Environmental Engineering Reference
In-Depth Information
processes should equal the volume of water stored in a wetland for any given
accounting period. By simply measuring the change in the elevation of the wetland
surface, and multiplying that change by the surface area of the wetland, we can
obtain a change in wetland volume over an accounting period and relate that change
to the hydrologic inputs and losses that occurred over that same accounting period.
Relative to the complexities associated with measurement of all of the input and
loss terms, measurement of wetland stage should be relatively simple and error free.
However, even small measurement error, and poor characterization of wetland
bathymetry and geometry, can still result in substantial errors (e.g., Winter
1981
).
3.3.1 Stage Measurement
The relative height of the wetland water surface commonly is referred to as wetland
stage, herein symbolized as
h
. This is sometimes confused with wetland elevation,
which is the height of the wetland water surface relative to a citable datum
(reference elevation); for example, North American Vertical Datum of 1988
(NAVD88). This also is not to be confused with wetland water depth, which is
the vertical distance from the sediment-water interface (herein, referred to as the
wetland bed) to the water surface. Stage typically is determined relative to a local
datum, such as a painted mark on a rock outcrop or stable concrete fixture, a pipe or
rod driven into the ground, a lag screw placed near the base of a nearby tree, or a
benchmark if one is located nearby. Wetland hydrologists commonly make the
assumption that the wetland surface is flat and that wetland stage can be measured
at any location in a wetland (see Sect.
3.3.3.3
below on how to address seiches for
large wetlands). Therefore, measurements typically are made either at a location
convenient to the observer or at the deepest point in the wetland if it is expected that
the wetland might go dry. In some cases, the wetland bed is artificially deepened at
the point of measurement so that the water level can be measured for a short
distance below the deepest portion of the wetland during drawdown. A water-
table monitoring well installed in the wetland is required to track further water-
level drawdowns during prolonged dry periods. Several of the more commonly
used methods for measuring stage are described below. Greater detail is provided in
a U.S. Geological Survey (USGS) report on methods for making stage measure-
ments (Sauer and Turnipseed
2010
).
3.3.1.1 Staff Gage
The simplest and most common method for measuring wetland stage is to visually
observe the value where the water surface cuts across a graduated plate placed
vertically in the water (Fig.
3.2
). Commonly made of fiberglass or enamelled metal,
the staff gage is bolted to a stable surface or placed on a pipe or solid rod driven into
the wetland bed. Data-collection interval commonly is variable and depends on the
