Is stormwater harvesting beneficial to urban waterway environmental flows?

The SDGs, Ecosystem Services and Cities: A Network Analysis of Current Research Innovation for Implementing Urban Sustainability

Ecosystem services are essential for cities and are key factors in achieving many of the Sustainable Development Goals (SDGs). Such services are best delivered through green infrastructure, which works in resourceful, multifunctional, synergistic, and environmentally sensitive ways to deliver ecosystem services and provide alternative cleaner pathways for the delivery of multiple urban services. It is unclear if current research supports the necessary linkages between ecosystem services, cities, and green infrastructure in order to achieve the SDGs. To answer this question, we conducted a systematic review analysing 3392 studies on the SDGs from the WoS database. The contents of 66 of those with relevance to ecosystem services and urban research were reviewed in depth. We applied network-analytic methods to map the relationships of different knowledge clusters of SDGs research (1) across time, (2) across disciplines, and (3) in relation to ecosystem services and cities. The results of our analysis show that research on the SDGs have developed stronger networks from 2010–2018, but this research has not been sustained. Further, whilst research on cities now occupies a central place in the SDGs literature, research on ecosystem services only shows tentative links to both green-infrastructure research and SDGs research. Such literature on urban green infrastructure remains peripheral to the central challenge of sustainable urban transitions. We conclude that when it comes to the SDGs, research articles typically consider urban services independently of green infrastructure. Further, it suggests that green infrastructure is not generally considered as a sustainable alternative to conventional urban infrastructures. To address this serious shortcoming, we recommend transdisciplinary approaches to link urban ecosystem and urban green infrastructure research to the 2030 global sustainability agenda.

A spatial planning-support system for generating decentralised urban stormwater management schemes

Current Water Sensitive Urban Design (WSUD) models are either purely technical or overly simplified, lacking consideration of urban planning and stakeholder preferences to adequately support stakeholders. We developed the Urban Biophysical Environments and Technologies Simulator (UrbanBEATS), which integrates stormwater management with urban planning to support the design and implementation of WSUD. This study specifically describes and tests UrbanBEATS' WSUD Planning Module, which combines spatial analysis, infrastructure design, preference elicitation and Monte Carlo methods to generate feasible stormwater management and harvesting infrastructure options in greenfield and existing urban environments. By applying UrbanBEATS to a real-world greenfield development case study in Melbourne, Australia (with data sourced from the project's water management plans and design consultants), we explore the variety of options generated by the model and analyse them collectively to demonstrate that UrbanBEATS can design similar WSUD systems (e.g. select suitable technology types, their sizes and locations) to actual infrastructure choices.

Quantifying the benefits of stormwater harvesting for pollution mitigation

Stormwater harvesting (SWH) provides multiple benefits to urban water management. Other than providing water for human use, it also reduces the volume of polluted stormwater discharge to the environment. There are currently no methods available to quantify the additional environmental benefits, which could encourage greater uptake of the practice. This paper investigates a number of factors (climate and catchment characteristics, pollutant reduction targets, etc.) that could impact upon the benefits of SWH for pollution reduction through sensitivity analyses. A method was developed and tested for quantification of the pollution mitigation benefits by SWH under different scenarios. A novel indicator, Impervious Area Offset (IAO), was proposed to reflect the additional impervious area that can be left untreated to achieve the equivalent pollution load reduction targets due to the introduction of SWH. Results indicate significant correlations (p < 0.01) between IAO values and extraction rate (proportion of total annual runoff removed due to the harvesting system and water use substitution), system type, and pollutant reduction targets. The proposed linear empirical relationships between IAO values and extraction rate for different types of system configurations and pollution reduction targets were well represented by observed linear regression (average R² = 0.98 for all tested scenarios). Empirical relationships were validated successfully against different scenarios, with differences between predicted IAO and baseline IAO values being only ±4.5% for the majority of the validation scenarios. Using this simple and reliable method to rapidly quantify SWH benefits can further add to the growing business case of adopting SWH practices.

