Environmental Engineering Reference
In-Depth Information
Fig. 3.34 Installations commonly used to determine wetland stage, elevation, and vertical
hydraulic-head gradient
venting, the sensor output is the sum of hydrostatic pressure of the water column
above the sensor port (the dwc or
d
epth of the
w
ater
c
olumn that we want to know)
and atmospheric pressure. Therefore, atmospheric pressure needs to be measured
and subtracted from the output of the submerged pressure transducer to obtain the
height of the water column above the submerged sensor. A barometer is suspended
in the piezometer casing, well above the water level, to provide atmospheric-
pressure measurements. If the well is susceptible to occasional flooding, the barom-
eter could instead be located anywhere nearby as atmospheric pressure does not
change appreciably over distances of several km.
Output from pressure transducers, as well as many other sensors, commonly is
converted to units in which field check measurements are made. In wetland settings,
that unit usually is feet or meters of water head. Meters will be used here. To
convert output in pressure to head, recall that Pressure
is density of
water (kg m
3
),
g
is acceleration due to gravity (m s
2
), and
h
, hydraulic head, is
the height to of a column of liquid that would exert a given pressure, in m. Output
from pressure transducers commonly is in units of Pascals. Recall that a Pascal is a
¼ ρ
gh
where
ρ
