Geoscience Reference
In-Depth Information
images represent only certain portions of the
light spectrum, as i ltered through the atmos-
phere and detected by cameras. No map can
depict all physical, biological, and cultural fea-
tures for even the smallest area. A map may
display only a few selected features, which are
portrayed usually in highly symbolic styles
according to some kind of classii cation scheme.
In these ways, all maps are estimations, gener-
alizations, and interpretations of true geographic
conditions.
Furthermore, all maps and GIS datasets are
made according to certain basic assumptions,
for example sea-level datum, which are not
always true or verii able. Finally any map is the
product of human endeavor and, as such, may
be subject to unwitting errors, misrepresenta-
tion, bias, or outright fraud. In spite of these
limitations, maps and now GIS have proven to
be remarkably adaptable and useful through
several millennia of human civilization. Maps
of all kinds are fundamentally important for
wetland research.
A
B
3.4 Physical ground-based methods
3.4.1 Surface methods
Ground-based means of study include all types
of traditional biological and physical methods
for sampling, measuring, and describing the
water, soil, vegetation, and wildlife of wetlands.
Detailed descriptions of these methods are
beyond the scope of this topic; the reader is
advised to consult specii c procedures for each
discipline. A few of the more basic ground
methods are presented here, beginning with
measuring the quantity of water l owing into or
out of a wetland system.
Surface water l ow through well-dei ned
channels is measured by stream gauging. Meas-
uring discharge at a gauging station depends on
knowing the cross-sectional area of the channel
and the velocity of water l ow at different depths
and positions across the channel. Once these
have been determined by preliminary i eld
measurements, the discharge may be calculated
from stream stage (height). Gauging stations
Figure 3-25.
Temporary gauging station (box in
center) erected on a glacial melt-water stream that
l ows into the lake in the background (A). View of
recording instrument that measures water height in the
pipe below the box (B). Photos by J.S. Aber at
Boundary Glacier, Alberta, Canada.
range from temporary, seasonal setups (Fig. 3-
25) to permanent, automatic stations that trans-
mit real-time data for online display (Fig. 3-26).
Direct water gain (precipitation) and loss of
water via evapotranspiration may be determined
