International Conference on Rain Water Cistern Systems
USA - June 1982
5: Current And Future Practices
Effect of Rationing
on Reliability of Domestic Rainwater Systems
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
- 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
- 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:
- The rationing policy to apply (duration of rationing and reduction of demand)
- The effect of a particular policy on the reliability of the supply
- 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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