| PWSC's
Modelling Philosophy & Application |
Development
of PWSC software has been guided by a number of underlying
concepts, beliefs and principles resulting from many
years experience in the analysis of 'real' systems
and the application of mathematical programming techniques.
The main components of this 'modelling philosophy'
are outlined below, together with the ways in which
our software strives to meet these objectives.
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(i) Optimisation
is necessary so as to ensure that alternative operating
policies or system development plans are directly comparable
in terms of meeting specified demands with the same
levels of supply security.
(ii) For
systems or operating policies of any complexity,
'manual optimisation' based on repetitive simulation
is unrealistic due to the number of interdependent
variables involved; some form of mathematical programming
algorithm is normally required in order to obtain
the 'optimal' solution.
(iii) The
optimisation method employed should reflect the nature
of the problem to be solved, rather than tailoring
the problem to comply with the limitations imposed
by a particular method.
PWSC
has wide experience in the development
and application of linear (LP), non-linear
and dynamic programming (DP) techniques,
many of a proprietary nature.
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(i) When
evaluating the operation of existing or proposed water
resource based systems, it is essential that adequate
consideration must be given to the effects of the 'persistence'
exhibited by historic flow series, particularly low
flow sequences.
(ii) For
systems of any complexity, the effects of spatial
and temporal stream flow variations can only be adequately
modelled by employing a simulation model with an
appropriately short time step.
(iii) Availability
of a detailed simulation model is a pre-requisite
for properly evaluating the performance of any 'optimised'
operating policy or planned system configuration.
(iv) Such
simulation models must be capable of representing
the behaviour of the system to the satisfaction of
those with practical knowledge of its characteristics,
and of simulating both existing and alternative operating
policies.
(v) While
many practitioners have advocated the combined use
of simulation and optimisation models, it is also
necessary to specify how they should be linked together.
Within
PWSC programs, the performance of 'optimised'
policies are evaluated by simulating
their performance using a detailed model
of the system, so as to ensure that specified
(supply) reliability criteria are satisfied
and costs accurately assessed. The emphasis
is thus on the derivation of practical
operating policies or development plans,
rather than producing a mathematically
'optimal' solution for an over-simplified
representation of the 'real' problem.
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(i) System
performance should be quantified in terms of criteria
that are comprehensible by lay people as well as 'experts'.
Thus, esoteric concepts such as '2% droughts' or 'firm
energy' should be avoided whenever possible.
(ii) Given
the uncertainties associated with the economic derivation
and justification of penalty functions applied to
unsatisfied demands, it is preferable to measure
supply security in terms of the imposition of supply
restrictions of quantified severity and acceptable
socio-economic effect.
PWSC
programs offer the user a variety of
supply reliability criteria, as quantified
by the simulation model. These include
the incidence of quantified supply restrictions,
maximum permissible supply deficits and
minimum reservoir draw down levels.
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(i) The
time step used in the Simulation Model should be short
enough to ensure that seasonal hydrological and cost
variations are adequately considered.
(ii) The
Simulation Model should preferably enable the user
to investigate the use of different time steps, so
as to evaluate the relationship between increased
accuracy and execution times.
(iii) The
time step used in the Optimisation Model should reflect
the practicalities of operating policy implementation
as well as hydrological and cost variations.
Time
Step
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Reservoir
Contents
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Relative
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Cost
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Reliability
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Speed
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Daily
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100
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95
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100
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Weekly
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90
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98
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30
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Monthly
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80
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100
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10
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It
is unlikely that such requirements will
be met by having the same time steps
in both the Simulation and Optimisation
models, and PWSC programs reflect this
situation.
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5)
Program Transparency and Outputs
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(i) The
workings of complex computer programs should be as
transparent as possible, so as to aid both user understanding
and future development.
(ii) Such
transparency requires the detailed monitoring of
program inputs, intermediate and final results in
annotated file form for presentation and archiving
purposes.
(iii) Understanding
of results and simulated system performance is greatly
aided by the graphical display of results, either
in plot or 'mimic' diagram form.
PWSC
has developed Windows based Run-Time
Environments (GUI's) for its DOS/FORTRAN
programs such as MOSPA (MOSES), SYSIM
(SYSIME) and EPSIM (EPSIME). For presentation
purposes these GUI's 'read' the annotated
files produced by the FORTRAN programs
and offer the user a comprehensive number
of display options. Programs such as
AQUARIUS and EXODUS, which are entirely
written in MS Visual Basic, store results
directly in an ACCESS database.
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(i) While
programs should ideally be generalised, i.e. data driven,
rather than 'bespoke', i.e. system specific, the development
of generalised programs is more complex and time consuming.
(ii) Many
systems exhibit 'unique' features which must be adequately
modelled if an adequate representation of the system
is to be obtained. It is thus inevitable that the
capabilities of 'generalised' programs may, from
time to time, need to be enhanced.
PWSC
is committed to the continued future
maintenance and enhancement of its software.
This is facilitated by having direct
access to all source code employed, including
the optimisation algorithms, and by taking
such requirements into consideration
during initial program design.
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(i) While
a high-level of 'user friendliness' is desirable, it
should not take precedence over providing the flexibility
required to model 'real world' system characteristics
and complexities.
(ii) The
true value of computer programs lies in the integrity
of the underlying concepts and calculations, rather
than its cosmetic appearance.
PWSC's
development of user friendly interfaces
only follows exhaustive testing of the
core analytical code.
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(i) New
software developments should seek to fill perceived
needs in the market place and incorporate improved
methodologies in terms of accuracy and optimality.
(ii) Advances
in programming languages should be exploited in the
interests of improved execution performance and user
friendliness.
(iii) Program
architecture and the choice of methodology incorporated
should reflect advances in computer performance,
in terms of both processor and data storage capabilities.
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(i) While
every effort should be made to eliminate program 'bugs'
during development it is recognised that, as with all
complex software, malfunctions may from time to time
become apparent.
(ii) Due
to the nature of the systems being analysed, no guarantees
can be given regarding 'fitness of purpose' or against
consequential losses arising from program applications.
PWSC
offers (annual) maintenance contracts
under which it provides prompt rectification
of any program malfunctions brought to
its attention, and supplies updated versions
of the software package without additional
charge.
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