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1st International Conference on Rain Water Cistern Systems
Honolulu, Hawaii, USA - June 1982

Section 5: Current And Future Practices

Page 308

Effect of Rationing on Reliability of Domestic Rainwater Systems

S.J. Perrens
University of New England, Australia


In a companion paper, Perrens (1982) presented the elements of a simulation model which has been used to study the main design parameters of rainwater supply systems at four locations in Australia. For that study, the model was run assuming that no rationing policy was opted when supplies were low. The results showed, however, that for all but the areas of subtropical rainfall with a mean annual greater than 1500 m, inadequate supply would-be available from all the roof areas on a typical farm or "station" to allow a reasonable standard of living for a family of four people. Under these circumstances, two strategies are commonly employed:

  1. Overall reduction in the demand for rain water either by lowering expectations or by seeking an alternative source of water to meet an element of the demand, for the use of water from a farm dam for toilet flushing or, with minor treatment, for clothes washing 
  2. Acceptance of a short-term reduction in demand by accepting a rationing policy which will increase the reliability of the supply for a given demand or, alternatively, allow increased demand for the same reliability.

The problem of allocating the rainwater resource given certain practical constraints on the physical elements of the system (storage size and catchment area) is one which regularly faces many Australian homesteads. Rationing may therefore be seen as an alternative to increasing the size of the physical components of the system. To make rational decisions about rationing, the user will need to know:

  1. The rationing policy to apply (duration of rationing and reduction of demand) 
  2. The effect of a particular policy on the reliability of the supply  
  3. The frequency with which rationing will be imposed under a particular policy.

This paper presents some preliminary analysis of these factors which have received very little attention to date.

Body (1968a, b) studied a range of rainwater supply systems in Australia and examined the effect of rationing policy on the reliability of supply. His work was never completed, however, and the results have not been made generally available. Body adopted more complex rationing rules than those used in this study and examined a three-stage rule requiring successive reductions of demand to 75, 50 and 25% of full demand when particular storage levels were reached. The study took the percentage of time that rationing could be accepted as one of the criteria for design and examined the probability of achieving this. The study also examined the variability of rationing with particular historic sequences of rainfall.

Like many engineering design tasks, the analysis can be made as complex as desired. The main factor requiring judgement is the appropriate level of sophistication required for the task in hand. In this apex, which is based on a study primarily aimed at domestic rainwater system, one simple rationing rule has been examined.

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