Environmental Engineering Reference
In-Depth Information
indicates zero wind speed. A sensor should be selected with the lowest threshold
that is affordable. Most sensors have cups or propellers that spin in response to the
wind and generate either a pulse per revolution or direct current (DC) volts propor-
tional to the rotational velocity. Ultrasonic anemometers operate based on resolving
the difference between times of transmission of ultrasonic pulses sent in both
directions between two transducers. Combinations of sensors are commonly ori-
ented along two or three axes to determine horizontal or 3-dimensional wind speed,
respectively.
Either dataloggers or field computers can be used to measure and store data from
analog and digital sensors that provide data to feed evaporation methods. Sensor
scan rate usually is determined based on the suite of sensors connected to a
particular datalogger as well as requirements specific to evaporation methods or
other products generated from the installation.
The most representative data are collected over the wetland in a location
designed to maximize fetch in all directions, or at least in the directions of the
prevailing winds. Sensors commonly are deployed from a floating raft or a platform
fixed to the bed, depending primarily on the wetland depth. Sensors that need to be
deployed at a constant height above the water surface, such as temperature-
humidity sensors and anemometers, need to be adjusted as the water level of the
wetland rises and falls. The temperature probe at the surface should be installed so it
rests as close to the water surface as practically possible. A float is commonly used
to keep sensors at the proper height above the water surface (Fig.
3.10
). Data from
land-based sensors often are used if a raft or platform is not available. In some
cases, use of land-based sensors results in little additional error (Rosenberry
et al.
1993
).
3.5.5 Measurement Errors
ET is difficult to estimate or measure in part because of the numerous sources of
error associated with the large number of sensors required. Error varies substan-
tially depending on the chosen measurement method.
The evaporation pan has sources of error associated with the actual physical
measurement as well as conceptually. A sensitive depth gage is very important
when adding or removing water in response to evaporation or rainfall. For a class-A
evaporation pan, evaporation of only 1 mm of water equates to 1.15 L of water that
needs to be added back to the pan to maintain a constant water level. Pans also
eventually develop leaks. If the pan is buried to minimize the effect of sidewall
heating, it would be difficult to diagnose a leak. Similarly, a floating pan is subject
to waves splashing over the side of the pan during windy periods. Numerous
modifications also have been made to minimize the effect of animals drinking
from the pan water. If a wire mesh is placed over the pan, a correction may need
to be made to account for the shading effect from the wire mesh. From a conceptual
perspective, a pan deployed in an arid environment, and elevated relative to the land
