Geoscience Reference
In-Depth Information
the wetland falls during the dry season or a
drought period, the situation may reverse and
become discharging.
On a regional basis, many wetlands may
exist at different elevations and may be con-
nected via various surface- and ground-water
l ows. Consider a series of linked wetlands at
the end of a drought phase when water
storage has been greatly depleted. As a wet
climatic phase begins, wetlands at higher ele-
vations must i ll their storage capacity before
they can begin to release surface water or
recharge aquifers that feed into lower wet-
lands. Thus, i lling of lower wetlands in the
series may be delayed in a cascade manner
(Welsch et al. 1995). The case of the Pantanal
wetlands in South America illustrates this
process nicely. The northern wetlands are
replenished by rain-fed streams and rivers
including the Paraguay River l owing from the
highland regions of Brazil during the wet
season of November through March. This
water slowly makes its way into the southern
Pantanal, nourishing the vast wetlands as it
l ows southward (see section 15.4).
Human diversions or extractions of either
surface l ow or ground water have the poten-
tial to impact integrated surface-ground-water
systems both locally and downstream. A common
scenario is overpumping of ground water, such
that local or regional water tables are drawn
down to a deep level at which springs, streams,
lakes, and wetlands are diminished or dry out
completely (Fig. 4-12). In coastal marine set-
tings, overpumping of ground water may lead
to ground subsidence and intrusion of salt
water, such that fresh-water marshes are con-
verted into open marine bays. Impoundment
and diversion of surface water for irrigation,
cities, or industry may reduce downstream
l ows substantially with severe impacts on wet-
lands. A famous case is the Colorado River,
which no longer reaches its delta in the Sea of
Cortez (see Color Plate 3-14); all the fresh
water is extracted along the way. The Colorado
River delta is considered among the world's
most severely affected ecosystems (Avila-
Serrano, Téllez-Duarte and Flessa 2009).
Figure 4-11.
Artesian well l ows into a marsh at
Russell Lakes State Wildlife Area in the San Luis
Valley, south-central Colorado, United States. Photo
by J.S. Aber.
at rates determined by hydraulic conductivity
and hydrostatic pressure. Because of these
movements, ground water has a close relation-
ship with surface water bodies
-
streams, lakes,
and wetlands; they are hydrologically con-
nected. In the case of gaining streams, discharg-
ing ground water moves into the stream channel,
and this basel ow maintains stream l ow between
surface runoff events. Conversely, water in a
losing stream soaks into the ground, thus
recharging the aquifer and reducing down-
stream surface l ow.
Wetlands have similar relationships to ground
water. In a recharging wetland, water moves
from the wetland into the ground-water system.
Thus, wetlands may be important sources of
recharge for aquifers that feed into surrounding
springs or supply well water. A discharging
wetland is the opposite; water migrates from the
aquifer into the wetland (Welsch et al. 1995).
The state of recharge or discharge may l uctuate
with the hydroperiod. During times of high
water level in the wetland, ground-water
recharge may take place. When water level in
