HEALTH AND SANITATION
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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