Hydrology and Physiography of Groundwater and Wetland Habitats - Freshwater Ecology: Concepts and Environmental Applications - page 56

(Fig. 4.7). Some areas have extensive continuous aquifers (e.g., the lower
Mississippi valley and the High Plains) and others have more sparse, lo-
calized aquifers (the Rocky Mountain re-
gion). Groundwater in many of these
aquifers is being depleted at rates faster than
the rate of recharge. Perhaps the most fa-
mous example of this is the Ogallala or
High Plains Aquifer (Sidebar 4.1). Ground-
water depletion is commonly associated
with irrigated land worldwide.
One of the major types of groundwater
habitat is found in limestone regions with
rough land surface called karst topographies
(White et al., 1995). Understanding specifics
of karst aquifer hydrology is important in
assessing the impacts of humans on ground-
waters (Maire and Pomel, 1994). Large
channels can form in these habitats because
the water can dissolve the limestone. If the
water subsides, caves are left (Figs. 4.1 and
4.5). Pools and streams in limestone caves
provide one groundwater habitat in which
the geological formation allows humans to
directly interact and sample the subsurface
habitat. Hydrology of karst aquifers is very
complex, in part because it is difficult to
predict the pathways of limestone dissolu-
tion (Mangin, 1994).

Sidebar 4.1.
Mining the Ogallala Aquifer

The High Plains or Ogallala Aquifer stretches
from Nebraska to the southern tip of Texas (Fig.
4.8). The aquifer underlies 450,000 km 2 and has
an estimated thickness of up to 300 m and an
estimated water volume of 4000 km 3 . The aquifer
supplies 30% of all irrigation water in the United
States (Kromm and White, 1992a). Mean
recharge rate is 1.5 cm per year, and withdrawal
rates average about 10 times this rate. Precipi-
tation to land above the aquifer is less than that
required to support the crops that are irrigated
from the aquifer (i.e., potential evapotranspira-
tion exceeds precipitation). Annual withdrawals
exceed the total annual discharge of the Col-
orado River (Kromm and White, 1992b). Some
regions of the aquifer are very thick and can
support withdrawals for decades. In many re-
gions the water table has dropped far enough
that it is not economically feasible to use the
groundwater for irrigation (Kromm and White,
1992b). Water is being withdrawn at greater
than sustainable rates, so the withdrawals can
be referred to as "mining" the aquifer.
In addition to loss of economic uses, there
are ecological impacts as the water table is
drawn deeper under ground. Depletion of the
groundwater has caused decreased water
supply and stream and river flow has disap-
peared in many regions (Kromm and White,
1992b). For example, the Arkansas River loses
water to the aquifer because agricultural ac-
tivity has lowered the water table, and now it
only flows during floods (Fig. 4.9). The loss of
flow has negative impacts on migrating water-
fowl that use the river and decreases the abil-
ity of the river to dilute and remove pollutants.
Conserving the remaining water makes good
economic and ecological sense. It remains to be
seen if more efficient irrigation technology and
dryland farming will allow the same level of eco-
nomic productivity as was made possible in the
region during the past few decades by exploit-
ing the High Plains Aquifer for irrigation water.

WETLANDS

Wetlands are crucial habitats for many
types of plants and animals (e.g., migratory
waterfowl) and provide many ecosystem ser-
vices, including flood control and the im-
provement of water quality. Wetlands are
used to treat wastewater in many places. In
addition, wetlands are globally important as
natural sources of methane to the atmosphere
(see Chapter 12), and this trace gas plays an
important role in the regulation of climate
(Schlesinger, 1997). Wetland sediments are
valuable because they preserve a long-term
record of environmental conditions, and sed-
iments in peat bogs are mined for use in gar-
dens (Fig. 4.10). The study of wetlands is rel-
atively new compared to that of lakes because
such study falls between the traditional disci-
plines of limnology and terrestrial ecology.