Document 12388657

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BY W. R. M^rrOON.
The large treeless.openings
or "parks"in the westernyellow
pine forestsof the southwest,which form a well known characteristic,afford an excellentopportunityfor a comparativestudy
of the effectof a forestcanopyuponlocal snowconditions. During the late winter and springof •9o9, the writer had an exceptionallyfavorableopportunityfor observingthe progressof snowfall and subsequent
melting in a virgin standof westernyellow
pine near the baseof the San Franciscopeakson the Coconino
National Forest in northern Arizona.
The observations include the measurement of each successive
snowfall,and the total depth of snow at intervalsof sevendays
under two entirely different forest conditions,namely,in a virgin
standof mature timber and on an adjacenttreelesspark, covering
au area of severalsquaremiles. The observations
were taken
during the period from February 26 to April 25, at an altitude
of approximately7,5oofeet.
On March zz, the averagedepth of the snowfall from a two
days' storm was 4.0 inchesin the park, as comparedwith 5.0
inchesin the forest, a differenceof 25 per cent. in favor of the
forest. A snowfallon March 23 measured•o.8 per cent. deeperin
the forest. Thesemay be takenas fair examplesof the difference
under the two conditions. As an explanation,it seemsprobable
that the sweepof wind acrossthe park carriesalong a certainexcessamountor load of snow from the snow gauge,which under
the quieteratmospheric
conditionsprevailingin the forestis ordinarily deposited a phenomenoncorrespondingin many respects
to the well known laws governingthe depositionof silt by water
currents. Over a forested area broken by parks the maximum
depositionoccursat the margin of the parks, the normal deposition in the forest body, and the minimum over the parks and
larger openings.
Contraryto the usuallyacceptedfact, during the early spring,
earlierand progresses
more rapidlyin the
forestthan in the opentreelessareas. This is due to differencein
radiation. Recordstakenin bothsituations
showa muchhigher
averagetemperaturein the forest,due to the fact that the night
are from 5 to t5 degreeswarmerthanin the adjacent
parks. Early in Marchit wasnotedthat the soilbeneaththesnow
in the forestgenerallycontainedno frost and consistedof soft
mud. The originof the.soil is from decomposing
basalticlava bed. At this time, however,a thick ice layer had
formedbeneaththe snowin the openpark and was constantly
thickening,dueto thelow dailyminimumtemperatures.By April
t, the ice layer had reacheda thicknessof 3 to 6 inches,and it is
safe to say that during the month of March a relativelysmall
amount of water from the surface melting reached the soil
throughoutthe park.
On March •7, the averagedepthof snowin the forestwas • •-5
inches,and of snow and ice in the park •9.5 inches,with water
equivalentsof 5.2 and 9.4 inches,respectively.The figuresare
significant,when it is recalled that the measurementsshow considerablyless snowfall in the park than in the forest. The disproportionof the ratio causedby the high water contentin the
park will be noted.
The distribution
of snowwasuniformin the smooth,openpark,
while in the forestthe groundsurfacewasexposedin manyplaces
and snow banks from 2 to 4 feet deep occurred in the natural
openingsand lanes betweentree groups. The depth for the
forestwas obtainedby averagingmeasurements
taken at ten snow
stationsspacedabout 5ø feet apart on a due north and south line.
A similarline of stationswasestablished
andusedin obtainingthe
park measurements.
The surfacerun-offin the two situations
is interesting
standpointof water conservation. By April •, bodiesof water
overlying the ice sheet had collectedin the depressionsin the
park, and a good-sizedstreamwas flowing at the outlet. No perceptiblesurfacerun-off from the forest (over the locality under
consideration)occurredduring March. The days of April •, 2
and 3 were unusuallywarm analquiet, and resultedin the only
run-off from the forest during the entire spring. The amount
was insignificantcomparedto the total water contentof the snow
mass. It is well to state, incidentally, that the writer made dailv
Effects o[ Forest Upon Snow Waters.
trips betweenthe two measuringstations,which afforded an
opportunityfor notingthe conditions.
A spell of warm weather occurred during the first half of
April. By April 8, the depthhad decreasedto an averageof 8.5
inchesin the park (6.5 inchesof snowand 2.o inchesof ice), and
3.8 inchesin the forest. In the followingfive days,o.f high temperatureand strongsouthwestwinds,practicallyall of this snow
and ice disappearedfrom the park, accompanied,
it is needlessto
state, by an excessiverun-off which continuedfor a few days
after the period. On April •5, no snowexistedin the park, while
throughoutthe forest there remained considerablesnow distributedin banksand ridgesover the north slopesand level surfacesas well. Photographsshownon the frontispiecegive a good
idea of the appearanceon this date of the measuringstationsin
the park and forest, and the remainingsnow,banksof snow, on
northerlyslopesin the mature timber. In the timber throughout
this regionthere remainedon April 25 a considerable
snowin shelteredsituationsfavorablefor late melting,while the
last trace of snowhad disappearedfrom the park by April •2.
The progressof accumulation
and later meltingof snowin the
two comparativesituationsmay be summarizedas follows:
(•.) The total snowfallin the forest is somewhatmore than
over the openparks, due chieflyto acceleratedwind velocityover
the parks,resultingin a lighter depositionof snow,a casesimilar
to the depositionof silt in streamcourses.
(2.) Due to protectionaffordedby the forestcoveragainstextremesof cold resultingin a higher averagetemperature,the processof meltingduringthe springcommences
in the forest than in the adjacentopenpark.
(3.) The low minimumdaily temperatures
in the park account
for the formation of a thick layer of ice at the base of the snow
duringthe earlyspring. This in turn servesto retainthe moisture
above the soil.
(4.) During the month of March, the park remainedalmost
entirely coveredwith a deepand quite uniœormlayer of sno,wand
ice, while in the forest the snow cover was much broken along
rockledgesand bankedhigh in the naturaltree avenues,and the
total amount of snow and water content above the soil surface
was decidedlylessper unit of area in the forestthan in the park.
Forestry Quarterly.
The conditionstronglysuggestedan apparentadvantageof a
treelessover a forestedarea in conservingthe winter snowfall
and storinga supplyof moisturefor distributionin the late spring
when most needed.
(5.)_With risingspringtemperatures
and absence
in the park
of protectionagainstextremesof heat, a point is reached--the
usual"warm spell"--whenthe layer of snowand ice in the park
"breaksup" very rapidly and the water goesoff with a rush,
resultingin a very small undergroundstorage,and the further
of a rapid drying or bakingof the soil.
(6.) In the forest,dueto the high efficiencyof the tree canopy
in modifying surface temperatures,and of the forest cover in
checkingthe velocityof the winds which at this seasonof the
year are strongand from the southwest,
and blowwith the regularity of the "trades,"the progressof meltingproceeds
moreuniformly and is prolongedinto late spring or early summerwith a
minimum lossof water by surfacerun-off and evaporationand a
relativelyhigh storagein the forestsoil.