Quantifying relationships between urban land use and flow frequency of small Missouri streams

Flow frequency is an important hydrologic statistic to consider in environmental flows assessment. However, there is a paucity of focused interdisciplinary hydrologic assessments that quantify human development influence on flow frequency of small streams (drainage area < 282 km²). Relationships between urban land use and land cover (LULC) and flow frequency were assessed for general trends at current gauged watersheds (n = 32) of Missouri, USA. Urban land use - flow frequency relationships changed from linear in developed areas with <50% total impervious surfaces (i.e. low density urban areas), to non-linear in developed areas with >50% total impervious surfaces (i.e. high density urban areas). Urban land use influence on flow frequency was not detected in events below median flow (0.02510 < R² > 0.03356; n = 32). Conversely, urban land use - flow frequency relationships were relatively strong above median flow (0.55500 < R² > 0.78703; n = 32). Further, explained variance generally increased to meso-scale flows (0.58350 < R² > 0.82470; n = 32), and then, decreased during high flows (0.34912 < R² > 0.61805; n = 32). More specifically, the greatest observed influence of urban land use on flow frequency increased from a 0.2 to 1 year return period in low density urban areas, to a 1 to 2 year return period in high density urban areas in small Missouri streams. Thus, results indicate that management efforts should focus on reducing the frequency of 1 year events in low density urban land use areas, and 2 year events in high density urban areas to secure environmental services of small urban streams in Missouri, USA. These results hold important implications for other regions globally, where urban land use has increased the frequency of streamflow response.

Quantifying relationships between watershed characteristics and hydroecological indices of Missouri streams

There is an ongoing need for multidisciplinary investigations that will lead to policy changes that target and reduce natural and anthropic alterations to hydroecological indices important for regional environmental flows management. The hydroecological indices assessed in this study were all deemed ecologically relevant due to causal linkages with hydrogeomorphology, physical habitat, water quality, and/or ecological processes. Watershed characteristics (i.e. topography, land use and land cover (LULC), soils, and geomorphic variables) and hydroecological data were assessed for general trends between ecoregions at gauged watersheds (n = 115) in Missouri, USA. Univariate ordinary least squares (OLS) and multivariate least absolute shrinkage and selection operator (LASSO) regression models were fit to selected hydroecological indices, and models were validated using a split-site approach. Key results included: 1) significant differences (p ≤ 0.05) were observed between hydroecological indices of different ecoregions, particularly low flows statistics; 2) urban land use was associated with moderate (0.25 < R² adj. > 0.75) to strong (R² adj. ≥ 0.75) influence on more hydroecological indices (31 of 171 indices) compared to other LULC indices and watershed characteristics assessed, especially urban land use - high flow frequency relationships (5 of 11 indices; 0.77 ≤ R² ≥ 0.85); and 3) univariate ordinary least squares (OLS) regression models performed better overall relative to least absolute shrinkage and selection operator (LASSO) regression models at validation sites. Given the ecological relevance of the hydroecological indices assessed in this study, results indicated management efforts should focus on mitigating urban land use influence on environmental flows in Missouri, USA. These results hold important implications for other regions globally, where urban land use has altered environmental flows.

Improving the Multi-Objective Performance of Rainwater Harvesting Systems Using Real-Time Control Technology

