Simulating the performance of rainwater collection and reuse systems using behavioural models

The collection of rainwater from roofs and its storage and subsequent use make a significant saving in the use of potable water. The capacity of the rainwater store is important because it affects both system and initial costs. The investigation concentrates upon the use of behavioural models to simulate the performance of rainwater collectors. The input data, in time series form, is used to simulate the mass flows through the model, and will be based upon time intervals of a minute, an hour, a day or a month. A preliminary mapping exercise is described that evaluates the accuracy of behavioural models for the sizing of rainwater collection systems using different time intervals and different reservoir operating rules applied to a range of reservoir capacities and collection areas. The preliminary analysis indicated that the yield after spillage (YAS) reservoir operating algorithm based on a hourly time interval could be used as a standard against which other models could be compared and calibrated. The detailed analysis enabled constraints to be proposed for the application of hourly, daily and monthly models expressed in terms of storage fraction, S/AR, where S = storage capacity (m 3 ), A = roof area (m 2 ) and R = annual rainfall (m). Hourly models can be used for sizing small stores with a storage fraction below or equal to 0.01. Daily models can be applied to systems with storage fractions within the range 0.01 to 0.125. Monthly models are only recommended for use with storage fractions in excess of 0.125.