Monitoring Plant and Groundwater Interactions - Introduction to Phytoremediation of Contaminated Groundwater - page 229

The VP saturation is the maximum amount of water the air can
hold at a given temperature; any more water vapor added at
this condition will condense from the air onto cooler
surfaces, and this temperature is called the dew point. The
VP saturation is directly related to air temperature; higher
temperatures result in the air being able to hold more water
vapor (Anderson 1936).
A similar variable that relates to the potential for air to
receive more water vapor and a parameter that is readily
obtained from meteorological studies is relative humidity,
but VPD accounts for the effect of air temperature on the
air's ability to hold or condense water vapor, whereas rela-
tive humidity is the ratio between the actual water vapor
content to potential vapor content. As the relative humidity
of the air increases every 20 F, the ability of the air to hold
water will double. Hence, the same relative humidity can be
stated for different air temperatures but will be of widely
different moisture contents.
Anderson (1936) gives an excellent illustration of this
point in his classic paper (1936) (Fig. 9.21 ). The amount of
water vapor in the air does not correlate to the humidity or
aridity often associated with various areas of the world.

Death Valley, California, for example, is an arid area but
has the same amount of water vapor in the air as a more
humid area, such as Minnesota at a given time of year. Even
the Moroccan desert has a relative humidity near 90% during
the summer. As such, Anderson (1936) stated that VPD
should be used rather than relative humidity when discussing
water dynamics for biological systems. What is important in
determining the degree of aridity is not the moisture content
of the air itself but that content relative to the amount that the
air could hold at a given temperature. This is similar to the
solubility of gases in solution that are dependent upon the
temperature of the solution, such as oxygen dissolved in
water. The VPD is an absolute measure of the air's ability
to hold water, rather than a relative measurement at a given
temperature.
The relation between the air temperature and relative
humidity also has a vertical gradient at most sites. The
degree of the gradient is dependent upon location, plant
type, and water status. An example of a vertical gradient in
relative humidity and air temperature is shown in Fig. 9.22 .
Even though the air temperature was greater than 100 F near
the grass, the relative humidity approached 50% there,
almost five times that of the air only 1 ft above the grass.
As might be expected from Eq. 9.11 , a lower VPD
equates to being closer to condensation conditions, and
higher VPD equates to more evaporation from an open
surface, or more transpiration through plants. This has
implications for groundwater use by plants in areas that

Fig. 9.22 Relation between air temperature and relative humidity at
different heights above unshaded but well watered grass in Columbia,
South Carolina, August 2007, during 10 consecutive days of record low
relative humidity. One foot is equivalent to 0.305 m.

Fig. 9.21 Relation between air temperature (in C and F), vapor
pressure (in pounds per square inch (PSI) and kilopascals (kPa)), and
relative humidity (in percent) (Modified from Anderson 1936).