Rapid urbanization since the mid-20th century has markedly increased the amount of impervious cover, resulting in gross changes to the natural water balance by decreasing infiltration and evapotranspiration [1
]. Such changes greatly increase the volume and frequency of surface runoff and concurrently decrease groundwater recharge [1
The increase in the volume and frequency of surface runoff from impervious surfaces greatly increases flooding risk, which has led to the construction of hydraulically efficient drainage infrastructure [4
]. However, constructed drainage networks, which directly drain the impervious surfaces to receiving waters, while further limiting the natural infiltration process, are widely recognised as severely altering both low and high flow aspects of the stream flow regime [5
], leading to urban stream degradation and biodiversity loss [7
Stormwater control measures (SCMs) have been used for many years to manage urban runoff. Perhaps the earliest of these were simple on-stream retarding basins to reduce peak flows, but a wide range of SCMs has since been developed, targeting event runoff volume, low flow behaviour and runoff quality in addition to peak flows. Processes that modify flow magnitude include storage (ponds, wetlands), infiltration (swales, infiltration basins, and biofilters) and water extraction for reuse (parkland irrigation and rainwater tanks). The scale of SCMs ranges from individual allotments (site scale) to substantial urban areas with established watercourses (catchment scale).
This study explores the behaviour of site scale rainwater tank systems. The water yield of such systems for domestic supply has been extensively assessed [14
], and their stormwater retention behaviour has also been investigated [23
]. However, their potential to restore baseflows depleted by urbanization seems not to have been explored in detail.
The aim of simultaneously mitigating peak flows and restoring lost baseflows is a response to the “natural flow paradigm” proposed in 1997 by Poff et al. [14
]. This paradigm suggests that aquatic ecosystems require a flow regime as close as possible to its natural level to remain in a healthy state. They propose aspects of the flow regime that should be considered, including the magnitude, frequency, timing, duration and flashiness. This paradigm implies that simply reducing peak flows from urban runoff through detention or retention systems will not be sufficient to protect or restore ecological function [11
]. There is also a need to ensure that the magnitude, duration and timing of low flows are maintained close to their natural levels.
Rainwater Harvesting Systems (RWH) collecting roof runoff for household use are a traditional form of water supply in rural areas, and more recently are commonly applied to supplement urban water supply due to growing water demand in many urban environments [10
]. They are also a type of stormwater control measure (SCM), designed to address flooding risk by capturing and storing stormwater runoff and supplying harvested rainwater to the household, essentially diverting rainwater from direct runoff to consumption [19
]. Such RWH systems are typically designed to have an inflow pipe to collect runoff from connected impervious surfaces and an outflow pipe to draw harvested rainwater for household consumption. There is an overflow pipe located at the top of the system that allows uncontrolled overflow to leave the system during spillage [20
The ability of conventional RWH systems to simultaneously provide the dual benefits of water supply augmentation and stormwater retention has been widely recognized and assessed through both modelling and empirical studies [24
]. Increasingly, RWH systems are being designed with a focus on low-impact stormwater management.
Low-impact stormwater management not only requires the alleviation and mitigation of flooding, the same as conventional urban stormwater management, but also has the more-recently recognised aim of restoring the pre-development flow regime and urban water cycle [9
]. Restoring pre-development flow regime at the catchment scale requires actions to be implemented mainly at the allotment scale, where the pre-development water fluxes (such as evapotranspiration and infiltration) have the greatest opportunity to be restored [31
]. Moreover, delivery of natural flow regimes is not just about mitigating high flows; it is about restoring the whole perturbed flow regime to a more natural state, including restoration of lost baseflows. Releasing stormwater to the stream in a temporal pattern consistent with the pre-development state, which counteracts the loss of baseflows due to loss of infiltration under impervious surfaces, can help to achieve such restoration.
