so that the latent heat of fusion of 1 cm3 of snow that has been drained to Swi will be l⋅Swi.
A correction to account for residual mobile water saturation was implemented, but the
effects were found to be minimal in light of the fact that the freezing front progresses
slowly compared to the speed of water percolation.
7.3 Surface Energy Balance
In generating melt, FASST currently uses a full surface energy balance to calculate the
volume of runoff that is generated from surface melt during each time step. In general,
the heat input at the top of the snow (Itop) is (Jordan 1991a)
Itop = I s ↓ (1 - α ) + Ii↓ - Ii↑ + H + L + Iconv
where I s ↓ is the net solar radiation at the surface, α is the surface albedo pertinent to
radiation, H is the sensible heat flux, L is the latent heat flux, and Iconv is the convective
heat flux. Solar radiation and albedo are currently not estimated in the module, and
neither is net longwave radiation, so I s ↓ (1 - α ) and Ii↓ - Ii↑ are taken as inputs to the
model. Details of the exchange follow Jordan (1991a, b). The resulting surface melt depth
∆hi,top = I top
If the ground temperature is above freezing, melting can also occur at the bottom of the
snow. The bottom melt depth is
∆hi,bottom = κ
∂z ρil f
where κ is the soil thermal conductivity (W/mK), T is the soil temperature (K), and z is
the soil depth (m).
7.4 Module Input and Output
The Snow Accretion/Depletion Module requires certain information about the snow
cover. If known, snow depth, snow water equivalent, initial water saturation (default
0.0288), effective porosity, and the snow surface temperature should be input. Otherwise,
the module will compute these parameters. The required meteorological data are air
temperature, wind speed, net solar radiation, net longwave radiation, and precipitation
(amount and type). If the type of precipitation (rain, snow) is not known, it will be
predicted based on the ambient air temperature. Net radiation can be obtained using the
two radiation modules described in Chapters 4 and 5. The output of the module is snow
depth and the volume of water (total and incremental) that has come out of the snowpack.