HEALTH AND SANITATION
30
A YAS model using either
hourly or daily input time series
could be used to predict
system performance.
Rainwater
harvesting
storage tanks
Several methods are used to size the tanks of rainwater
harvesting (RWH) systems. The main methods include
simplified methods, critical period, probability matrix
methods, statistical methods, and procedures based on
stochastic data generation.
By Water Research Commission
SIMPLIFIED METHODS
The simplified method, used for preliminary design, is
based on user defined relationships (Ward et al., 2010).
Though they are used with relative ease, their results are
not reliable due to poor modelling of rainfall and/or water
storing processes (Raimondi and Becciu, 2014). The demand side approach assumes that the storage
requirement is equal to the largest demand to be
supplied by the tank. The method does not take into
account water demand and uses only the water
availability for the design of the storage tank (Ward et al.,
2010). The main limitation is the assumption that there
will be enough rains to fill the tank before the dry period
commences.
The supply side approach assumes that the required
tank is large enough to store the maximum amount of
rainwater in the wet season. Limitations of the method
are that it ignores the water demand, which lead to
inaccuracies when sizing the system. The method does
not account for the seasonal variation in rainfall. The tank
design is only based on the water needs and the period
of water shortage. CRITICAL PERIOD METHOD
The critical period method is based on the continuity
equation wherein the required storage is equal to the
maximum difference between the outflow and inflow of the
reservoir during a critical period (McMahon and Adeyole,
2005). The term critical period refers to the period from
a full reservoir condition to emptiness (McMahon and
Adeyole, 2005). It identifies and uses sequence of flows
Methods used to size RWH systems:
January 2019 Volume 25 I Number 1
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