Page 122 - Biosystems Engineering

Soil and W ater Conservation 103

3.4.4 Pan Evaporation
Evaporation from free open-water surfaces is commonly measured
by evaporation pans. The class A pan is the accepted standard by the
U.S. Weather Bureau. These pans are 47.5 in (121 cm) in diameter and
10 in (25.4 cm) in depth. Evaporation estimates from these pans over-
estimate actual evaporation by 20 to 40 percent as energy enters the
pan from through sides and bottom. The evaporation values obtained
from these pans are typically multiplied by a correction factor of 0.7
for a more accurate estimation of actual evaporation from free open-
water surfaces.

3.4.5 Evapotranspiration
Evaporation and transpiration are often combined together for con-
venience and termed evapotranspiration (ET). There are various
methods to determine evapotranspiration (Fangmeier et al. 2005):
• Tank and lysimeter experiments
• Field plot experiments where the quantity of water applied is
controlled to prevent deep percolation and runoff is measured
• Soil water studies
• Analysis of climatological data
• Integration methods where water use by plants and evapora-
tion from water and soil surfaces are combined
• Water balance method for large areas over long time periods
Estimation of ET is important for crop management, especially irriga-
tion. Soil moisture content is a function of water supply in forms of
rainfall or irrigation, and ET. During dry periods, soil water deficit can
be estimated and irrigation can be planned accordingly by estimating
ET. ET depends on many factors such as amount of solar radiation
reaching the surface, amount of wind, aperture of stomata, soil water
content, soil type, and plant characteristics, among others (Ward and
Trimble 2003). For simplification, researchers introduced the potential
evaporation (E ) and potential evapotranspiration (ET ) concepts. Jensen
p p
et al. (1990) defines E as “evaporation from a surface when all surface–
p
atmosphere interfaces are wet so there is no restriction on the rate of
evaporation from the surface. The magnitude of E depends primarily
p
on atmospheric conditions and surface albedo but will vary with the
surface geometry characteristics, such as aerodynamic roughness.”
Penman (1956) was the first to define ET , and he defined it as “the amount
p
of water transpired in unit time by a short green crop, completely shad-
ing the ground, of uniform height and never short of water.” In general,
actual ET is different than ET , and we need the actual value, not the
p
potential one. In practice, ET is calculated first and actual ET is
p