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Chapter VII. Water Detained on the Soil Surface

Chapter VII. Water Detained on the Soil Surface

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156



VII.



WATER D E T A I N E D O N THE SOIL SURFACE



precipitation-generating airstream of a frontal storm, or at high

altitudes (Chapter 11).

After the snowstorm ends the survival of the snow deposited

depends on how much energy the new snow receives. Energy comes

to it from solar radiation (of which a small fraction is absorbed), from

the soil, as atmospheric (long-wave) radiation from the poststorm

airstream, and by convection from it. The snow surface, by reason of

the upward limit on its temperature, cannot radiate away more than

315 W m-2. If the energy inputs listed above add up to more than 315

W mP2,the snow will start to melt. If thin, it soon vanishes, if thick, it

may hold out until cooler weather or a new snowstorm comes. In the

latter case, the formation of a winter snow cover has begun.



Environments of Snow Cover

Considering the survival of a layer of new snow, we see that in

addition to considering terrain factors that affect deposition and

subsequent drifting we must also consider factors that affect exchanges of thermal energy between snow mantle and environment.

Some of these environmental factors are atmospheric: transparency

of the air, allowing short-wave radiation to reach the snow mantle;

clouds that emit long-wave radiation; and turbulence that determines

downward fluxes of sensible and latent heat from air to snow. These

energy flows to the snow are also affected by terrain: the sheltering in

valleys, the exposure to wind and sun, and the trees towering above

the snow mantle that both reduce the incoming short-wave (or solar)

radiation and augment downward long-wave radiation to the snow.

On a large scale, environmental conditions favoring survival of a

snow cover are found in regions of a generally cold atmosphere, or

weak solar radiation, or both. There are several classes of such

regional environments.



Maritime Mountain Ranges In many mountains near cooler parts of

the ocean, snow is deposited in amounts so great that in spite of

warmth in interstorm periods a large fraction of each deposit survives

to form a base for the next layer. The snow mantle of winter represents

a series of large accretions separated by the effects of melting periods

during which translocation processes produce changes in crystal size

and shape tending toward large rounded grains. Many of these

changes are accelerated by percolating meltwater, some of which

reaches the ground and becomes an outflow from this particular water

storage.



SNOW COVER



157



This type of storage requires large snowstorms. Typical climatic

conditions also include the likelihood that some storms may bring rain

instead of snow. The warmth of the intervening periods when

maritime airstreams remain dominant is also characteristic. For these

reasons the snow mantle in maritime mountains is closely associated

with the variation of temperature with height in the atmosphere.

The temperature conditions in storms affect the probability of

receiving snow rather than rain, as we saw earlier. The warmth of the

air in interstorm periods affects poststorm melting. Windward slopes

of coastal mountains display a rapid change from zero snow cover at

their bases to great depths that last all winter a t the higher elevations

where the air during and between storms is cold.

Altitude of the lower edge of the snow cover fluctuates during the

winter. Between storms it recedes upward; cold storms bring i t back to

lower altitudes. The snow line lies close to sea level in the Alaskan

Panhandle, and at about 1.2-km altitude in the latitudes of central

California.

Such mantles, often called "snowpacks" because of the high density

of h e stored water, are typical of the new mountain ranges around the

Pacific Ocean-the ranges of North America, the Andes, the Australian and New Zealand Alps, and the Japanese Alps. Vapor from the

North Atlantic Qcean nourishes the snow covers of the European

Alps, Scotland, and Scandinavia.

Field measurements of the water equivalent (or mass) of the snow

cover at 2.2-km altitude in central California (Table i and Fig. VIJ-1)

illustrate the way the amount of water in cold storage, nearly zero

until the winter solstice, steadily grows until early April, about 100

days after the solstice. After April the storms are smali, and the mantie

melts away faster than new snow is added to it. In late winter and

spring it is several meters thick, very dense, and affords good skiing

for several months. It forms a new landscape that differs in almost

every visual feature from the landscapes where no snow cover comes

into existence, and this contrast is a further attraction to visitors from

outside the snow zone (see Plates 12 and 13). A model of the

accumulation and ablation processes (Riley et a [ . , 1973) gives satisfactory reconstitutions for both the California and Montana basins of

Table 3 , and also for another snow laboratory basin, in Oregon.

In maritime mountains, with their heavy snowfall and steep rise

above adjacent lowlands, the snow cover possesses enormous potential energy. Consider its mass and altitude-a ton mp2 poised 2 km

above sea level. The energy equivalent, renewed every year, is 20 MJ



158



VII.



WATER D E T A I N E D O N T H E S O I L S U R F A C E



Fig. VII-I. Courses of basic hydrometeorological elements during the period of

snow-mantle accumulation and melting in 1947-1948, a t the Central Sierr‘i Snow

Laboratory, California ( U . 5.Corps Engineers, 1952, p. 13).



