WATER AND WATER VAPOR ENVIRONMENT
Introduction
Water movement in soil is much more complicated than that of CO2 because not only
is water present in two phases, one liquid and the other gas, but also water movement is
related to heat flow. However, in order to simplify the problem and understand the system
clearly it is, again assumed that vapor flow in soil is independent of heat flow, as we assumed
in the chapter on heat flow analysis. Furthermore, we will focus only on vapor movement
in the present chapter, because in most cases water flow is assumed to occur in parallel with
water vapor flow and they are combined. This can be condensed into what is the diffusion
coefficient for both.
When soil is covered by vegetation, as .is usual in agricultural fields, water is transported
from the soil via roots, stems and leaves. This transport system, in the form of a soil-plant-
atmosphere continuum (SPAC), was been introduced by Slatyer (1963) using water potential.
it is convenient to use water potential to describe the water flow in SPAC. The concept of
SPAC is clear, and sophisticated models for open fields have been developed since then; the
power of CSMP for modeling has also been demonstrated. Since evaporation from bare soil
is the most important part of those systems and those systems are rather complicated, a
system without plants is considered in this chapter. In a later chapter, the relationship
between transpiration and environmental conditions is discussed in a physiological model of
plant leaves.