Modeling - Field Sampling: Principles and Practices in Environmental Analysis - page 180

FIGURE 7.4 A lysimeter containing an undisturbed soil and a water collection
sump.
7.1.4. Evapotranspiration
In the field, especially in a field with plants, measuring and separating evaporation and
transpiration is extremely difficult. Evaporation is the loss of water from the bare soil
surface. The soil surface is under continually varying conditions of cover, shade, direct
sunlight, and so on. Transpiration is the loss of water from stems and leaves. Stems and
leaves lose water at various rates, depending on air temperature, relative humidity,
sunlight intensity, wilting point, stage of plant growth, and so forth. At a minimum there
are six interdependent, constantly changing variables that must be measured to predict
water loss. It is much easier and more accurate to measure the total water loss during the
growth of the plants, called evapotranspiration, and even this is generally very
complicated.
To simplify estimating or modeling the loss of water during a period of time an open or
evaporation pan method is usually used. A pan filled with water is placed in the field and
the amount of water lost each day is measured. This is then related to environmental
conditions and water loss from evapotranspiration using a pan coefficient. The concept is
that the evaporation pan integrates the effects of temperature, wind, relative humidity,
solar radiation, and so on, and so can be used to estimate evapotranspiration. Today other
more sophisticated instrumentation is becoming more widely used.
The pan coefficient would seem to be unrelated to models needed or used in field
sampling. With volatile contaminants, however, loss from the surface may be significant.
Although it is often assumed that contaminants can only move deeper in soil, this is not
true. When evapotranspiration exceeds precipitation, components can move up the soil
profile. Evapotranspiration data can be used to predict where components will be in a soil