VOL.
6, NO. 2
WATER
RESOURCES
RESEARCH
APRIL
1970
The Disposition
o[ SnowCaughtby ConiferCrowns
DONALD
R.
SATTERLUND
WashingtonState University,Pullman,Washingtern
99163
HAROLD
F.
HAUPT
Forestry SciencesLaboratory, Moscow,Idaho 838•3
Abstract. Snow interception studiesduring the warm winters of 1966-1967and 1967-1968
in northern Idaho revealed that Douglas fir and westernwhite pine saplingscaught about
one third of the snowthat fell in 22 storms.More than 80% of the snow initially caught in
the crownsultimately reachedthe groundbeing washedoff by subsequentrain, falling by
direct massrelease,or drippingas meltingsnow.Only a small portionwaslost by evaporation.
INTRODUCTION
This paper is a continuationof a previous
study in which the developmentof interception
storageof snowon saplingDouglasfir (Pseudotsuga menziesii var. glauca [Beissn] Franco)
andwesternwhitepine (PinusmonticolaDougl.)
was described[Satterlundand Haupt, 1967]. It
reports on the dispositionof interceptedsnow
to the ground and the atmospherefrom seven
trees of eachspeciesduring the periodJanuary
10, 1967 to March 27, 1967, and December4,
rainstormswas evaporated,and that all the
snowreleasedfrom the treesreachedthe ground
in solidor liquid form.
Snow also fell from the trees en masse after
a storm ended, and could be identified by a
sharp, vertical trace on the intereeptograph.
Whenever one pound or more of snow fell we
could measurethe quantity directly from the
trace. We refer to these directly measured
quantities of snow released from the trees as
'large mass release.' Snow also fell from the
trees at intervals too dose, and in quantities
1967 to April 8, 1968.
too small, to measure individually from the
METHOD
Or STUDY
intereeptograph.During periodsof above-freezThe experimentalsite, recordingapparatus, ing temperatures,drip from melting snow on
and field procedurehave been previously de- the branchesoften accompaniedthe releaseof
small massesof snow.Either way the snow fell
scribed [Satterlund and Haupt, 1967; Haupt
and Je•ers, 1967]. Basically,a continuous
weight resulted in an irregular decline in the trace.
record of snow load on suspendedtrees (an We call the release of water from the trees
'interceptograph') was maintained except for which is too small in quantity to measure intwo storm periodsin December1967, and Jan- dividually (whether as snow, drip, or both)
'minor mass release.'
uary 1968, when recordswere lost becauseof instrument malfunction.
Minor mass release was often accompanied
.The catch, retention, and release of snow by conditions conducive to evaporation, and
by the trees was followed on the intercept- frequently extended over periods of several
ograph (Figure 1). During snowstorms,the hours or more. Though the total loss of water
from the trees could be measured from the inbuildup of the snow load was accompaniedby
a rise of the trace. Many of the snowstorms tereeptographduring such periods,the amount
turned to rain, or were followedcloselyby rain. that reachedthe ground and the amount that
An irregular, decliningtrace during rain indi- evaporated could not. Therefore we measured
catedthat snowwas beingwashedoff the trees. the quantity reachingthe ground by catching
It was assumed,based upon considerations
of it on a large plasticsheetspreadbelowthe trees
vapor pressure,that none of the loss during and weighingit directly. The differencebetween
649
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
650
BRIEF REPORTS
.10•
e bxJ•
0 o. o
0
l;! Snow
BRain
and
.-
snow
BRain
2
ß lb
•
1/4
I
I
i
I
I
I
I
I
I
I
I
20
M
21
M
22
M
23
M
24
M
26
DAY
Fig. 1. A compositeinterceptographcombiningportions of several actual interceptographs
to illustrate different aspectsof the snow interception water balance. Days 20 and 21' snow
beginsand accumulateson :•hebare tree. As the storm turns to rain, part of the intercepted
snow is washed to the ground. Day 22' warm temperatures result in minor mass release
(irregular decline) and large massrelease (vertical trace) of snow.Day 23: slight decline due
to evaporation of intercepted snow. Day 24: rain washed remaining snow from tree leaving
it bare and wet. Day 25: liquid water evaporates leaving tree bare and dry.
the amount lost from the trees, as revealedby
the interceptograph, and that caught on the
sheet was attributed to evaporation.
The measurements
of minor mass release to
the ground were frequently nullified by additional precipitation. Consequently, we were
forcedto estimateits quantity by extrapolating
our few successful measurements
over the en-
from two major snowstorms,with a water
equivalent of 2.33 inches,were lost becauseof
instrument malfunction
in the 1967-1968 season.
A completetabulation of the dispositionof interceptedsnowis presentedfor eachtree species
in Table
1.
0nly about one third of the snowfall was
temporarily retained in the tree crowns at the
tire period that minor mass release occurred.
Temperature, wind, humidity, and radiation
records were checked against the interceptograph to delineate the periods when minor
mass release and evaporation were concurrent.
Evaporation during periodswithout massrelease could be identified by a slow, regular decline of the trace but occurred with surprising
infrequency.Less than one third of the snowstoms were followed by one or more identifiable periods of evaporation without concur-
end of snowfall.Lossesback to the atmosphere
from intercepted snow representedonly 4.5%
and 5.2% of total snowfallfor the Douglas fir
and white pine, respectively.
Most of the intercepted snow (86% of the
initial catch) ultimately reached the ground.
The greatestquantity (about 46%) was washed
off by rain. Large mass release of snow from
the branchesaccountedfor an additional 25%,
and minor massreleaseand drip from melting
rent mass release.
mainder.
