Plumbing Africa February 2019 | Page 37

HEALTH AND SANITATION 35 << Continued from page 33 estimate the optimum tank sizes of households across the country using existing daily rainfall time series 1989 to 1998, with the exception of SamSamWater rainwater harvesting tool which has its own rainfall data set. Four sites where selected for this analysis namely: Pretoria University, Cape Town fire service station, Stellenbosch, and Durban botanical garden. The following parameters were used for various sites across the country: roof area of 150m 3 , daily demand of 50 litres per capita per day (an average of four inhabitants per household), and a run-off coefficient of 0.8. The results of the modelling exercise provided a variety of optimum tank sizes for the range of models used. While there was no agreement between most model outputs, the raincycle and YRA models were in agreement in some areas. The SamSamWater rainwater harvesting tool underestimated the optimum tank sizes because it uses monthly rainfall data. A number of models have been developed to optimise RWH systems and quantify their water savings. There are however no guidelines to verify which model is more relevant for which reasons, given that they are not validated against observed data. Model validation is the process of demonstrating that a given site-specific model is capable of making sufficiently accurate predictions (Refsgaard, 1997). A model is said to be validated if its accuracy and predictive capability in the validation period/area have been proven to lie within the predefined acceptable limits. It is therefore a necessary step that is almost totally overlooked by the developers of the RWH models discussed above. Capturing the processes operating in RWH systems is not sufficient to ensure that the model is good, as the functional form for the processes, and how these link together to form a system, cannot be derived from first principles. Several methods are indeed used to size RWH systems and, there is no way of determining which one is the most relevant. To verify which model is more relevant, model outputs have to be compared with observations. Unfortunately, there is a tremendous lack of data, both with which to run the models, especially demand from the tank, and to validate them. Next month we look at tank material. Sources Development of resource guidelines for rainwater harvesting, a report to the Water Research Commission by Jean-Marc Mwenge Kahinda, Shirley Malema, Eunice Ubomba-Jaswa, Luther King Akebe Abia and Adesola Ilemobda. PA February 2019 Volume 24 I Number 12