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Newsletter and Technical Publications
Lakes and Reservoirs vol. 2
The Watershed: Water from the
Mountains into the Sea
The Watershed: The Water collector
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| Photo 7: Section of the Amazon Watershed; satellite image, Brazil |
The quantity and quality of water on the land surface depends to a large
degree on land usage and human actions within watersheds (or
catchments,
drainage basins) (Photo 7). In much the
same manner as a drop of water along the lip of a cup will flow into the cup,
all the water in a watershed will flow down to its lowest part, which is usually
a river, lake or other water-body. Within this context, the Earth ’s land
surface can be viewed as a series of irregularly-shaped watersheds contiguous to
one another. The characteristics of an individual watershed depends on the
cumulative effects of all the water, land and land-based activities, people,
plants, animals, farms, cities, factories, etc., contained within it. Further,
other than where water is artificially transferred from one watershed to
another, each watershed is independent of all others, even those lying adjacent
to it.
Land use typically refers to the specific use or purpose for which humans use
the land surface, examples being agricultural fields, urban areas, roads,
forests, etc. Human actions on different land uses further dictate the quantity
and quality of fresh water within a given watershed. Agricultural land, for
example, can be used for a number of different purposes, including row crop
production, livestock raising, orchards, pastures, etc. As discussed below, land
use and land-use activities are primary determinants of water pollution.
Nature largely dictates the absolute quantity of available water (in the form
of precipitation) in a given watershed via the hydrologic cycle. However, human
water demands fundamentally influence the relative quantity of available water
within a given watershed. Arid and semi-arid regions, for example, do not
receive a large quantity of precipitation over the annual cycle and may cause
excessive demands on existing water supplies. The result is water scarcity
(Photo 8).
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| Photo 8: Yemeni Desert in the South Arabian Peninsula |
The converse is true for watersheds receiving large quantities of
precipitation and/or having fewer water demands. Thus, the availability (or
scarcity) of water in a watershed represents a balance between the water supply
(the volume of water supplied by nature) and the water demand (the volumes of
water needed by humans for different land-based activities).
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| Photo 9: Water pollution in an urban lake, Jakarta, Indonesia |
Human settlement of a watershed involves the building of cities, industrial
complexes, farms, streets, etc., the use of raw materials and chemicals used in
industrial activities, fertilizers and pesticides used in agriculture, and
alterations in land cover vegetation, natural drainage networks, etc. These
types of activities invariably result in the production of liquid, solid and
gaseous wastes of various types and quantities, which can subsequently find
their way into rivers, lakes and groundwater aquifers in the watershed. This
phenomenon is called “pollution ” which, depending on the types and quantities
of materials, can fundamentally define the quality of water (Photo 9).
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| Photo 10: Pipeline discharges; a point source of pollution |
Specific human water uses require adequate supplies of water of acceptable
quality for its intended use. Drinking water, for example, requires the highest
quality in order to be safe for human consumption. In contrast, water used for
irrigation or to generate hydro- power does not have to be of the same high
quality. Within this context, water pollution also constitutes a type of water
“scarcity ” in that it decreases the range of potential water uses for a given
water supply without some degree of pre-treatment prior to its use.
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Photos 11, 12, and 13: Non-point source of pollution originating from storm
run-off water entering Lake Biwa, Japan
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Pollutants can be characterized as point P12 or non-point (or diffuse) in
origin. Point sources P13 comprise “pipeline ” discharges of wastes or other
pollutants to rivers or lakes draining a given region, examples being municipal
wastewater treatment plant and factory effluents (Photo 10). Non-point sources
represent storm-induced water drainage or
runoff
over the land surface in which the flowing water dissolves or picks up
pollutants and other materials and carries them to rivers and other water
systems draining the region (Photos 11, 12 and 13). Accordingly, non-point
source pollutant loads are closely tied to precipitation or snowmelt events.
Further, the quantities and types of non-point pollutants are a function of the
characteristics of the land surface over which the water passes, as well as of
human activities on the land surface. Agricultural and urban areas typically
produce large non-point pollutant loads, and are the major pollutant sources in
many regions of the world (Photos 14 and 15).
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| Photo 14: Mixed urban and agricultural lands causing point
and non-point pollution in rivers and streams. |
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Photo 15: Polluted stream with urban waste
and agricultural run-off. |
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