Conditioning of the snowpack follows the work
A conceptual simulation of "cold content" and liq-
of Anderson (1973). A "cold content" is continu-
uid water characterizes the condition or "ripe-
ously accounted for using an antecedent index.
ness" of the snowpack. The model also includes
The "heat deficit" must be made up by above-
an index approach to dealing with frozen ground
freezing temperatures before snow is allowed to
(Anderson and Neuman 1984).
melt and enter the soil system. A liquid water
The energy balance of the snow cover is
holding capacity of snow can be specified, and is
expressed as
usually taken as 25%. Snowmelt resulting from
M = Qn + Qe + Qh + QPx
(4)
ground heat can be specified as a constant melt
rate in inches per day, for each month.
where M = snowmelt,
Strengths of SSARR
Qe = latent heat transfer,
A canopy interception factor may be mod-
Qh = sensible heat transfer,
eled as a reservoir and reduced by evapo-
transpiration. Interception quantity can be
varied monthly to account for seasonal
The major assumptions are
changes in vegetative cover.
The ratio between sensible and latent heat
Gage-catch deficiency can be accounted for.
is given by the Bowen ratio,
The surface energy balance equation is for
Outgoing longwave radiation can be calcu-
partly forested areas (eq 3) and contains
lated using Stefan's law with a snow surface
coefficients that can be used to account for
temperature of 0C during melt,
forest cover and solar angle effects on short-
Incoming solar radiation is negligible dur-
wave radiation, wind environment effects
ing overcast conditions,
on condensation and convection, and forest
Incoming longwave radiation is equal to
cover effects on longwave radiation melt.
blackbody radiation at the temperature of
the bottom of the cloud cover, assumed
Limitations of SSARR
equal to the air temperature,
The albedo can be varied over the melt season,
Relative humidity is 90%,
but must be provided.
Atmospheric pressure can be computed
from elevation,
NWSRFS
Saturation vapor pressure can be estimated
The National Weather Service's centralized
as a function of air temperature.
library of computerized forecasting techniques is
known as the National Weather Service River
Snowmelt during rain periods
Forecast System (NWSRFS). The Generalized
Snowmelt during a 6-hr rain-on-snow period
Streamflow Simulation System (Burnash et al.
is described by Anderson (1973) as
1973), frequently identified as the Sacramento
Catchment Model is a major component of the
M = 3.67 10-9 (Ta + 273)
4
NWSRFS (Peck 1976). The snow accumulation
and ablation routines used in NWSRFS are based
- 20.4 + 0.0125PxTa + 8.5 f (ua )
primarily on the efforts of Anderson (1973, 1976)
and Anderson and Crawford (1964). Anderson
[(0.9esat - 6.11) + 0.00057 PaTa ]
(5)
developed a combined energy and temperature-
index method that used only temperature and
where Ta = air temperature (C),
precipitation as input meteorology. A difference
from other temperature index methods is that
each physical process is represented separately,
(mm),
rather than using a single melt index. Processes
f(ua) = wind speed function at a height (za)
above the snow surface (mm mb1
are conceptually modeled and include snowpack
6 hr1),
accumulation, heat exchange at the air/snow in-
terface, areal extent of snow cover, heat storage
esat = saturation vapor pressure at the air
within the snowpack, liquid water retention,
temperature (Ta) at height za,
lagged transmission of melt through the pack,
Pa = standard atmospheric pressure at a
and heat exchange at the ground/snow interface.
given elevation (mb).
4
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