Plate 12. Shelter hut i n Lower Meadow, Central S i e r r a Snow Lahoratorj, Californi,i,

in July 1957 The prolectlon is called r? ”Santa Claus chimney” ( U 5 Forest 5ervice

photo)



159



S N O W COVER



TABLE I

Duration, Mass, and Density of the Snow Cover of Experimental Basins In a Maritirnt

Mountain Range (the Sierra Nevada) and an Interior Range (the Northern Rockies)"

Depth

(cm)



Mean duration of snow

cover (days)



Castle Creek ,basin, California (Central Sierra Snow Laboratory)

Latitude 39N, altitude 2.2 km

>25

166

>loo

120

Date:

Mass (kg m-'1

Density (kg m-3)



21 Nov 21 Dec 21 !an

60

200



195

260



500

330



10 May

30 April



21 Feb 21 Mar 21 Apr 21 May 21 Jun

645

340



760

370



460

400



Bear Creek basin, Montana (Upper Columbia Snow Laboratory)

Latitude 48N, altitude 1 7 km

225

177

,100

129

Date



Mass (kg IT-')

Density (kg m-y



Last date



70

460



0

-



16 May

28 April



21 Nov 21 Dec 21 Jan 21 Feb 21 Mar 21 Apr 21 May 21 J u n

65

160



230

230



340

260



405

280



485

310



380

380



65

430



0

~



' U . S. Corps Engineers (1956)



(or 6 kwh) from each m' of a mountain ecosystem. The economic

equivalent is usually far greater than the value of any other product of

these ecosystems.

In the Sierra Nevada this potential energy was put to use d century

ago to power one of the most advanced technologies of the time,

hydraulic gold mining. When excessive sediment outputs stopped this

enterprise, the hydropower was converted to hydroelectric power that

supported one of the first electrified economic regions of the country,

centered on urban activities, and irrigation from pumped ground

water.

In the Southern Hemisphere, most of the 2 million km' of transient

snow cover lies in such maritime mountains as the New Zealand Alps,

interior Tasmania, and the 5nowy Mountains of Australia. Its potential energy gives it far greater economic importance than its area

indicates, and in addition all three highlands provide attractive

recreation.



160



Plate 13.



VII.



WATER D E T A I N E D O N THE S O I L SURFACE



Same shelter hut on 9 April 1958, when the snow cover is near its deepest



(U. 5. Forest Service photo by Kenneth Knoerr).



Interior Mountains Airstreams reaching mountains in the interior of

the northern continents are depleted in moisture and bring less snow

than they did to the maritime ranges. As a result the altitudes of snow

cover are higher in these mountains than in those nearer the oceans at

the same latitude: for instance, 2.5 km in the Colorado Rockies versus

1.2-1.5 km in the Sierra Nevada.

Snow is deposited under much colder conditions, which has significant effects on sizes and characteristics of its crystals. Densities, as

shown in Table I, are smaller than in maritime snow covers,* and high

winds produce more drifting. These make it feasible to attempt to

control f i n d placement of the snow being added to the mantle, for

example, by erecting snow fences or taking other measures that

enhance topographic "snow trapping efficiency and capacity" (Martinelli, 1967).

Interstorm periods, although sunny, are cold; the snow suffers little

melting and remains low in density. The winter's accumulation builds

to appreciable totals. The Rockies and the high mountains of central

* A t altitudes of 2 . 5 3 . 0 km, snow-cover density in the Tian-Shan and Pamir

Mountains averages 280 kg m-3 as against 340 in the eastern Alps (Kotliakov, 1968, p.

132).



S N O W COVER



161



Asia enter spring with considerable water storage, which has great

value to the surrounding dry lowlands.



Interior Lowlands The snow mantle in the interior of the Northern

Hemisphere continents, as in the mountains, often is not established

much earlier than the winter solstice since the ground must first be

cooled to the freezing point. This cover often has a shorter life than

mountain snow. It is more likely to disappear and to be reinstated in

correspondence with the alternating reigns of tropical and polar

airstreams, especially nearer to the Atlantic.

Individual snowfalls usually are light; any depth greater than 10 cm,

which scarcely would be noticed in the mountains, has a crippling

effect on lowland activities. The difference in snowstorms is the

difference between a system of vertical air motion anchored in one

area and a system that moves several hundred kilometers a day,

spreading its snow thinly over a wide expanse. Heavy snowstorms can

occur, however, leaving up to 20- to 30-cm depth. Such a deep deposit

disrupts communications and may last for months. Figure VII-2 (U. S.

Weather Bureau, 1964) shows the 0.1 frequency (once in 10 years) of

water equivalent in the upper Midwest and Plains in early March.

