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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