Modelling the performance of rainwater collection systems: towards a generalised approach

The collection of rainwater from roofs, its storage and subsequent use make a significant saving in the use of potable water. This paper investigates how spatial and temporal fluctuations in rainfall can be incorporated into behavioural models, which simulate the performance of rainwater collectors. Temporal variations are considered at two different time scales, that is daily and monthly intervals. The first model uses a daily time interval to model system performance. A set of rainwater collector performance curves for five geographic locations is developed. From the location-specific curves, a set of average curves is determined which is shown to be sufficiently accurate for estimating rainwater collector performance. The second model uses a larger time interval of one month. The effect of daily fluctuations in rainfall is incorporated into this model using a storage operating parameter. Values of the parameter were determined by matching the output from the daily model with that of the monthly model. Generally the rainwater collector performance predicted by the monthly model using average values of the storage operating parameter is shown to correlate well with the corresponding values determined using a daily time interval model. The monthly model provides a simple and versatile method of modelling the performance of rainwater collectors.