RESULTS
snow in the
branches
accounted
for
the
re-
No important quantitative differencesin total
snow catch or in the mode and timing of loss
of snow from the crowns were evident by
species.Though no statisticaltests were undertaken, the obviousstorm-to-stormvariability in
The weather during the winters of 1966-1967
and 1967-1968 was warm and moist, and unfavorable atmospheric conditionsfor evaporative lossesof intercepted snow predominated. catch and loss insures that the small differences
Precipitation was much above normal in the in Table I are not significant.
1966-1967 seasonand near normal during the
DISCUSSION
1967-1968 season,but most of it occurred as
I)espite the rather great differencesin form,
rain. 0nly 22 snowstorms,
with a water equivalent of 8.65 inches,were sampled.The records needlestructure and arrangement,branch stiff-
BRIEF REPORTS
651
TABLE 1. The Dispositionof Intercepted Snow from Douglas Fir and Western White Pine Saplingsat
the Priest River Experimental Forest (winters of 1966-1967 and 1967-1968)
Douglas Fir
Water Equivalent,
inches
Total
snowfall
8.65
White Pine
Water Equivalent,
inches
%
%
100
8.65
100
Grossinterception storage*
2.78
32.1
3.07
35.5
Throughfall during storms
Snow washedoff by rain*
Large mass releaseto ground*
Minor massreleaseand drip to groundt
5.87
1.33
0.68
0.38
67.9
15.4
7.9
4.4
5.58
1.36
0.80
0.46
64.5
15.7
9.2
5.3
8.26
95.5
8.20
94.8
0.13
1.5
0.15
1.7
0.26
3.0
0.30
3.5
0.39
4.5
0.45
5.2
Total to ground
Evaporation*
Evaporation during minor massreleaseand
drip t
Total evaporation loss
* Measureddirectly from interceptograph.
• Estimated on basisof samplemeasurements.
ness, and other characteristicsof Douglas fir
andWhitepinecrowns,
therewaslittlediffer-
the atmospherecould not changethe essential
conclusionof a small evaporative lossfrom snow
ence in either their
interception.
catch or release of snow.
This suggeststhat meteorological conditions
rather than morphologicalcharacteristics
of the
trees themselvesdominate in the interception
of snow from evergreenconifersof similar size.
The dominanceof the meteorologicalfactor is
further indicatedby the similar timing of snow
lossesby both speciesto the ground by rainwash, mass release,and drip.
The evaporative lossesfrom snow interception were surprisingly small from either an
absolutestandpoint or relative to total snowfall.
Several factors are involved.
Instrument malfunction during two storm
periodswhen a total of nearly 2« inches,water
equivalent,of snow fell prevented measurement
of more than one fifth of the total snowfall;
thus, the absolute measure of loss is less than
the actual loss.However, sincethe proportion
of intercepted snow released to the ground,
particularly by rainwash and large mass release,was greater following heavy storms than
light storms,the percentageof evaporativeloss
may be slightly overstated.
Further, the estimatesof minor massrelease
and drip reaching the ground contain an unknown amount of error. But even a substantial
error in partitioning between the ground and
The primary cause of the small evaporative
losswas the warm, moist winter weather at low
elevations(2400 feet, MSL) at the Priest River
Experimental Forest. This is evidencedby the
substantial
removal
of snow from the crowns
by rain and by the large mass release that
almost always occurred during above-freezing
weather. (The mechanicalaction of wind in this
protectedvalley site was not important to either
catch or releaseof snow).
The small evaporative loss from intercepted
snowsupportsthe observationsof Miller [1962]
in the Sierra Nevada
of California.
He
con-
cluded that warm daytime temperatures,which
were accompanied by the rapid release of
snowby massreleaseand drip, sharply limited
interception losses.However, such results may
not be representative of somewhat cooler and
drier conditions,under which snowmay persist
on the trees exposedto evaporative lossesfor
much longer periods of time. Snow seldom remained on the trees for more than a day or two
after each storm, so caution must be used in
extendingthese data to colder climatic conditions.
Though mass releaseof interceptedsnow or
drip to the ground is the major factor in the
652
BRIEF REPORTS
small interceptionloss,the slowaccumulationof
interceptionstorageat the beginningof snowstorms is another. Unlike rainfall, small storms
contribute little to snow interception losses,
for throughfall is high during the early stages
of eachstorm [Satterlundand Haupt, 1967].
The negligiblecatch of snowin small storms,
combined with the rapid mass releaseof snow
or drip to the ground followinglarge storms,
was sufficientto eliminatemost opportunityfor
evaporative lossesto occur in this study. The
dominance of the meteorologicalfactors also
maskeddifferencesbetweenthe Douglasfir and
western white pine which might have been expected in view of their considerablydifferent
crown morphology.
Acknowledgments. Scientificpaper 3134, Washington State University College of Agriculture,
Pullman, project 1849. This investigation was
supported in part by Cooperative State Research
Service funds from the McInti,'e Stennis forestry
researchprogram and was conducted in cooperation with the U.S. Forest Service, Intermountain
Forest and Range Experiment Station, Moscow,
Idaho.
The
contribution
of Bud
L. Jeffers and
Boyd G. Hill in obtaining field data in the face
of adverse conditions is gratefully acknowledged.
REFERENCES
Haupt, H. F., and B. L. Jeffers, A system for
automatically recording weight changes in
sapling trees, 4 pp., U. $. Forest $erv. Res. Note
INT-71, Intermountain Forest and Range Experiment Station, Ogden, Utah, 1967.
Miller, D. H., Snow in the trees--Where does it
go?, Proc. $Oth West. Snow Co•f., 21-27, 1962.
Satterlund, D. R., and H. F. Haupt, Snow catch
by conifer crowns, Water Resour. Res., $(4),
1035-1039, 1967.
(Manuscript received August 11, 1969;
revised December 1, 1969.)