θmax - θr
θw = θr +
(9.5)
(1 + αΨ )
mvG
nvG
where θr is the residual volumetric water content, θmax is the maximum volumetric water
content, αvG (cm1) is a constant related to the reciprocal of the bubbling pressure head,
nvG is a constant dependent on the distribution of pores, and mvG = 1 1/nvG. The
hydraulic conductivity also depends on the volumetric moisture content such that (van
Genuchten 1980)
2
(
)
K = K sat w ⎡1 - 1 - w1/ mvG
⎤
mvG
0.5
(9.6)
⎢
⎥
⎣
⎦
where
θ -θr
w=
(9.7)
θ max - θ r
is the relative soil moisture.
9.3 Initial Conditions
FASST requires very little information concerning the initial conditions. At a minimum,
it needs to know the initial snow depth and/or ice thickness on the surface. If the initial
soil temperature and moisture profiles are known, the user can also input this
information.
9.4 Site Information
Several parameters describing the site are needed by FASST. They are
Site Latitude (+ = North)
Site Longitude (+ = East from Prime Meridian)
Site Elevation (m, ft)
Time Offset between Local and Universal Time Conversion (UTC) (hr)
[local UTC]
Slope (Degrees from horizontal)
Aspect Angle (Degrees from North, + = clockwise)
Surface Vegetation 0 = unknown
8 = grass/pasture/steppe/meadow
50 = mixed trees
999 = other
The vegetation enumerations are based on SEDRIS nomenclature. At present, the
vegetation simply changes the surface albedo and emissivity. The next release of the
model will include a two-layer vegetation model including root uptake of soil moisture.
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