Chapter 10
Model Initialization
10.0 Introduction
Before any calculations can be made, the surface conditions as well as the temperature,
volumetric moisture content, and soil properties profiles must be determined. The initial
nodal spacing and number of nodes are also calculated.
10.1 Surface Conditions
The presence of snow or ice affects both the soil temperature and the volumetric moisture
content. It also influences the radiative properties of the surface, specifically the albedo
and emissivity.
The albedo is a measure of the amount of solar radiation that is reflected by the surface. It
is the ratio of the reflected to the total incoming solar radiation. The greater the albedo,
the less shortwave radiation is absorbed by the surface. Value of the albedo for a snow-
covered soil range between αs = 0.78 or 0.55 following Pomeroy et al. (1998), depending
on the age of the snow (see Chapter 7.2.7). For ice-covered soil αi = 0.70. The albedo
values for the USCS soil types are given in Table 9.2.1.
The emissivity of a material is the ratio of its emitted energy to emitted infrared radiation
in the spectral band from xxx to yyy to that of a black body, or perfect absorber, at the
same temperature. It influences the amount of longwave radiation that the surface loses
to the atmosphere. If the surface is snow- or ice-covered, the emissivity is εs = 0.98 or
10.2 Temperature and Volumetric Moisture and Ice Content Profiles
If the user has any initial soil temperature or volumetric moisture content data, FASST
incorporates these into the calculated profiles. For both parameters FASST assumes a
linear profile as a function of depth between any consecutive fixed values.
If no initial surface temperature is given, FASST assumes that the soil surface
temperature is equal to the air temperature if there is no snow or ice on the ground. If
snow or ice is present, FASST sets the soil surface temperature to 0.0C. If the
temperature of the lowest model layer is not provided, FASST assigns a value equal to
the surface temperature or the lowest given nodal temperature.
FASST presumes that the initial volumetric soil moisture content at the surface is equal to
the residual moisture allowed plus 30% of the difference between the maximum and
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