Environmental Engineering Reference
In-Depth Information
Fig. 3.8 Two standard class-A evaporation pans in use near a lake in northern Minnesota. One is
serviced daily and the other weekly (Photo by Donald Rosenberry)
evaporation-chamber measurements and were considered to represent actual ET with
no correction factor (Masoner and Stannard
2010
). Comparisons with the empirical
Priestley-Taylor method (discussed in the Combination methods section) indicated
that the Priestley-Taylor method over-estimated ET during the day because the air
over the hot, dry landscape surrounding the wetland did not represent atmospheric
conditions directly over the evaporating water in the wetland. The study also
indicated that the floating-pan measurements over-estimated ET during mid to late
afternoon and under-estimated ET during nighttime to early morning. This was
attributed to the shallower depth of the water inside of the pan being more sensitive
to diurnal air-temperature changes than the deeper water column adjacent to the
floating pan. However, the errors were largely offset so the floating pan provided
daily ET values with little bias.
3.5.2.2 Eddy-Covariance Method
The process of evaporation can be viewed as vapor-rich rotating eddies of various
sizes that rise because they are less dense than other volumes of drier air that descend
to occupy the volume that the moist, rising air just vacated. The process is 3-
dimensional and also occurs on horizontal axes, but for the purpose of determining
evaporation from a wetland surface we are most concerned with the vertical axis.
Sensors measure the vertical velocity of these air packets, as well as their
“concentrations” (either temperature or absolute humidity) to obtain the vertical
velocity of the upward or downward flux of these properties. Vertical flux is then
