30
HEALTH AND SANITATION
Rainwater harvesting
models
This is the latest in our series on rainwater harvesting. This
month we look at models for calculating various aspects
of rainwater harvesting to create the perfect system.
By Water Research Commission
A number of scholars have developed RWH models
through the years: Jenkins and Pearson (1978)
developed the yield after spillage (YAS) and yield before
spillage (YBS) release rules; Dixon (1999) developed a
daily time-step model that simulates tank size, water
quality, quantity, and cost of the RWH system; Van der
Zaag (2000) developed a spreadsheet-based model
which calculates storage capacity when daily water
use and roof area are known; Roebuck and Ashley
(2007) developed an Excel-based mass balance
transfer model that predicts future financial and
hydraulic performance of RWH systems.
Ndiritu et al. (2011) developed a daily time-step
simulation of household water supply and a frequency
analysis of the resulting number of days that the
household gets supply for every year of the analysis;
Raimondi and Becciu (2014) developed a model
which uses analytical probabilistic approaches to the
modelling of rainwater tanks.
As previously alluded, the storage tank greatly affects
the initial capital cost and the volume of the rainwater
Table 5.2: Existing RWH models, their developers, spatial and temporal
resolutions, as well as their availability
collected because it is the most expensive component of
a RWH system. Proper tank sizing is therefore important
in order to avoid extra costs incurred when the tank
is over sized and for avoiding low efficiency when it is
under sized (Ghisi, 2010).
It is a daunting task to review all existing RWH models.
Thus, while a number of models presented in the
literature are listed (Table 5.2), only those that were
accessed are discussed in more detail.
Roof
Roof is a spreadsheet-based water balance model that
calculates storage capacity when daily water use and
roof area are known; the model requires a complete
series of daily rainfall data for at least three consecutive
years. Roof uses the following water balance equation:
dV dt=Q r +Q t -Q abs -Q 0
Equation 5.5
Where: V = volume of water stored in the tank [m 3 ], Q =
roof run-off into the tank [m 3 d -1 ], QT = additional inflow
into the tank [m 3 d -1 ], Q abs = water abstracted from the
tank [m 3 d -1 ], Q o = Overflow from the tank [m 3 d -1 ], and t
= time [day].
With roof run-off equal to:
Q r = P × A r × c r
Equation 5.6
Where: P = precipitation (m d -1 ), A r = roof area (m 2 ), and
c r = roof run-off coefficient.
The model produces a storage capacity graph which
provides guidance in selecting the best tank size for a
given geographical area. The relative storage capacity
graph consists of the relative water consumption (RWC)
expressed in mm water layer per day on the horizontal
axis, and the relative development of resource guidelines
for rainwater harvesting storage capacity (RSC)
expressed in mm water layer on the vertical axis. The
model is based on the following equations:
RWC=QA
Equation 5.7
RSC=SA
Equation 5.8
Where: RWC = relative water consumption [mm d -1 ], RSC =
relative storage capacity [mm], Q = daily water consumption
February 2019 Volume 24 I Number 12
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