Environmental Engineering Reference
In-Depth Information
degree of instability of the lower atmosphere. If the atmosphere is stable, in which
case, temperature and density both decrease with increasing elevation, there is very
little vertical movement that could otherwise aid the vapor-removal process. There-
fore, most methods for quantifying ET also measure the vapor-pressure gradient,
wind speed, or both, just above the surface supplying water for ET.
3.5.2 Direct Measurements
Of the options available for quantifying ET, only two can be considered capable of
directly measuring the process. The evaporation pan is very simple, but requires a
coefficient that can be difficult to determine. The eddy-covariance method often is
considered a direct method because it has a sound theoretical foundation and
requires no assumptions regarding atmospheric stability or the wind-speed velocity
profile. It also requires extensive instrumentation and data processing.
3.5.2.1 Evaporation Pan
The evaporation pan is perhaps the most direct method for quantifying ET.
It consists of a cylinder nearly filled with water with the top open to the atmosphere.
Evaporation is determined by totalling the water added to the system, minus water
removals following rainfall, required to maintain the water at a constant level. Pans
of a variety of shapes, sizes, and depths have been used over many decades. Since
the 1950s and 1960s, many national networks (e.g., Jovanovic et al.
2008
) have
adopted the class A pan configuration of 1.21-m diameter and 0.25-m height with a
stilling well positioned inside the tank to facilitate accurate measurement of the
water level (Fig.
3.8
). To ensure uniformity, the standard installation is located in an
open area at least 20 m by 20 m with close-cropped grass on a wooden platform
15 cm above ground (Allen et al.
1998
). Some sites also measure wind speed just
above the rim of the pan as well as air and water temperature to make additional
empirical corrections for those environmental variables (e.g., Harbeck et al.
1958
).
The amount of water lost from an evaporation pan almost always is larger than
water lost from a nearby lake or wetland because of enhanced wind flow across the
pan surface and radiational heating of the sides of the pan. A pan coefficient of 0.7 is
generally applied to convert measured ET to actual ET on an annual basis (Dingman
2002
), although values have been reported ranging from 0.64 for the Salton Sea,
California, to 0.81 for Lake Okeechobee, Florida (Linsley et al.
1982
). Numerous
studies have determined site-specific monthly coefficients that vary over a much
larger range to achieve greater accuracy (Kohler
1954
;Abtew
2001
;Masoner
et al.
2008
). Several studies have employed a floating pan situated in the water
body of interest. For small wetlands, this method works well because waves do not
become large enough to overtop the floating pan and corrupt the data. Data from a
floating pan situated in a small wetland near Norman, Oklahoma, were within 3 % of