Many studies have identified the potential of rainwater harvesting (RWH) systems to simultaneously augment potable water supply and reduce delivery of uncontrolled stormwater flows to downstream drainage networks. Potentially, such systems could also play a role in the controlled delivery of water to urban streams in ways which mimic baseflows. The performance of RWH systems to achieve these three objectives could be enhanced using Real-Time Control (RTC) technology to receive rainfall forecasts and initiate pre-storm release in real time, although few studies have explored such potential. We used continuous simulation to model the ability of a range of allotment-scale RWH systems to simultaneously deliver: (i) water supply; (ii) stormwater retention; and (iii) baseflow restoration. We compared the performance of RWH systems with RTC technology to conventional RWH systems and also systems designed with a passive baseflow release, rather than the active (RTC) configuration. We found that RWH systems employing RTC technology were generally superior in simultaneously achieving water supply, stormwater retention and baseflow restoration benefits compared with the other types of system tested. The active operation provided by RTC allows the system to perform optimally across a wider range of climatic conditions, but needs to be carefully designed. We conclude that the active release mechanism employing RTC technology exhibits great promise; its ability to provide centralised control and failure detection also opens the possibility of delivering a more reliable rainwater harvesting system, which can be readily adapted to varying climate over both the short and long term.

Improving the Viability of Stormwater Harvesting through Rudimentary Real Time Control

Stormwater Harvesting (SWH) to alleviate water scarcity is often hindered by the lack of suitable available storage in urban areas. This research aimed to discover an economically viable strategy of storing runoff in existing stormwater ponds with the assistance of rudimentary Real Time Control (RTC) techniques to increase the effective storage capacity. The Diep River sub-catchment situated in the southern suburbs of Cape Town, South Africa, that has several stormwater ponds that were largely constructed for the purposes of flood mitigation, was used as a case study. Six SWH scenarios utilising three distinct RTC strategies coupled with two alternative water demand alternatives were simulated with the aid of 10 years’ of historical rainfall data with a view to determining the unit cost of supplying selected developments with non-potable water. The use of RTC to increase the effective storage of the ponds was shown to improve the volumetric yield without significantly impairing the flood mitigation provided by the system at a cost that was comparable to what the local residents were already paying for potable water. This finding is important as it suggests a cost-effective way of overcoming one of the greatest limitations associated with stormwater harvesting.

A metabolism perspective on alternative urban water servicing options using water mass balance

Urban areas will need to pursue new water servicing options to ensure local supply security. Decisions about how best to employ them are not straightforward due to multiple considerations and the potential for problem shifting among them. We hypothesise that urban water metabolism evaluation based a water mass balance can help address this, and explore the utility of this perspective and the new insights it provides about water servicing options. Using a water mass balance evaluation framework, which considers direct urban water flows (both 'natural' hydrological and 'anthropogenic' flows), as well as water-related energy, we evaluated how the use of alternative water sources (stormwater/rainwater harvesting, wastewater/greywater recycling) at different scales influences the 'local water metabolism' of a case study urban development. New indicators were devised to represent the water-related 'resource efficiency' and 'hydrological performance' of the urban area. The new insights gained were the extent to which alternative water supplies influence the water efficiency and hydrological performance of the urban area, and the potential energy trade-offs. The novel contribution is the development of new indicators of urban water resource performance that bring together considerations of both the 'anthropogenic' and 'natural' water cycles, and the interactions between them. These are used for the first time to test alternative water servicing scenarios, and to provide a new perspective to complement broader sustainability assessments of urban water.

Validation of stormwater biofilters using in-situ columns

Stormwater harvesting biofilters need to be validated if the treatment is to be relied upon. Currently, full-scale challenge tests (FCTs), performed in the field, are required for their validation. This is impractical for stormwater biofilters because of their size and flow capacity. Hence, for these natural treatment systems, new tools are required as alternatives to FCT. This study describes a novel in-situ method that consists of a thin stainless steel column which can be inserted into constructed biofilters in a non-destructive manner. The in-situ columns (ISCs) were tested using a controlled field-scale biofilter where FCT is possible. Fluorescein was initially used for testing through a series of continuous applications. The results from the ISC were compared to FCT conducted under similar operational conditions. Excellent agreement was obtained for the series of continuous fluorescein experiments, demonstrating that the ISC was able to reproduce FCT results even after extended drying periods (Nash-Sutcliffe coefficient between the two data sets was 0.83-0.88), with similar plateaus, flush peaks, slopes and treatment capacities. The ISCs were then tested for three herbicides: atrazine, simazine and prometryn. While the ISC herbicide data and the FCT data typically matched well, some differences observed were linked to the different climatic conditions during the ISC (winter) and FCT tests (summer). The work showed that ISC is a promising tool to study the field performance of biofilters and could be a potential alternative to full scale challenge tests for validation of stormwater biofilters when taking into account the same inherent boundary conditions.

