Assessing and Measuring Wetland Hydrology - Wetland Techniques - page 118

Despite its simplicity, the Priestley-Taylor method applied to data from a wetland in
the prairie-pothole region of North Dakota produced better data that the Penman
method when compared to results from the energy-budget method (Rosenberry
et al. 2004 ).
Another type of combination method that may work well for small wetlands, or
wetlands that are moisture limited, is the complementary relationship method. First
proposed in 1963, the complementary-relationship method makes the assumption
that actual evapotranspiration ( AET ) is reduced relative to evapotranspiration in a
wet environment ( ET wet ) to the same extent that potential evapotranspiration ( PET )
is larger than ET wet ; the drier the environment, the greater the difference between
AET and PET . Following this logic,

AET þ PET ¼ 2 ET wet (3.20)

By calculating ET wet based on available energy, and measuring PET , one can then
determine AET with

AET ¼ 2 ET wet PET (3.21)

Any number of methods can be used to determine ET wet and PET (e.g., Morton
1983a ). One approach is to determine ET wet using the Priestley-Taylor method and
determine PET using the Penman method (Brutsaert and Stricker 1979 ). As one
might expect, the Brutsaert-Stricker method applied over a wetland in North Dakota
(Rosenberry et al. 2004 ) and over a small lake in New Hampshire (Rosenberry
et al. 2007 ) gave results very close to either the Penman or the Priestley-Taylor
methods alone because the environment was wet; ET was occurring at the potential
rate. However, wetlands do not always have an ample water supply. During times
when wetlands are relatively dry, the complementary relationship method likely
would produce better results than other methods designed to indicate ET at the
potential rate. The method also can be used to determine the extent to which warm,
dry air may increase ET along the upwind edge of wetlands, a particular concern for
wetlands that are small or situated in arid environments (Morton 1983b ). This also
is a concern for other ET methods that rely on the assumption that vapor and
temperature gradients are adjusted to the wetland surface at the point of measure-
ment. However, at a wetland in North Dakota, the open-water portion of which
varied in size from 1.5 to 3 ha, insufficient fetch was found to cause errors in ET
estimates of less than 2 % (Stannard et al. 2004 ).

3.5.4 Measurement Parameters for Estimating ET

Quantification of net radiation is required for most of the ET methods. This involves
measurement of downward shortwave and longwave radiation from the atmo-
sphere, upward reflected shortwave and longwave radiation, and upward longwave