Environmental Engineering Reference
In-Depth Information
Figure 5.17
The relationship between the ratio of actual to
potential evapotranspiration AE/PE and soil moisture.
Encircled 1 and 2 are schematic curves for a vegetation-
covered clay loam under low evaporation stress and a
vegetation-covered sandy soil under high evaporation stress
respectively.
Source: After Barry (1969).
from moisture stress and nutrient deficiencies, and in some cases the stomata in the
leaves may close, reducing transpiration further. But the drain upon the soil moisture
store continues, so the moisture stress gets worse. Progressively the rate of actual
evapotranspiration falls below PE to develop a soil moisture deficit.
It now seems clear that the effect of declining moisture availability depends upon a
variety of conditions, including vegetation type, rooting depth and density, and soil type.
In a heavy clay soil, for example, it seems that evapotranspiration rates fall only slightly
as the soil dries out until the point is reached where no more water is available to plants.
Evapotranspiration then falls rapidly. Conversely, in sandy soil the decline in actual
evapotranspiration rates is much more regular (Figure 5.17), as the sandy soil's capacity
to retain moisture is less than that of clay.
PLANT RESPONSES TO MOISTURE STRESS
The reduction in evapotranspiration as the soil dries out has a number of implications.
Eventually, of course, the plant experiences severe nutrient deficiencies and the yield is
reduced. Thus we often see a close relationship between the degree of moisture stress and
crop yields. In addition, the moisture in the plant helps to control its temperature; the
energy used in transpiration cannot heat the plant. As transpiration declines, more energy
