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This indicator should
be obtained from the water administration authorities. Nevertheless
the rationale of calculation can be summarized with the aid
of the above figure taken as an example from the Piedra river
in Spain:
1. The black curve represent
the natural situation. The horizontal light blue line represents
the minimum naturally regulated discharge guaranteed at about
1.3 m3/s. The ratio between the surface under this light blue
line and x axis divided betweem the surface under the dark
blue line and the x axis represent the natural regulation
of the river.
2. Because the naturally
regulated discharge of 1.3 m3/s is not enough to satisfy the
water demand a reservoir is built. The red curve corresponds
to the typical discharge after reservoir construction. The
area between the red curve and the X-axis represent the total
natural resources and is coincident with the area between
the black line and the X-axis (total natural resources are
the same with or without reservoir).
3. The graph shows the
annual hydrographs of the Piedra River in Spain, in the natural
situation (black) and the artificially regulated situation
(red). The regulated situation corresponds to the outlet of
a reservoir built and the natural situation is the same outlet
before the reservoir construction. Once the reservoir is constructed
the Piedra river basin upwaters is artificially regulated.
The real rating curve (red) is adapted to the demand providing
high discharges in summertime (irrigation), so we have 100%
of artificial hydrological regulation.
4. In practice the question
is more complex as we are dealing with a basin with several
demands in several points and several tributaries of a main
river, with different natural resources, proportions of water
uses, which return to the rivers as waste water (which may
be used downstream after purification) or for other uses less
dependent in water quality.
The scheme of the Fig.
2 depicts a complex real situation as described. The % of
artificial regulation will be determined considering the global
system of demands, reservoirs and wastewater releases and
reuses.
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Fig.
2 Complex combination of water supply and use to be found
in most large river basins |
5. From a strict point
of view the indicator defined here should be the difference
between the percentage of regulation which characterises the
natural situation and the percentage of regulation in the
second situation, meaning thus the increase in regulation
obtained. In practice as Mediterranean and semi-arid lands
rivers very frequently present a near zero degree of natural
regulation, the natural regulation is neglected and then we
speak of regulation in absolute terms.
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