Environmental Engineering Reference
In-Depth Information
3.3.1.7 Remote Sensors
Several other sensors are capable of measuring wetland stage without coming in
contact with the water, some with considerable accuracy. Acoustic sensors transmit
an acoustic wave to the water surface and record the time of transmission upon
reflection of the acoustic wave back to the sensor. This provides a useful method to
monitor the stage of a seasonally frozen wetland when a pressure transducer could
suffer damage caused by freezing (Hayashi et al.
2003
). Corrections need to be
made for air temperature and density to maintain a high level of accuracy. Sensors
that transmit and receive a radar pulse operate under the same assumptions. For the
radar sensors in particular, the diameter of the water surface over which stage is
being determined depends on the transmission beam angle as well as the distance
the sensor is mounted above the water surface. Therefore, any object(s) projecting
above the water surface that are within the cone of influence can corrupt the
measurement. Several acoustic and radar sensors can provide water-level measure-
ments that are within 3 mm of the true value. Laser-based devices also are available,
but for water that is particularly clear, the laser beam may penetrate the water rather
than reflect off it. Use of a floating reflector positioned inside a stilling well may
minimize this problem.
High-resolution satellite images or aerial photographs provide reasonably accu-
rate estimates of inundated wetland areas under ideal conditions. If the relation
between wetland area and stage is known (e.g., Eq.
3.4
below), then the wetland
stage can be estimated with reasonable accuracy. However, the accuracy of this
method depends on the delineation of inundated area, which may be difficult with
the presence of emergent vegetation (e.g., Fig.
3.1
), and on the accuracy of the
stage-area relation.
3.3.2 Converting Stage Change to Volume
Measurement of wetland stage commonly is determined on a short time interval,
perhaps every 15 min or once an hour, unless the measurement is made manually.
This allows quantification of stage in response to individual precipitation events if
precipitation also is determined on a short time interval. However, for the purpose
of determining a water budget, change in stage should be determined on the same
time interval as the hydrologic component with the longest measurement interval.
In most situations, the time-limiting parameter will be evaporation, which rarely
is determined on less than a daily interval. Therefore, assuming that all other
hydrologic components are determined at least on a daily basis, stage change should
be determined based on subtracting wetland stage at midnight from wetland stage
during midnight of the subsequent day. In this way, daily change in wetland
stage will be integrated over the day, just as is the case for measurement of the
rest of the hydrologic components.
