lution during summer snowmelt on the Greenland

temperature in frozen strata using thermody-

ice sheet (Rowe et al. 1995), an energy balance

namically derived freezing curves for typical

study of a continental, midlatitude alpine snow-

sand, silt, and clay soils. This conceptualization

pack at Niwot Ridge, Colorado (Cline 1997), and

of water infiltration through the snow is one of

beneath canopy energy balance studies in the

an even, horizontal wetting front proceeding

boreal forest in Saskatchewan, Canada (Hardy et

downward. In reality, finger flow occurs and

al. 1997). The model is currently being applied to

tends to accelerate the arrival of melt to the bot-

snow cover mapping in the boreal forest of Canada

tom of the snowpack (Colbeck 1979, Marsh and

(Davis et al. 1997), snowmelt forecasting in Bosnia

Woo 1984), while capillary tension draws water

(Melloh et al. 1997), integration with mesoscale

along finer grained snow layers. Deformation of

meteorological data (Melloh et al., in prep), and in

the snow cover over time takes into account set-

distributed snow model studies in Sleepers River

tling due to metamorphism, and compaction due

Research Watershed in northern Vermont (Melloh

to overburden, melt, or sublimation. Vapor flux

and Jordan, in prep.). SNTHERM is applicable to

through snow is assumed driven by diffusion of

a full range of meteorological conditions such as

saturated air and is computed by Fick's law. The

snowfall, rainfall, freeze-thaw cycles and transi-

residual water content of snow (irreducible

tions between bare and snow-covered ground.

water saturation) is assumed to be 4% of the snow

The meteorological boundary conditions in

pore volume.

SNTHERM require air temperature, dew point

The numerical solution is obtained by sub-

temperature, wind speed, precipitation, and

dividing into snow layers, each represented by the

either incoming values of solar and infrared radi-

governing equations for heat and mass balance.

ation, or cloud cover and site information (solar

SNTHERM uses a control volume numerical pro-

aspect and inclination of the surface). The surface

cedure (Patankar 1980) for spatial discretization

energy balance is

that allows for compaction of the snow cover. Use

(

)

of the control-volume technique conserves the

quantities over a finite control volume (∆*V) *rather

than at an infinitesimal point as with a finite-

+ *I*sen + *I*lat + *I*conv

(23)

difference scheme. The rate of change of these

quantities within a control volume ∆*V *must equal

where *I*s↓(1αtop) = downwelling shortwave radi-

their net flow across the boundary surface plus

ation,

their rate of internal production. As snow com-

α = albedo or shortwave reflec-

pacts over time, the one-dimensional grid is

tance

allowed to compress, so that volume elements

continue to correspond with the original element

ation,

of snow. The rate of flux is taken with respect to

the deforming grid. A Crank-Nicholson central

tion,

difference scheme is used to solve partial differ-

ential equations in the time domain. A new hydro-

logic version of SNTHERM limits the number of

nodes in the bottom two-thirds of the snowpack

falling snow.

while maintaining detail near the surface to gain

efficiency for water resource applications.

Ground heat flux and soil temperature profiles are

The sum of the constituent bulk densities is the

modeled, but soil moisture is kept constant. The

total density (ρt) written as

steady-state bottom boundary condition is set by

the initial soil temperature and moisture profile

ρ*t *= ∑ θkρk = ∑ γ k

specified by the user.

(24)

k

k

SNTHERM was based initially on the mass and

where θk = individual volume fractions of *k *con-

energy-balance snow model of Anderson (1976);

it incorporates the mixture theory approach

stituents,

espoused by Morris and Godfrey (1979), Morris

(1987) and Morland et al. (1990), and the tech-

(*v*), and air (*a*),

ρk = density of each *i*, *l*, *v*, and *a *constitu-

nique for gravitational flow of water through the

snowpack of Colbeck (1971, 1972, 1976, 1979). The

ent,

9

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