to 2 m) measured by Koerner (1969), and noted by HansenBristow (1981:38): "In this form the tree is dwarfed to a mat,
usually no higher than 1.5 m, but up to 5 m long." Similarly, tree
10 10 10 40 10 10 70 10 80 100 110 120 110 140 110 110 170
Snow Depth {em}
Figure 2.--Krummholz apex snow depths on Niwot Ridge, Colorado.
Table 1.--Krummholz and lee snowdrift dimensions on Niwot Ridge,
Colorado, February-March, 1975.
Variable
Krummholz width
Krummholz height
at apex
Krummholz length
Drift length
Drift depth at
krummholz apex
Mean Median
Error Assessment
Sample
Min. size
(Qm)
(run)
(run)
S.D.
(Qrn)
Max.
(Qm)
269
255
123
755
55
170
112
375
605
115
320
560
45
245
282
295
2,000
1,750
12
60
100
131
171
171
76
75
34
168
8
171
The magnitude of errors associated with several components of this analysis needs explanation. Errors associated
with the krummholz frequency estimate are probably the
largest in this study. Even with the optical magnification
available for the aerial photograph interpretation, trees less
than 20 cm tall may not have been identified and the distinction
between mat and flagged forms was not always completely
reliable.
142
The half-cone equation (Tabler 1975) was formulated for
an area with unidirectional winds, uniform terrain sloping less
than 15%, sufficient snow for the drifts to reach equilibrium
profiles, crown width approximate.ly twice crown height, and
drifts approximately 10H in length. These conditions were
generally satisfied within the krummholz communities
sampled on Niwot Ridge. In this application, apex snow depth
is the effe.ctive crown height, since the top portion of the
krummholz is sparsely foliated and relati.vely ineffectual as an
aerodynamic barrier. On the average, measured tree width
was slightly more than twice tree height and drift length was
slightly less than 10 times snow depth at the apex. The
importance. of barrier hei.ght in determining lee drift volume
is supported by numerous snmy fence studies (Tabler 1986)
and by the moderate.ly high Pearson c.orrelation coefficients
for associations between drift length (a proX}'for drift volume)
and both krummholz height (r= 0.82) and apex snow depth
(r= 0.78). The shape and porosity of krummholz and sagebrush differ; krummholz are wedge-shaped whereas sagebrush are spherical. Not enoughi.s known about the magnitude
of these. variations to quantify the e·rror.
Use of a constant value for snow density is a simplification.
Snow deposited i.n the interstices of the vegetative mat, while
relatively low in volume, i.s not accommodated, nor is the
interaction between krununholz drifts and topographic snow
traps. Snow firnification, drift erosion due to high velocity
winds, variations in tree island aspect and the consequent
variations in ablation rates due to differences in insolation a.re
likewise not c.onsidere.d. These jnflue.nces are essentially
impossible to quantify on a basin scale. The calculations
de.scrIbed here are a conservative "first approximation" of
drift snow water equivalent.
probably only a few days after the date of peak stream
discharge. This may be due to their relatively shallow depths.
Drifts behind flag krummholz., on the other hand, can persist
several weeks longer, often well into July. The 1974 measurements support this claim. ~faximum depths at terrain drifts
were greater than 3.8 m on April 17, 1974, a date when the
maximum depth recorded at a nearby flag krummholz drift
was 3.7 m. Snow over 1.5 m deep persisted at the terrain drifts
as late as mid-July 1974, several weeks after the June 24 peak
streamflow discharge in nearby Green Lakes Valley. Although
measure·ments of the flag krummholz drift were not made in
July, given the near ~quality of snow depths on April 17, it is
reasonable to hypothesize. that the flag krummholz drift had
not completely melted in mid-July, 1974. The flag form of
krummholz is generally larger than the mat form, so that
deeper drifts form which take longer to melt. It may be that the
optimal condition for both drift accumulation and melt delay
occurs when krummholz, either flag or mat, are situated
immediately upwind of terrain snow traps, as on ridge-terrace
line.s (Koerner 1969).
Alpine. snowfields gene.rally supply late season runoff to
lowland areas, and the preliminary observations reporte,d in
this paper suggest that drifts associated with krummholz
contribute to runoff even later than the. average from alpine
c.atchments. Although the quantity of water from the drifts is
relatively small, about 1%. of the summer streamflow from a
191-ha alpine. basin adjacent to the study area (or one-quarter
of the. peak single day discharge), a 7-to-10 day delay in release
of snowmelt water from the larger krummholz drifts adds to
the importance of this water source in an otherwise "summer
dry" lowland environment. Much of this water would be lost
to sublimation if the vegetation did not accumulate snowdri.fts.
Timing of Water Availability
Acknowledgements
Even if the actual water equivalent is twice the estimated
amount, the total water contained in the drifts is relatively
small when compared to typical streamflow volumes. The
importance of this water is not primarily in its magnitude, but
may be in the timing of its availability.
Estimation of the timing of water release. from the drifts on
Niwot Ridge has relied on two data sources: fie.ld measurements of several mat krummholz lee drifts made throughout
the 1965 ablation period (Koerner 1969), and of a flag krummholz and terrain drifts during 1974 (Berg 1977).
Extrapolation of Koerner's data suggested drift disappearance by June 25,1965. Extreme snowpack ablation rates
occurred in 1965 (Rennkk 1966), therefore the late-June
melt out date was probably earlier than average. Even so, in an
above-normal snowfall year, 1974, peak stream discharge.
occurred on June 24, suggesting that krummholz drifts may
melt out after the stream flow peak. C~asual observations over
several years by Daly (per. comm.1987) suggest, however, that
mat krummholz drifts melt relatively quickly, and disappear
This study was supported by the Institute of Arctic and
Alpine Research, Unive.rsity of Colorado, and the. Roc.ky
Mountain Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture.. Katherine HansenBristow, Christopher Daly, Bruce McGurk, Richard Minnich,
and Ronald Tabler made valuable review comments on an
earlier draft of the manuscript.
Literature Cited
Berg, Neil H.1977. Prediction of natural snowdrift accumulation on alpine ridge sites. Ph.D. Dissertation, Departme.nt
of Geography, University of Colorado, Boulder, CO.
:310 p.
Carroll, Thomas Ray. 1974. The water budget of an alpi.ne
catchment in central Colorado. :M:.A. Thesis, Department
of Geogra.phy, University of Colorado, Boulder, CO.
124 p.
143
Koerner, John Marvin. 1969. Krummholz influences on alpine
snow accumulations. J\f.A. Thesis, Department of Geography, lJniversity of Colorado, Boulder, CO. 115 p.
Rennick, Kenneth 8.1966. Floods of May-June, 1965 in eastcentral Wyoming. Open-file report, U.S. Ge.ological
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I
144