where M = snowmelt (in. day1)
Output of modeled snow information in
k′ = shortwave radiation melt factor
HEC-1 is limited to melt contribution to
"rainfall" excess. Temporal changes in the
(dimensionless)
snowpack depth or snow water equivalent
F = average basin forest canopy coverage
are not given.
(dimensionless),
Temperature lapse rates are fixed and can-
Ii = solar radiation incident on a horizon-
not be varied with actual weather condi-
tal surface (langley day1),
tions.
α = albedo (dimensionless),
k = condensation-convection coefficient
SSARR
(dimensionless),
SSARR (Streamflow Synthesis and Reservoir
Ta′ = difference between the air (10 ft) and
Regulation) was developed by the North Pacific
snow surface temperatures (F),
Division (NPD) Corps of Engineers beginning in
Td′ = difference between the dew point and
1956 to provide hydrologic simulations on snow-
surface snow temperatures (F),
melt-dominated river systems for planning,
C2 = coefficient used in HEC-1 to account
design, and operation of water control works (U.S.
for variation from the generalized
Army Corps of Engineers 1991). SSARR was later
snowmelt equation (dimensionless).
expanded to provide operational river forecasting
Term 1 represents melt due to direct solar radia-
and river management for the Columbia River.
tion in the open, term 2 represents convection
SSARR was developed during a time when elec-
melt, term 3 is condensation melt, and term 4 is
tronic digital computers first made continuous
longwave radiation melt in the forest. The short-
stream flow hydrograph simulation practical. The
wave radiation melt factor (k′) depends on aver-
philosophy of the model developers was that limi-
age exposure of an open area in comparison to a
tations in data quantity and quality, and in devel-
horizontal surface and is assumed equal to 1.0 in
opment of fundamental relationships, prevented
HEC-1, implying a horizontal surface. The forest
the development of all-purpose, physically based
canopy coverage (F) is fixed at 0.5. Albedo (α) is
models. SSARR, thus, was conceptually based and
reduced from 0.75 to a minimum of 0.4, using the
of sufficiently limited detail to allow operational
inverse square of days since the last snowfall to
application on a daily basis. SSARR was one of 11
account for factors that reduce albedo as the snow
models from eight countries evaluated in a world-
ages, such as increased snow grain size. The con-
wide comparison of snowmelt runoff models
densationconvection coefficient (k) is taken as 1.0,
(WMO 1986), and has been used extensively in
and the snow surface temperature is taken as 32F.
operational snowmelt modeling in the Pacific
Northwest. The basic snowmelt equations are the
Limitations
same as those used in HEC-1, though with fewer
The HEC-1 methods do not allow for impor-
restricted coefficients.
tant snowpack processes such as snow rip-
ening, pore water retention, and flow of
Temperature index method
water through the pack.
The degree-day, temperature index method is
The use of wind speeds measured at 50 ft
the same equation as used in HEC-1 (eq 1), except
(15.2 m) is awkward, since many weather
that the degree-day melt coefficient (Cd) can be
stations measure at 2- or 10-m heights. It is
varied during the model run on as much as a daily
uncertain whether methods developed for
basis.
wind speeds at 50 ft convert well to other
measurement heights.
Energy balance method
The HEC-1 snow methods have been used
The SSARR energy balance method equations
more often in planning studies than in fore-
are the same as HEC-1 (eq 2 and 3) with the fol-
casting, and in situations where snowmelt
lowing important additions:
runoff is not a primary contributor.
Fractional forest cover canopy (F) is not
Radiation, convection, and condensation
fixed, but can be varied from 0 to 1,
coefficients that vary to represent a range of
k′ and k can be varied,
watershed conditions in EM-1110-2-1406 are
Albedo (α) can be specified.
fixed to midrange values in HEC-1.
3
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