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.

1) The Need For Optimisation and Mathematical Programming Methods

(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.

2) Integrated Application of Simulation and Mathematical Programming Algorithms

(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.

3) The Use of Appropriate Measures of Supply Reliability

(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.

4) The Choice of Simulation and Optimisation Model Time Steps

(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.

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.

5) Program Transparency and Outputs

(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.

6) Program Generality

(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.

7) User Friendliness

(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.

8) Program Development

(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.

While PWSC`s original computer programs were written in FORTRAN to minimise execution times, recent packages such as AQUARIUS and EXODUS are written in MS Visual Basic (VB). With the advent of `native code` VB compilers, execution speeds of compiled programs are now similar to those obtainable with FORTRAN. VB also facilitates `seamless` communication with standard database structures for the storage and retrieval of program results. Advances in computing power have also made feasible the use of Linear Programming in EPSIM, AQUARIUS and EXODUS for optimising source/supply allocations within each simulation time step, rather than the less exact methods used previously. For more information on this area see `An integrated simulation & dynamic programming approach for evaluating the performance of complex water resource systems and optimising operating policies: Methodology & Applications`.

9) Software Maintenance

(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.