Note how the mass increases to the east. Also note that compared to

mountain snow cover the values are small, even in this snowy year.

The interior lowlands of North America are interrupted by one

feature that tends to steer and strengthen passing snowstorms and

create new ones and so to create areas where snow is deeper and lasts

longer than would otherwise be the case. This feature is the tremendous heat storage formed by the Great Lakes, which produces a

special intensification of snowfall downwind from them. The duration

of six major snow belts to their lee (Lake Superior supports two belts)

(Muller, 1966) is shown in Fig. VII-3 (Phillips and McCulloch, 1972).

Snow in these belts lasts up to 150 days, whereas in adjacent locations

without lake effect it may last as little as 100 days. The lake effect also

appears outside the true snow belts; snow lasts 100 days at Muskegon,

to the lee of lower Lake Michigan, about 50 days at Milwaukee directly

across the lake. Farther east, frequent storms that draw in Atlantic

vapor bring heavy snow. Melting periods occur near the coast, and

altitude effects are marked.

The final disappearance of the snow mantle of continental interiors,

with 100-200 kg m-' mass, is a less well-defined event than that of a

deep mountain snowpack with 1000 kg mP2.Melting rates are high in

strong southerly airflow in spring and the stored water may be

released in a short time. In some years melting is sporadic and



105



100



45



85



80



Fig. V11-2. Mnxirnum mass of water stored in sinow cover (ntm) in e x l y M a l r h , exceeded i n 171 t i f the springs (from Ll. 5. Weather Bur., 1964, p .

17).



S N O W COVER



163



Fig. Vll-3. Mean number of consecutive days of snow cover 2 5 cm or more in depth

in the basin of the Great Lakes, showing malo1 snowbelts to the lee of the lakes (from

Phillips and McCulioch, 1972. Chart 32)



interrupted by periods of cold cioudy weather, and extends over

several weeks.

On the drier portions of the interior plains the extensive snow

mantle is, in spite of its thinness, an important contributor to the

water needs of summer crops. In such areas various practices are used

to retain this water storage on crop land. For example, standing wheat

stubble has been observed to trap enough snow to improve spring soil

moisture by 50 mm, an important amount in Dakota circumstances.

Similar methods of holding the snow mantle in place have long been

standard practice in drier parts of European Russia and western

Siberia.

In range country of eastern Montana, snow fences trap drifting snow

on small (8 x 8 m) catchment basins covered by butyl rubber, the

runoff from which is stored for stock water in the summer. Catchment

without snow fences produced an average of about 25 kg m-L (Saulmon, 1973), whereas catchments with fences produced an average of

195 kg m-' (12.5 tons of meltwater from each catchment).



164



VII.



W A T E R D E T A I N E D O N T H E SOIL S U R F A C E



Heavy, extensive snowfalls early in the season establish a large

snow-covered territory that steers later storms in such a way that cold

air is more than normally present over the snow cover. No melting

then occurs until late in spring. Such a winter was 1842-1843 in the

upper Midwest (Rosendal, 1970). Snow cover was established about 10

November and in most areas remained until February, when it was

deepened by heavy snowfalls that spread “a deep and extensive snow

cover. . . over much of the eastern two-thirds of the country.” The

severe cold of February and March (10°C below the 1931-1960 normal)

enabled the snow cover to remain in Wisconsin until mid-April. Its

duration was about 155 days, which is nearly as much as that of the

mountain snow covers. In a farming region, it occasioned widespread

suffering, and many cattle starved to death. It is far from impossible

for such winters to come again, with even more severe effects in our

transport-dependent economy.



Other Regional Environments of Snow Cover On Arctic lowlands

snow is thin because the snowstorms are poor providers. The meager

deposits do not, however, evaporate or melt during the long dark

winter, but are incessantly shifted about the landscape by the winds.

The earth’s surface becomes one of streamlined wind-sculptured

shapes. Wind action reduces the crystals to small grains that pack into

a cover of moderately high density and good bearing strength. From a

water-storage standpoint this snow cover is significant in a regional

water budget in which all the water fluxes are small.

Snows deposited on the high ice caps, which have been built-up 2-3

km above the level of the polar seas, are likewise thin and windblown,

but they do not melt in summer and next year are covered under the

snows of later storms. Transformed into firn and then ice, and sinking

into the continent itself, the deposits of each annual cycle retain such

individuality that in drill cores the snows of 10,000 years ago can be

distinguished.

Low-latitude mountains that are high enough to be well within

below-freezing air accumulate snow in the wet season; this cover

melts or evaporates in the dry season. The response pattern follows

the precipitation regime since there is little annual change in energy

supply. The environment is dry, cold air; weak incoming long-wave

radiation and intense short-wave radiation from a sun nearly at the

zenith produce sharp points of snow facing the sun. Evaporation of

snow is more important here than it is in any other environment of

snow cover.



