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WATERSHED
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Watershed defnedA watershed is a geographicalunit in which the hydrological
cycle and its components can beanalysed.
Usually a watershed is defnedas the area that appears, on thebasis o topography, to
contribute all the water that
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Watershed-any sloping landsurace that sheds water; more
functionally-all land enclosed bya continuous hydraulic drainagedivide and lying upslope rom aspecifc point on a stream.
USS-!U"s #defne$
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%he surace trace o the boundary that delimitsa watershed is called a divide.
%he hori&ontal pro'ection o the area o awatershed is called the drainage area of astreamat that cross section.
%he location o the stream cross section thatdefnes the watershed is determined by the
analysis
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(elineation) a permeable soil covers an impermeable
substrate, the topographical division owatershed will not always correspond to theline that is e*ectively delimiting thegroundwater.
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Figure 2.2. Articial changes that occur in awatershed [Musy, 2001]
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WATERSHED
CHARACTERISTICS
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+hysical characteristicsGeomorphology
Watershed SuraceWatershed Shape
Watershed orientationTopography
Watershed hypsographical curveWatershed average slope
HydrographyHydrographic networ topology
Agro-pedo-geological factorsSoil types and vegetal coveringWatershed geology
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!ydrological
characteristicsConcentration time
Isochrones
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Watershed Surace
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WA%S!( A/A%he rain water ater infltering into the ground may
percolate through the subsoil and build up theground water table #W%$. %his ground water
through the water bearing strata may 0ow into thead'acent basin or directly into the sea i the waterbearing strata outcrops into the sea. %his is calledwatershed lea1age. %here may be even accretion oground water into the basin rom another nearby orremote basin i there is hydraulic interconnectionthrough a water bearing strata. )t is this subsurace,in0ow or out0ow that poses problem in the waterbalance studies o the basin.
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When the topographic divideand phreatic divide do notcoincide watershed lea1ageoccurs and is e2ual to the
ground water 0ow rom thearea between them.
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WA% 3AA4"%he input items into a basin are essentially
precipitation #+$ and subsurace in0ow #i$while the water losses are evaporation #$,evapotranspiration #t$ and subsuraceout0ow #o$. %he balance goes to rechargeground water #r$, increase the soil moisture#S5A$ and as surace runo* #stream0ow, $.
%he water balance e2uation can be written as
+ 6 i 7 6 t 6 8 6 S5A 6 r 6