Environmental Engineering Reference
In-Depth Information
is density of water (kg/m
3
), and
g
is acceleration of gravity (m/s
2
). Since pressure is force per area and can be
expressed as kg m/s
2
/m
2
, units for
where
ψ
is pressure head (m),
P
is pressure (Pa),
ρ
g
(kg/s
2
/m
2
), which
yields m. As long as the pressure transducer is positioned somewhere within the
open column of water in the wetland (i.e., not buried in the sediment beneath the
wetland), the pressure head is directly proportional to wetland stage because, within
a water column, pressure head is exactly offset by the elevation head (i.e., the height
at which the pressure head is being measured). The deeper the transducer is
positioned in the wetland, the smaller the elevation head but the larger the pressure
head. For example, if the transducer is mounted at a depth 0.1 m above the wetland
bed, and the converted output from the transducer is 0.3 m, then wetland depth is
equal to elevation head (0.1 m) plus pressure head (0.3 m), which is equal to 0.4 m.
If instead the transducer is mounted at 0.25 m above the bed, then the height of the
water column above the transducer would be smaller because the transducer would
be immersed at a shallower depth in the wetland. In this case, elevation head would
be 0.25 m, output from the transducer converted to pressure head would be 0.15 m,
and wetland stage would still be 0.4 m.
are kg/s
2
/m divided by
ψ
ρ
3.3.1.6 Manual Measurement with a Graduated Rod
In seasonally frozen wetlands, a staff gauge needs to be surveyed annually to
account for movement due to frost action or moving ice. Conly et al. (
2004
)
presented a simple and practical method to eliminate this requirement using a
water-depth measurement rod. In this method, permanent markers, typically
metal stakes, are driven into the wetland sediments at locations where standing
water commonly is present. The elevation of the top of the marker stake is surveyed
once, which is used to establish the elevation of the wetland bed, which is in turn
used to calculate the wetland stage from depth measurements. An observer places a
wooden measuring rod, approximately 2 cm in diameter and 1.5 m in length and
graduated to 1 mm resolution, on the bed next to each marker stake. A 6-cm
diameter metal or plastic disk attached to the base of the rod prevents the rod
from being pushed below the sediment-water interface, ensuring greater repeatabil-
ity of measurements. A notch is cut into the base so the rod can be slid along the side
of the metal stake, ensuring consistency of the measurement location among site
visits. The observer slides the rod downward until the base touches the bed.
Distances from the marker base to the water surface, from the marker top to the
water surface, and from the marker top to the wetland bed are recorded, providing
redundancy in the water-surface measurement. Conly et al. (
2004
) reported the
accuracy of this method to be on the order of 1-2 cm. While this degree of
uncertainty may be too large for daily water budgets, the method provides a useful
approach to maintaining inter-annual consistency during long-term monitoring of
wetland stage and in determining monthly to seasonal water budgets.