Therefore, the application of allotment-scale RWH systems has great potential to simultaneously reduce or eliminate excess runoff volume and provide water conservation benefits, while also mimicking natural baseflow regimes by means of carefully controlled discharge [10
]. There are two innovative RWH systems that can deliver these multi-objectives: the passive release system
and the active release system
. Both can return some of the lost baseflow to streams, by providing a controlled slow-release either back to the landscape or directly to the stormwater system (and subsequently the receiving water).
The passive release system
divides the RWH system into two segments, the stormwater detention volume and the retention storage volume, by adding a passive discharge orifice at an intermediate depth [36
]. The retention storage volume is designed to supply the domestic consumption, and comprises the bottom portion of the storage, while the detention storage occupies the top portion of the system. Stormwater runoff held above the passive discharge orifice slowly drains out to contribute to in-stream baseflow [36
In contrast to the passive release system, the active release system
places an automated outlet at the bottom of the system which is operated by a novel approach—Real-Time Control (RTC)
—which can control the RWH systems remotely via a wireless connection [37
]. RTC technology can optimize RWH system performance, by the managed release of water from the system to reduce the volume of uncontrolled stormwater runoff, and/or to supply water for restoring baseflows in streams. This technology has been widely used in wastewater systems to monitor and control water quality [40
] and address combined sewer overflow (CSO) and sanitary sewer overflow (SSO) issues [42
]. However, the potential to incorporate RTC into RWH systems remains largely untested.
The active release system can utilize RTC technology to receive rainfall forecast data in real time and automatically trigger a pre-storm release through a customized valve according to the forecast precipitation and water level within the RWH system. Water in the system is only released if there is insufficient storage capacity to capture the forecast amount of precipitation. Consequently, this customization would preserve the water conservation function, and would significantly reduce or even eliminate the uncontrolled stormwater runoff that discharges into the storm drainage system creating a risk of flooding [38
]. While the pre-storm release aims to reduce the risk of flooding, the baseflow release has the objective of restoring the infiltration lost at the source due to the impervious area at each allotment. It requires a constant and controlled release to satisfy the volume and frequency of instream baseflow. In doing so, these systems are likely much more effective than simple RWH systems in delivering a more natural flow regime.
While applied at the individual house scale, the real-time-controlled active release rainwater harvesting systems would likely be implemented by a water authority or private water company, who would install, operate and maintain the systems as part of their overall water service provision. Such technology could also be used to meet regulatory requirements for on-site detention (OSD). Such site-based OSD requirements are common in Australia [44
] and many parts of the world (e.g., [45
]). Combining this functionality with rainwater harvesting and restoration of stream flow regimes provides an attractive integrated water cycle solution.
A few places around the world, such as North Carolina, United States [38
] and Seoul, South Korea [21
], have deployed and monitored RWH systems operated by RTC technology to deliver and optimize the dual benefits of water conservation and stormwater retention. However, no studies to date have explored the potential to adapt such RTC systems to facilitate baseflow restoration. Such potential needs to be explored in the context of its associated impact on water supply and stormwater retention.
Here, we compared the modelled performance of RWH systems equipped with RTC technology versus both conventional systems and also systems designed to passively release water. System performance was characterized using metrics related to water supply, stormwater retention, and baseflow restoration.
Baseflow release provided by active and passive release systems is essentially a form of yield (use of water specifically to provide environmental flows) which is theoretically in conflict with the water supply objective. However, the specific impact of baseflow release on other objectives, along with the effect of RTC technology on system performance, remain unknown. Therefore, our paper addresses the following questions:
How does the addition of baseflow release affect the water supply and stormwater retention performance of RWH systems?
How does the addition of RTC operation affect the water supply and stormwater retention performance of RWH systems?
How do active and passive release systems compare in achieving multi-objectives?
Our work shows that RWH systems employed with RTC technology are generally superior in achieving the triple objectives compared with passive release systems, although the differences are relatively modest. Active release systems exhibit great promise in retaining stormwater runoff with only a small detriment to water supply, compared with conventional systems. Our work highlights the substantial potential of equipping RWH systems with RTC technology in a range of contexts.