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Watersheds
Today,
rather than looking at land and water resources as separate,
unrelated parts, water managers consider the connections within a
watershed or drainage basin. Every part of the Earthís land
surface is within a watershed. Divides (ridges, peaks or areas of
high ground) separate watersheds. Because water flows downhill,
rain falling on these divides may flow in opposite directions,
becoming part of different watersheds. For example, from the Great
Divide in North America the continentís river systems flow in
opposite directions.
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A watershed is the
land area that contributes runoff, or surface water flow, to a
water body. The water resources within a watershed are affected
primarily by what happens on the land within that watershed.
Anything on the land within the watershed, however far from the
water body, can eventually reach and impact that water resource.
Some examples of contaminants that may be picked up by water in
the watershed are soil particles (suspended materials) and
chemicals (dissolved materials), such as nutrients, pesticides,
oils and gasoline residues.
The
shape of the land defines a watershed. Water flows both above and
below the ground from points of higher elevation to points of
lower elevation through the force of gravity. Rainfall that is not
absorbed by the soil but flows to a larger body of water is known
as runoff; runoff collects in channels such as streams, rivers and
canals. The small channels, in turn, flow to larger channels and
eventually flow to the sea. These channels or streams are also
known as tributaries. The slope of the land, as well as the amount
and type of vegetation and soil and the type of land use,
determine the rate and amount of runoff that enters a water body.
More water soaks through sandy soils than through clay soils;
gentle slopes allow more time for rain to soak into the ground or
to evaporate than do steep slopes; and natural areas generally
allow more water to enter the ground than areas that are covered
with houses or pavement. Vegetation also absorbs water and slows
its movement.
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Florida ís karst terrain and flat topography sometimes
make determining watershed boundaries difficult. In some places
the drainage pattern is best described as ìdisjointedî because
streams and rivers do not form continuous channels on the land
surface (Mossa 1998) ó they may disappear underground in sinks or
depressions. Large rivers may form from springs issuing from the
aquifer, and surface water watersheds may be quite different from
groundwater watersheds. Some portions of Florida are poorly
drained (Mossa 1998). There are few or no streams or channels in
these areas, and water flows across the surface through extensive
swamps or marshes. This is known as sheetflow.
In much of south Florida, the natural landscape has been altered
with huge public works projects, making the region a managed
watershed. Canals, pumping stations and water-control structures,
such as dikes and weirs, have altered the watershed. The historic
swamps, marshes and associated sheetflow are greatly altered or
are replaced by urban development and agriculture and drained by
canals. Public and private entities are responsible for water
movement, especially the discharge of floodwater.
Aquifers
Aquifers are underground rocks that hold water. In Florida, three
aquifers are used for water supply: the Floridan aquifer, the
intermediate aquifer and the surficial aquifer. In northwest
Florida, the surficial aquifer is called the sand and gravel
aquifer, and in southeast Florida it is called the Biscayne
aquifer.
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The Floridan aquifer has been called Floridaís rain barrel
(Parker 1951) and is one of the most productive aquifers in the
world. Each day Floridians use about 2.5 billion gallons of water
from the Floridan aquifer. It underlies 250,000 square kilometers
(100,000 square miles) in southern Alabama, southeastern Georgia,
southern South Carolina and all of Florida. Over most of Florida,
the Floridan aquifer is covered by sand, clay or limestone that
ranges in thickness from a few feet in parts of west-central and
north-central Florida to hundreds of feet in southeastern Georgia,
northeastern Florida, southeastern Florida and the westernmost
Panhandle. Within the aquifer, water may travel quickly or very
slowly. In parts of the aquifer with caves and large conduits,
water may travel several miles in only a few hours. Where
water-filled spaces are small and underground routes convoluted,
it may take days, weeks or even years for water to travel the same
distance.
In the past several
decades, increased pumping of ground water has lowered water
levels in the Floridan aquifer in several places in Florida and
Georgia, including the Panhandle, northeastern and southwestern
Florida, and southeastern and coastal Georgia (Berndt et al.
1998). In some parts of
Florida, the Floridan aquifer is not a suitable or drinkable
source of fresh water. In some places, it is too far below the
surface; in other places, the water is salty. The surficial sand
and gravel aquifer is the major source of fresh water in Escambia
and Okaloosa counties in northwest Florida, and the surficial
Biscayne aquifer is the major source of fresh water in Dade and
Broward counties in southeast Florida. Between the surficial
aquifers and the Floridan aquifer in some parts of the state is
the intermediate aquifer. This aquifer is an important source of
fresh water in Sarasota, Charlotte and Glades counties. The
remainder of the state uses the Floridan aquifer as its main
source of drinking water.
Recharge
& Discharge
Water is
replaced in the Floridan aquifer by rainfall that soaks into the
ground. This is referred to as recharge. Recharge does not occur
everywhere. In some places (mostly along the coasts and south of
Lake Okeechobee) water flows out of, rather than into, the
aquifer.
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This is referred to as discharge. In other areas, thick clay
covers the aquifer and slows or stops the downward flow of water.
Areas of high recharge only occur in about 15 percent of the state
and include the well-drained sand ridges of central and
west-central Florida. Sand is porous, which means water can easily
flow through it. Limiting intensive development in high recharge
areas is critical for maintaining water supplies: water cannot
soak through pavement.
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