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3.6
DETERMINATION OF MINIMUM ACCEPTABLE STREAMFLOW IN
CZECHOSLOVAKIA
Two terms are used to describe minimum flow: minimum desirable discharge and minimum acceptable discharge.
The minimum desirable discharge is the discharge which satisfies minimum acceptable discharge and all other constraints:
-
domestic water supply, irrigation, industrial, etc;
evacuation of waste water;
-
navigation: transport of passengers of goods;
-
hydropower generation;
recreation and water sports;
transport, decomposition and dilution of residual pollution of
treatment plants and pollution arising in uncontrolled sites,
excessive use of fertilizers in agriculture, etc.;
influence on groundwater levels along the river course;
fish and water poultry farming;
-
water for landscaping, town planning, etc.
Water requirements vary along the length of individual rivers and must
be analyzed and evaluated individually.
Minimum acceptable discharge is the remaining flow after priority has
been given to all other water needs. It should be sufficient to
maintain normal biological life in the river and its close environment
and not interfere with common water use, as defined by the
Czechoslovak Water Act concerning water uses for which no licence is
required from the water authorities: bathing, washing, extraction of
water without mechanical devices.
Values of minimum acceptable discharge are evaluated according to the
following principles.
a
Streams influenced by reservoirs:
the minimum flow released from the reservoir is that which was
exceeded 355 days per year prior to erection of the dam (Q, ) ;
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-
in the reaches downstream of the reservoir the value of the minimum
acceptable discharge is gradually reduced in the direction of the
point where the influence of the reservoir on the flow is no longer
as certain. Downstream of this point the value of the minimum
acceptable discharge is determinated according to the principles
established for other streams.
b
Other streams
Minimum acceptable discharge is defined as half of the sum of the
98 percent minimum monthly flow and the 100-year minimum daily flow.
The flow must not be less than Q-,, and need not exceed Q o c c .
n
364
355
c
Influence of water quality
The value of the minimum acceptable discharge is increased to the
value of the flow exceeded 355 days per year for class IV quality of
water, the lowest class by Czechoslovak standards.
d
Exceptional cases:
-
at international borders, the minimum acceptable discharge is fixed
- where necessary - by agreement between adjacent countries;
-
reaches downstream of diversion channels;
reaches downstream of reservoirs situated near the confluence with
much larger rivers;
in rivers for which the 98 percent mean monthly discharge and the
100-year daily discharge cannot be evaluated with acceptable
accuracy;
in rivers having a great degree of flow regulation or on which very
large reservoirs have been built;
in rivers with very small flows.
Derogations must be approved by the highest water authorities (Ministry
of Forestry and Water Management).
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Some comments on the principles used to determinate minimum acceptable
discharge
a
The principles outlined above have been applied in Czechoslovakia
for over ten years with satisfactory results. They will not be
revised before 1990.
b
It is useful to discuss the reason for the selection of the
characteristic values used:
the 355 and 364 day exceedence values have been used by water
authorities for granting water licences for many decades and
they have therefore been adopted for the determination of the
minimum acceptable discharge. It is interesting to note that
practical experience applied in the past seems to be in close
concordance with the scientific results of hydrobiological
surveys undertaken in recent years which show that the
distribution of different species of water fauna is to a very
great extent dependent upon the flows in the 355 to 364 day
exceedence range.
The 100-year minimum daily flow is used as a control value only.
Its value is in most cases very low and it has little influence
on the value of the minimum acceptable discharge;
the 98 percent minimum monthly flow is used as a water resources
characteristic in state balances of water resources and needs in
Czechoslovakia whose principles were established some ten years
ago, at the same time as the principles used to define the
minimum acceptable discharges.
c
Another method for evaluating state balances of water resources and
needs has an influence upon the determination of minimum acceptable
flow.
Water demands are divided into two groups: use of the water course
itself and water extracted from the river for use elsewhere
(Table 3.6.1).
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Table 3.6.1
Total water demand
Group 1
Group 2
Use of the watercourse:
Abstraction from the watercourse:
- navigation
- drinking water supply
- recreation and water sports
- industrial water
- evacuation of waste water
- irrigation water
- fish and water poultry farming
- water for other purposes
- minimum acceptable discharge
- hydropower generation
Other constraints, such as the influence of river levels upon
groundwater levels and the use of water for landscaping and town
planning, call for the control of river water levels rather than
the discharge. They can often be satisfied by the construction of
weirs.
It is evident that quantities of water abstracted from the river
must be accounted for when evaluating their influence on flow
conditions. Use of the watercourse itself may well dictate flows
which automatically meet all other water demands. Mere addition of
demands is not sufficient when evaluating minimum permissible flow.
d
Flow of groundwater in Czechoslovakia is very complex due to the
hydrogeological conditions and it is very difficult to determine if
the abstraction of groundwater influences river flow. When
evaluating state balances of water resources and needs it is
assumed that the abstraction of groundwater always influences the
streamflow in the corresponding river reach, unless data is
available proving the contrary. This assumption is on the safe side
and therefore the values of minimum acceptable discharge can be
relatively low as the principles mentioned above for the
determination of minimum acceptable discharge in watercourses show.
e
According to the Czechoslovak Water Act the term watercourse
includes not only rivers but reservoirs and ponds also.
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REFERENCES
Manual for the compilation of balances of water resources and needs.
ECE/WATER/5, New York, 1974.
Metodiceskoje rukovodstvo po sostavleniju vodochozjajstvennych
balancov i vendenije vodnogo kadastra (Methodological manual for
the compilation of balances of water resources and needs and for
management of water register). CMEA, Moscow, 1981.
PLAINER, J., 1977. Vodohospodârskâ bilance (Balance of water resources
and needs). Prague.
PLAINER, J., 1983. Vyuzïvâni a ochrana vodnïch zdrojù (Utilization and
protection of water resources). Prague.