### Modeled Snow Cover Product (Snow Water Equivalent) 

The snow water equivalent model is a spatially distributed snowmelt
model driven by remotely sensed and assimilated meteorological data. The
model is based upon the Utah Energy Balance (UEB) snowmelt model
developed by Tarboton (Tarboton, 1994; Tarboton et al., 1995). The UEB
model is very parsimonious with only three state variables. The model
uses a lumped representation of the snowpack with two primary state
variables, namely, water equivalence (or total amount of water,
expressed in depth, in the snowpack), and energy content relative to a
reference state of water in the ice phase at 0 C. This version of the
model runs in a spatially distributed mode using a grid resolution of 10
km.

The energy content is used to determine average snowpack temperature,
and/or fraction of the snowpack that is in liquid form. Age of the snow
surface is retained as a third state variable, used for the calculation
of albedo (or reflectance properties of the surface). The model is
driven by inputs of air temperature, precipitation, wind speed,
humidity, and radiation at a 6-hourly time step, a time step that is
sufficient to resolve the diurnal cycle. The model uses physically-based
calculations of the radiative, sensible, latent and advective heat
exchanges (i.e., exchanges, or fluxes, of energy due to incoming and
outgoing short- and long-wave radiation, thermal or heat conduction,
evaporation and condensation, and horizontal heat transport). An
equilibrium parameterization of snow surface temperature accounts for
differences between snow surface temperature and average snowpack
temperature, without having to introduce additional state variables.
Melt outflow is a function of the liquid fraction, and is parameterized
according to Darcy\'s law; this parameterization allows the model to
account for continued outflow even when the energy balance is negative.

The daily rainfall input grids are the [satellite rainfall estimates
(RFE)](http://igskmncnwb015.cr.usgs.gov/adds/readme.php?symbol=rf)
provided by NOAA/CPC (Climate Prediction Center) (Xie et al., 1997). The
RFE is calculated on a 0.1-degree latitude/longitude grid. The remaining
input variables required by the model (solar radiation, air temperature,
wind, humidity, and atmospheric pressure) are estimated from downscaled
output fields from NOAA's Global Data Assimilation System (GDAS) and the
Air Force Weather Agency's MM5 (Mesoscale Model, version 5) weather
forecast model.

The (daily) snow water equivalent maps show the spatial distribution of
the modeled water content of the snowpack. As such, the maps also
portray the spatial distribution of snow cover extent, and provide an
indication of relative snow depth and water available for irrigation
when the snow melts.

Tarboton, D. G., T. G. Chowdhury and T. H. Jackson, (1995), \"A
Spatially Distributed Energy Balance Snowmelt Model\", in
Biogeochemistry of Seasonally Snow-Covered Catchments, ed. K. A.
Tonnessen et al., Proceedings of a Boulder Symposium, July 3-14, *IAHS
Publ*. no. 228, p.141-155.

Tarboton, D. G., (1994), \"Measurement and Modeling of Snow Energy
Balance and Sublimation From Snow\", in Proceedings, International Snow
Science Workshop, Snowbird, Utah, October 31 to November 2, *Utah Water
Research Laboratory working paper no*. WP-94-HWR-DGT/002.

Xie, P. and P.A. Arkin, (1997), "A 17-year monthly analysis based on
gauge observations, satellite estimates, and numerical model outputs",
*Bulletin of the American Meteorological Society* 78(11): 2539-58.