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forces, the water in the soil has a soil water potential less than
zero, thereby reducing the equilibrium vapor pressure from that of pure
water. The following relationship (Baver et al., 1972) can be used to
determine the saturated vapor pressure above a water surface with a
potential other than zero.
ev() = evs(T) exp(- -) (2-29)
where:
ev(0) = saturated vapor pressure of water with chemical
potential, 0 [Pa]
= soil water potential [m]
g = acceleration due to gravity [mns-2]
Using the ideal gas law, the vapor density over a water surface with a
chemical potential other than zero can be obtained by:
P = Pvs(T) exp(- -) (2-30)
A set of simultaneous equations (2-22, 2-25, 2-26, 2-30) describe
the conservation of thermal energy, water vapor and liquid water for
the soil continuum and formed the basis of a coupled mass and energy
model for the soil. The assumption of thermodynamic equilibrium
between the liquid and vapor states yields the constitutive
relationship expressed in equation (2-30) and provides a fourth state
equation to be used in the model.
A well-posed problem also includes boundary conditions and, in
the case of transient problems, the initial conditions must also be
prescribed. The system of governing equations was one-dimensional and
therefore required two boundary conditions. The first boundary is the