are: using water-wise plants, mulching, efficient irrigation systems,
irrigation scheduling, rain water harvesting and recycling waste
water [21].
Research on irrigation scheduling in urban lawns has mainly
concentrated on water conservation while rooftop water harvesting
(RWH) of rain water has concentrated on making the water safe
and reliable to the rural areas of South Africa [24,25]. However,
another benefit of RWH is improving water quality through runoff
reduction [26]. Evapotranspiration based controllers are superior
to time-based controllers in irrigation scheduling [27]. However,
in case of precipitation, they are programmed by the manufacturers
to pause for a certain period of days before resuming irrigation
irrespective of whether the precipitation was sufficient or not [28].
In addition, the scheduling considered so far makes use of water
solely from the utility [29]. Bocanegra-Martínez et al. [30] designed
an optimal rainwater collecting system in residential areas and
showed its viability to use to meet certain residential water
demands. However, the model was not optimized to specific end
uses.
This paper reports the first attempt to design a practical and
economically attractive optimal irrigation scheduling model using
the harvested rain water and the TOU electricity price tariff. The
harvested water from the rooftop is stored in a reservoir which
only gets the potable utility water whenever the stored water
has been depleted through the irrigation scheduling. There has
been several studies seeking to address the reliability of RWH
systems meant for supplying water to various demands like lawn
irrigation and toilet flushing using solely the harvested water
[31,32]. The municipal water sources are used as a back up to
improve the reliability of the system and ensure that the tank
never runs dry. Further, optimal control strategy has the ability
to predict the dynamics of water in the tank effectively minimizing
the chance of spillage taking place. This paper presents an
optimal scheduling breakthrough that can reduce both water
and energy consumption leading towards achieving more sustainable
buildings. Furthermore, the optimal model with RWH is
useful in developing the nations’ cities where utility potable
water is unreliable due to the high demand that surpasses the
existing supply infrastructure. This optimal model, if widely
adopted, would reduce the demand for potable water and energy
from the utilities, lower waste water drainage and purification
cost and at the same time lowering the bills associated with both
resources.