Transpiration and the Ascent

MeasurementofTranspirationand Evaporation 233
eta!., 1965) with such a low rate of transpiration that when a mature stand
completely shades the ground the rate of evapotranspiration may be lower than
that of a younger stand which leaves the soil partly exposed (Ekern, 1965; also
see Chapter 12).
. Over the entire United States about one-fourth of the total precipitation escapes
as stream flow and three-fourths returns to the atmosphere by evapotranspiration
from the soil and vegetation. Some precipitation is intercepted and is
evaporated directly from the surface of vegetation, but most reaches the soil and
is returned either by direct evaporation from the soil surface (49% in an Illinois
cornfield, Table 7.1; 45% in a North Carolina deciduous forest, Table 7.2) or
by transpiration. The relative losses by evaporation from the soil and from the
vegetation are of special interest on watersheds in respect to vegetation management.
For example, experiments in humid southwestern North Carolina indicated
that conversion from mature, deciduous forest to low growing vegetation
increased stream flow 12.5 to 40 cm the first year after cutting; the increase was
much greater from north-facing than from south-facing slopes (Hewlett and
Hibbert, 1961). According to Bosch and Hewlett (1982) in nearly 100 experiments
with catchment basins removal of vegetation always resulted in an increase
in runoff. Clear cutting lodgepole pine in Colorado resulted in a 30%
increase in stream flow and conversion of chaparral to grass in the central valley
of California even doubled runoff. Instances are reported where the removal of
forests resulted ina measurable rise in a shallow water table. McNaughton and
Jarvis (1983) discussed methods of measuring the effects of vegetation change
on transpiration and evaporation, and Kaufmann (1984) developed a canopy
model for estimating transpiration of forest stands.
It is fairly common for evapotranspiration to exceed precipitation during the
growing season for a year or two, even in an Illinois cornfield, as shown in
Table 7.1. The deficit was supplied from stored soil water; the water content of
the upper 2.3 m of soil was reduced from 31.0 to 17.7 cm during the growing
season. This deficit would be made up during an average winter. An extreme
instance was the 18- to 20-year-old apple orchard in eastern Nebraska, studied
by Wiggans (1938) during the drought of the 1930s. The trees removed 25 to
38 cm more water per year from the soil than was returned by precipitation,
and it was estimated that in 3 more years they would exhaust all the available
water to a depth of over 9 m. Presumably the trees eventually would have died
from desiccation, but unfortunately they were killed by a severe autumn freeze,
prematurely terminating the observations. Many tree plantations in the prairie
and Great Plains states flourished for several years on stored soil water, but died
when it was exhausted (Bunger and Thomson, 1938).
Another example is the deep-rooted perennial alfalfa, which made excellent
growth in virgin soil on stored water in certain areas of Nebraska for the first
3 years, quite independently of current rainfall, after which the yield declined