Environmental Engineering Reference
In-Depth Information
reading. Clear water makes it difficult to see specifically where the water cuts across
the staff plate. Corrosion or algal growth can obscure the values on the staff plate.
A tilted staff plate causes the indicated stage change to be larger than the actual
stage change.
As mentioned earlier, a staff gage mounted on a pole or stake driven into the bed
can move over time, resulting in a bias in time-series trends. This problem is greatly
enhanced in locations where the surface water freezes during winter. Staff gages
commonly are pulled upward in the spring when recharge to the wetland causes the
floating ice to rise before the ice melts enough to release contact with the staff gage.
If wind causes the ice to move horizontally during the melting process, the staff
gage can be tilted or even sheared from the pole or stake. In these cases, staff gages
need to be resurveyed to a stable datum annually.
3.3.3.2 Local Datum Errors
Elevation of the local datum to which the staff gage is related also can change over
time. Lag screws at the base of large trees are surprisingly stable, but a strong wind
can cause a tree to lean or fall to the ground. Therefore, many installations make use
of multiple datums and annual surveys are made from the staff gage to each one.
Some studies have used nearby monitoring wells as stable reference marks for
maintaining inter-annual elevation control. Wells associated with wetland studies
commonly are shallow because the water table is close to land surface. Shallow
wells are not well anchored, leaving them susceptible to frost expansion; some have
been observed to move 10 cm or more in a single winter. Therefore, use of a well as
a stable reference point is not recommended in environments where soil frost occurs
(Rosenberry et al.
2008
).
3.3.3.3 Automated Sensor Errors
Automated sensors also are subject to error. Each sensor has specifications that
indicate sensor resolution and accuracy. In addition to stated sensor limitations,
problems can develop that are specific to particular types of sensors.
Systems that include a float are subject to hysteresis, as described earlier. Errors are
proportional to the square of the float and pulley diameters and generally will be less
than 3 mm if the float diameter is greater than about 60 mm and the pulley diameter
is 0.1 m. Floats also can ride deeper in the water or even sink due to a leak. Debris
accumulation on the float can cause the float to ride deeper in the water, creating a bias.
A potentiometer connected to a float can fail (basically, wear out) over specific depth
increments if the float remains at essentially the same height but waves cause the float
and potentiometer to move back and forth (“paint”) for extended periods. During
exceptionally high or low water levels, the counterweight may exceed its length of
travel, resulting in faulty measurements.
