Agriculture Reference
In-Depth Information
Boundary Condition at the Bottom of the Soil Profile
The boundary condition at the bottom of the soil profile is defined by a flux to the
open water system via drains and a flux to the underlying aquifer:
V b =
V d + V a
(10.26)
In which
V b =
bottom boundary flux (cm/d)
V d =
flux to the open water system via drains (cm/d)
V a =
flux to the aquifer (cm/d)
Subsidence and Cracking
The shrinkage characteristic is defined in the model as the relationship between
moisture ratio (volume of moisture/volume of solids) and void ratio (volume of
voids/volume of solids) of the soil. Introducing the shrinkage characteristic of a soil
into model calculations allows each soil compartment to have its own relationship
with water content, pressure head, hydraulic conductivity, and matrix volume. Each
water content change (in comparison with saturation) in the model can be converted
into a volume change of the soil matrix
V
can then be converted into a change in compartment thickness and a change in crack
volume by Oostindie and Branswijk ( 1992 ) :
V using the shrinkage characteristic.
V 2
V 1
1
r s
z 1
z 1
z
=
(10.27)
z 1 ·
V cr =
V
z
(10.28)
where
change in soil matrix volume (m 3 )
V
=
z
=
change in compartment thickness due to shrinkage (m)
change in crack volume (m 3 )
V 1 , V 2 =
V cr =
volume of cube of soil matrix at saturation and after shrinkage (m 3 )
z 1 =
compartment thickness at saturation (m)
r s =
geometry factor (-)
Time Step
The model computes the maximum allowable time step. The time step depends on
the wet range of the hydraulic conductivity curve. The critical value for the time
step is calculated as
e α z
< ∂θ
1
t
z
e α z
k
+
1
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