the warm end of the column and a decrease in the level at the cold end. After 8-10
hours, this level difference disappeared, after which the water continued to move, but
now from the hot to the cold end, creating a difference in the hydraulic head.
The gradient of the electric potential and the temperature gradient are capable of
causing an electric current, or a gradient of the moisture potential and a temperature
gradient can cause a moisture flux in the soil. Irreversible phenomena are expressed by
a linear phenomenological relationship of a general type
X
L
І
k
ik
і
=
, (1)
where І – flow; L – a phenomenological coefficient, or a constant, which depends
only on the physical state and geometry of the system; Х – function that acts as an
acting force; і, k – indices for the designation of components, which states that the total
flux can be created with the participation of all force fields (i = 1,2 ... n) [10].
As applied to the heat flux, this means that the change in temperature along the
mean free path, which has an average length, should be much less than the average
temperature itself. This expression shows that the difference in pressure in water arises
in a porous medium under the pressure of a constant temperature difference depends
on the enthalpy of water and on the ratio of the moisture transfer coefficient in the same
material [10].
The motion of the soil solution occurs when the chemical potential of the particles
and the temperature are equal in the different parts of the solution. The known
conditions for such an equilibrium are the constancy of the chemical potential of
molecules, ions, and temperature:
μ = cоnst, Т =const.
Under conditions of incomplete saturation of the soil with moisture, or the
presence of an evaporation front of the soil solution that fills the pores completely, the
basic forces determining the movement of moisture are usually capillary forces that
manifest themselves as capillary pressure, depends on the curvature of the water-air
interface and on the surface tension on this boundary [11, 12].
S. Sanoyan [12] proved that the relative contribution of the steam flow to the total
moisture flux increases with decreasing soil moisture. These conclusions are valid
under the condition that for different soil moisture the concentration of salts in the soil
solution does not change. With an increase in the concentration of salts in the soil
solution, the relative contribution of the vapor stream decreases.
It should be noted that the effect of the concentration of salts on the rate of
evaporation is much less if they do not take into account the value of α, which
characterizes the decrease in the effective evaporation surface.
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