WE = water-equivalent of the solid portion
PxTp
Dm = -
of the snow cover (in.).
(11)
160
The amount of snow cover outflow it computed
where Dm = heat deficit of snow due to mass
on an hourly basis is
change (mm),
OS = R1(S1 + El )
(14)
(mm),
where OS = snow-cover outflow (mm hr1),
equal to air temperature (C).
S1 = excess water storage at the begin-
ning of the period (mm),
The heat deficit consists of liquid water that
El = lagged excess liquid water entering
will refreeze and snow that is at temperatures
storage during the current period
below 0C, including new snow. Melt and rain
(mm).
continue to refreeze within the snow cover until
the heat deficit reaches zero.
The change in liquid-excess stored in the snow
cover is computed as
Retention and transmission
of liquid water
S2 = S1 + El - tOs
(15)
A constant liquid water holding capacity is
defined as the amount of liquid water that the
where S2 is excess water in storage at the end of
snow can hold against gravity and is expressed
the period (mm), and t is the length of the time
as a percentage by weight of the solid (ice) por-
period (hr).
tion of the snow cover. Excess water not held
Melt due to heat exchange at the snow/soil
within the snowpack is both delayed and damped
interface is assumed in the range of 0.3 to 0.15 mm
as it moves through the snowpack. The relation-
day1, and is added to the snow cover outflow.
ship used was that observed in April and May
1954, from the Central Sierra Snow Laboratory
Limitations
(CSSL) lysimeter. Though applied to all snow
Processes not considered include vapor exchange
types, the observed relationship was observed in
due to condensation and sublimation, snow inter-
well-aged snow at 0C with a spherical crystalline
ception due to forest canopy, and redistribution
structure. Flow through the finer, drier snow
of snow due to wind. The equation for lag of melt-
would need to be lagged and attenuated more.
water through the snowpack is based singly on
The equation for lag is
the Central Sierra Snow Laboratory lysimeter
observation.
-0.03WE
L = 5.33 1 - e E
SRM
(12)
The simple degree-day method, SRM, was
developed in small European basins by Martinec
where L = lag (hr),
(1975) to simulate and forecast streamflow from
WE = water-equivalent of the solid portion
mountainous basins. A more recent, restricted
of the snow cover (mm),
degree-day version can utilize radiation input
E = excess liquid water (mm 6 hr1).
(Martinec 1989, Kustas et al. 1994, Brubaker et al.
1996). SRM uses remote sensing derived input of
A rate of outflow equation was determined by
snow cover distribution for operational use, and
curve matching to lysimeter data, measured in
this gives it a diagnostic capability beyond the
English units,
simple degree-day method for basins where such
operational snow cover maps are available. The
-1.0
R1 =
model has been tested in over 60 basins world-
-500Els
(13)
1.3
wide, including number of basins in the United
5.0e WE
+ 1.0
States (Rango 1995). SRM (along with SSARR)
where R1 = one-hour withdrawal rate (hr1),
was one of eleven models from eight countries
Els = amount of lagged excess liquid water
evaluated in a worldwide comparison of snow-
for the period (in.),
melt runoff models (WMO 1986).
6
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