Towards sustainable urban water management: a critical reassessment

Within the literature, concerns have been raised that centralised urban water systems are maladapted to challenges associated with climate change, population growth and other socio-economic and environmental strains. This paper provides a critical assessment of the discourse that surrounds emerging approaches to urban water management and infrastructure provision. As such, 'sustainable urban water management' (SUWM) concepts are scrutinized to highlight the limitations and strengths in the current lines of argument and point towards unaddressed complexities in the transformational agendas advocated by SUWM proponents. Taking an explicit infrastructure view, it is shown that the specific context of the urban water sector means that changes to infrastructure systems occur as an incremental hybridisation process. This process is driven by a range of factors including lock-in effects of legacy solutions, normative values and vested interests of agents, cost and performance certainty and perceptions of risk. Different views of these factors help explain why transformational agendas have not achieved the change SUWM proponents call for and point to the need for a critical reassessment of the system effects and economics of alternative service provision models.

Reuse of urban runoff in Australia: a review of recent advances and remaining challenges

The degradation of aquatic ecosystems due to hydrologic and water quality impacts of urbanization, combined with increasing water scarcity, has generated increasing interest in the harvesting of urban storm water. This paper reviews the rationale for integrated storm water treatment and harvesting and synthesizes recent advances and trends and knowledge gaps that limit its application. Storm water harvesting is shown to be a viable alternative water supply and to provide a potential solution to the increases in runoff frequency and peak flows that occur as a result of catchment urbanization. In general, treatment technologies for storm water harvesting have been adapted from existing "water-sensitive urban design" approaches, with limited use of traditional water supply and wastewater technologies. Risk management is often lacking, in part due to a lack of relevant guidance. Reported performance shows variable levels of potable water savings, with cases of up to 100% substitution recorded. Costs of storm water harvesting systems are shown to be inversely related to their scale. The limited cost data show the importance of context, with the harvested water costing more or less than alternative supplies, depending on the cost of the alternative. Limited data exist on environmental benefits, such as reductions in pollutant loads and flow peaks. Implementation of storm water harvesting systems is impeded by inadequate data on risk, lifecycle costs, externalities, and water-energy tradeoffs. Furthermore, retrofit of storm water harvesting into existing urban areas is proving to be a challenge, creating an urgent need for specific technologies for use in retrofit situations.

Understanding, managing, and minimizing urban impacts on surface water nitrogen loading

The concentration of materials and energy within cities is an inevitable consequence of dense populations and their per capita requirements for food, fiber, and fuel. As the world population becomes increasingly urban over the coming decades, urban areas will dramatically affect the distribution of nutrients across the face of the planet. In many cities, technological developments and urban planning have been effective at reducing the amount of waste nitrogen that is ultimately exported to downstream surface waters, largely through investments in sanitary sewer infrastructure and wastewater treatment. There are, however, still large cities throughout the developed world that have failed to take advantage of these obvious innovations to reduce their impact on downstream ecosystems. In addition, very few cities have adequately addressed the problems of diffuse nitrogen pollution, instead city infrastructure is often designed to route this N directly into downstream ecosystems. In the developing world, many of these problems are more acute, as rapidly growing urban populations exceed the capacity of limited municipal infrastructure. Reducing urban N pollution of groundwaters and surface waters both locally and globally can only be achieved through cultural and political adaptation in addition to technological innovations. In this review, we will focus on the implications of an increasingly urban world population on local, regional, and global nitrogen cycles and propose a variety of approaches for minimizing and mitigating the impacts of urban N concentration.