HEALTH AND SANITATION
[10 -3 m 3 d -1 ], A = roof area [m 2 ], S= storage
capacity [10 -3 m 3 ], A = roof area [m 2 ]
The storage capacity graph represents a
combination of the RWC and RSC for a given
satisfaction level. The satisfaction level refers
to the days where the daily water demand
will be fully satisfied and it is expressed
in a percentage; a 30% satisfaction level
means that daily water demand will be fully
satisfied for 30% of all the days. For a given
combination of daily water demand and roof
area the RWC value is known, therefore the
RSC value can be obtained from the graph by
multiplying the obtained RSC value by the roof
area, thereby obtaining the storage capacity in
the certain situation. The roof model was used
on a study by Mwenge Kahinda et al. (2010)
where the optimum tanks size for the area
studied was found to be 0.5m 3 .
The SamSamWater rainwater
harvesting tool
SamSamWater rainwater harvesting tool is a
global online RWH tool used for determining
the optimum size of a rainwater harvesting
system. The tool requires inputs of the location,
roof size, roof type, and water demand of
the household. The rainfall data set used in
the tool is based on the CRU CL 2.0 dataset
which is adopted from New et al. (2002). The
tool provides outputs of monthly rainfall for
an average year, water availability and water
demand throughout the year, water level in
the tank throughout the year. The value of the
volume of water harvested at a certain month
is represented in equation 5.
V h = A × r c × R avg
Equation 5.9
Where: V h = volume harvested by the roof
[m 3 ]; r c = run-off coefficient, and R avg is the
average rainfall [m]
These outputs are then used to conclude the
size of the tank that would be suitable for the
specific location.
Yield reliability analysis model
The yield reliability analysis (YRA) is a daily
continuous simulation model developed by
Ndiritu et al. (2011a), the model is based on
an approach towards volumetric reliability by
Su et al. (2009) who applied daily continuous
simulation modelling to obtain relationships
between storage size, deficit, and its
exceedance probability, which allows for the
selection of a confidence level associated with
a specific yield and tank size. Ndiritu et al.
(2011a) used the above-mentioned approach
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by Su et al. (2009) determining exceedance
probabilities using the frequency analysis of the
number of days of supply each year using a
plotting position formula. The model calculates
the number of days in a year that a household
water demand is met by a RWH system, ROR
(run-of-river), and the combination of the two.
An optimal tank volume is described as the
minimum tank size giving the highest number of
days of supply for the specific roof area.
The yield reliability analysis model has been
used in three case studies to date:
• The yield-reliability analysis of rural
domestic water supply from combined
rainwater harvesting and run-off river
abstraction in Nzhelele village, Limpopo
province (Ndiritu et al., 2011a)
• Incorporating hydrological reliability in
rural rainwater harvesting and run-of-
river supply (Ndiritu et al., 2011b)
• Probabilistic assessment of the rainwater
harvesting potential of schools in South
Africa (Ndiritu et al., 2014).
Raincycle
Raincycle is a Microsoft Excel-based mass
balance transfer model that predicts future
financial and hydraulic performance of RWH
systems (Roebuck and Ashley, 2007). The model
is applicable for RWH systems in domestic,
commercial, public or industrial buildings, and
makes use of the YAS spillage algorithm as
described by Jenkins and Pearson (1978).
The model accounts for the change in water
demand throughout working days, weekends
and holidays. The models can provide daily
simulations of the proposed design for up to
100 years of operation. The main result of
the model is expressed as the percentage of
demand fulfilled by harvested water. The model
was validated by comparing the hydraulic
outputs of the model with the methodology
described by Fewkes and Warm (2001).
EVALUATION CRITERIA OF RWH MODELS
To evaluate RWH models, a set of evaluation
criteria has been developed, after Cunderlik
(2003) who developed evaluation criteria
for hydrologic models. The selection of an
existing model to be used in this project
depends therefore on a range of criteria
rather than the personal preferences of
the project team. A number of criteria are
informative while others are ranked and
included in the evaluation process.
Criteria are ranked from either 1 to 3 or 1 to
2, with: rank 1-Bad, 2-Average and 3-Good.
Continued on page 33 >>
www.plumbingafrica.co.za
February 2019 Volume 24 I Number 12