S N O W COVER



165



Ecosystem Environments of Snow Cover As we saw in Chapter VI,

snow accumulates in shrub ecosystems differently than in ecosystems

of grass or annual species, because aerodynamic deposition processes

are affected by the greater roughness. These aerodynamic characteristics, little studied, also affect the turbulent energy fluxes that bring

about metamorphosis and melting of the snow cover. Characteristics

of these ecosystems also affect radiation components; the intimate

mixture of branches, foliage, and snow increases absorptivity and

accelerates melting.

Snow cover under forest canopy lies in an environment that is

characterized by dominant diffuse-source radiation and weak turbulent transfers. Deposition is less an aerodynamic and more a gravitational process; organic debris is mixed into the snow cover, and the

snow cover is pitted by snow clumps and drip water falling from the

canopy overhead. In a south-slope cedar-hemlock forest in Idaho,

Haupt (1972, p. 9), using a special lysimeter in the snow, measured

104 mm of dripwater percolate during winter. Aerodynamic and

radiative processes in metamorphosis and ablation are also modified

in the forest environment.

Snowfalls in urban ecosystems were discussed in Chapter 11; these

systems also form a special environment for snow cover. Obvious

characteristics are deposition of contaminants, patchiness resulting

from snow removal activities, and the effects of urban aerodynamics

on accumulation and melting. These effects operate even in small

settlements. For example, Barrow Village on the Arctic Coast of Alaska

experiences "meltout" 2 weeks earlier than adjacent tundra ecosystems, due to lower albedo of the urban snow cover, its patchiness, and

the aerodynamic roughness of the village. These factors increase both

the radiative components of the snow energy budget and its turbulent

fluxes in warm-air advection. Computer simulation of the energy

budget of the urban and tundra ecosystems (Outcalt et a l . , 1975)

indicates that the radiative effects are more important in ablation than

the aerodynamic.

Characteristic Changes in Snow Cover

Snow has many properties that are rare in nature: a high albedo or

reflectivity of solar radiation; small thermal conductivity; and a small

net loss of energy by exchange of long-wave radiation because the

temperature of its surface cannot exceed 0°C. The upper limit on its

surface temperature also affects its exchanges of energy with the



166



VII.



WATER D E T A I N E D O N T H E SOIL S U R F A C E



atmosphere, which carry heat or vapor downward more commonly

than is true over most types of surface. Furthermore, some of these

properties can change radically.



Metamorphosis Ecosystems change with age; rock formations metamorphose; water bodies age and die; but few natural bodies age as

fast as a snow cover. Albedo and bulk density are qualities that

epitomize the history of accumulation and weathering of the deposits

of snow made during preceding weeks or months. Snow crystals

undergo rapid metamorphosis into a large-grained snow of high

density and low albedo where winters are relatively warm. In the

Sierra Nevada of California, the density" of spring snow reaches 500

kg mp3 and the albedo 0.45 or lower. In the coastal mountains of

British Columbia snow-cover density increases to 420 kg m--" (standard deviations of 50 kg m-3) by mid-April, as compared with 330 kg

mp3i n the Great Lakes area, 260 kg mp3in the taiga, and 330 kg IT-:' in

the wind-packed snow- of the tundra (McKay and Findlay, 1971).

One equation for density of lowland snow cover relates i t to factors

that express winter melting or energy availability, its own mass

(gravitational settling), wind packing and wind-powered translocation

of mass within the snow cover, and effects of new-snow density and

of rain:

p = 10

T + 0.1 k + 24n - Apo + 100ukp' + 210,



2



in which p is density, kg m-3; C T accumulated sum of positive air

temperature; h depth of snow cover, cm; n number of days when wind

speed exceeded 6 m sec-'; po density of new snowfalls on the cover;

and r rain amounts in kg m-' (Kotliakov, 1968, p. 109). The fluffiest

snow lies in the quiet cold forests of Canada and Siberia, where no

rain or warm air intrudes and wind-packing is minimal. In eastern

Siberia density in open sites is 180-190 kg mp3and in sheltered forest

ecosystems only 160-170 (Kotliakov, 1968, p. 111).The equation shows

that mechanical and thermodynamic forces are more important than

gravity in increasing snow density; they are the factors that bring

about metamorphosis.

Figure VII-4 depicts how a winter snow cover is built up from

successive layers of individual storms. The heavy lines show how each

layer of stratum shrinks as time passes, and gets denser (stippling)

under conditions of temperature (light lines) that are generally high.

* High density means relatively high thermal conductivity. Because the entire mantle

is warm, however, the gradient of temperature through it is small and little heat moves

by conduction.



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