Environmental Engineering Reference
In-Depth Information
3.4.8 Summary
Precipitation measurement provides critical input data to water-budget analysis.
With proper installation and operation of precipitation gages, and sufficient atten-
tion to calibration and site maintenance, it is feasible to achieve an accuracy of
5-15 % in point precipitation measurement. Spatial variability adds another degree
of uncertainty for larger areas of interest, but the uncertainty can be reduced by
using multiple gages and appropriate spatial interpolation techniques.
3.5 Evapotranspiration
Evapotranspiration (ET) is the combined flux from surface water to the atmosphere
resulting from evaporation and transpiration from plants. Evaporation is the process
of converting liquid water to water vapor, along with the transport of that vapor
from the water surface to the atmosphere. Transpiration is a similar process, but one
that occurs in plants. Liquid water is pulled through roots from the soil and
transported to the plant leaves. The vaporization process occurs within plant leaves
and the release of water vapor to the atmosphere occurs via small openings on the
leaf surface called stomates. We usually cannot distinguish between evaporation
and transpiration so hydrologists generally combine the measurement of these
processes (e.g., Shoemaker et al.
2011
). Most of the time, this is sufficient from a
water-budget perspective. Unless loss of wetland water to groundwater is unusually
large, ET usually is the largest water-budget loss term for wetlands that do not have
a surface-water outlet and is, therefore, an important term to quantify as accurately
as possible.
3.5.1 Commonly Used Methods
Vaporization of water is an energy-intensive process; 4.2 J are required to raise 1 g
of water 1
C whereas approximately 2,500 J are needed to vaporize 1 g of water.
Both evaporation and transpiration are dependent on the amount of energy available
to drive the process. They also depend on the relative availability of liquid water as
well as on the ability of the atmosphere to remove the water vapor once it is formed,
thereby allowing for the formation of additional water vapor at the wetland surface.
Since wetlands by definition are settings that generally (although not always) have
an ample water supply, it is usually assumed that ET occurs at a rate that is
unlimited from a water-supply perspective. Therefore, ET is assumed to occur at
the maximum potential ET rate based on available energy. This same assumption
cannot be made regarding the ability of the atmosphere to remove the recently
vaporized water, however. The vapor-removal component of ET depends on the
