**1. Introduction**

Urban stormwater is considered a nuisance because it causes flooding and has an impact on aquatic ecosystem health [1–4]. This mindset overlooks the potential to transform stormwater into a safe-to-use resource. While population increase and the impact of climate change exacerbate water scarcity, authorities commonly continue to fail to see urban stormwater as the last available uncommitted water resource for our cities. For example, in 2015, the Australian Senate recommended formulating strategies to optimise stormwater management [5]. Nevertheless, the National Water Account for 2018 (Australia) disregarded treated stormwater as a source of recycled water [6]. The significance of stormwater reuse is related not only to its contribution to meeting the water demand in urban areas, but also to safeguarding conventional water resources [7,8]. Given the inevitability of climate change effects on the amount of water resources available to urban settlements, Goonetilleke et al. [9] have urged taking advantage of the opportunities offered by stormwater.

Turning this potentially valuable source of water into a safe-to-use resource requires the removal of pollutants entrained in stormwater. To remove pollutants, understanding the processes that determine their loads/concentrations, including build-up on urban surfaces during dry weather periods and wash-off during wet weather, is a fundamental need. Even though these processes have been investigated under the influence of urbanisation [10–12], pollutant behaviour is subject to variations due to the impacts of climate change [13]. Global warming (or more critically, regional warming) results in changes to typical patterns of dry and wet weather periods [14–17], increasing the complexity of changes in stormwater pollutant loads and their characteristics [17,18].

Most studies on the impact of climate change on stormwater only address the changes in runoff quantity in response to changes to rainfall patterns [19–21]. Further, the studies that assess or develop stormwater management measures such as Low-Impact Development (LID) mainly address the control of runoff volume in response to climate change [22–27]. Only a limited number of studies have highlighted the adverse impacts of climate change on stormwater quality [28,29]. However, these studies fail to draw attention to the changes in the patterns of pollutant build-up and wash-off during dry and wet weather periods. The paucity of information and guidance makes it difficult to accurately predict future changes to stormwater quality essential for planning and management decision making in the context of safeguarding stormwater quality and thereby enhancing its reuse.

Predicting stormwater quality is undertaken using mathematical models. The two primary modelling approaches currently used are: (1) physically-based modelling, which replicates temporal and spatial variations in stormwater quality using established physical theory; and (2) statistical modelling for simulating approximations of scenarios subject to a set of observed (field) data. For accurate stormwater quality predictions, the models are required to: (a) encompass robust physical relationships between the changes in stormwater quality and influential factors; and (b) be able to quantify uncertainty in model predictions [30].

Currently, neither (a) nor (b) as articulated above can be measured accurately. Regarding requirement (a) above, the current mathematical formulations of stormwater pollution processes consider dry and wet weather as a static system and do not allow for their dynamic nature resulting from changes driven by global warming [31,32]. Consequently, the accuracy of such mathematical formulations is questionable as weather altered by global warming can change the behaviour of pollutants. For example, large amounts of particulate solids, which carry toxic pollutants, can accumulate on impervious surfaces over longer dry periods, while heavy rainfall can wash-off increasingly large shock-loads of pollutants into receiving waters, exceeding their assimilation capacity. Regarding requirement (b) above, it is inevitable, given the limitations in requirement (a) noted above, that current stormwater quality models do not account for uncertainties that can arise due to the effects of global warming.

In short, stormwater treatment measures lack resilience given that global warming continues to alter dry and wet weather patterns [17]. Hence, the receiving waters remain vulnerable to degradation, and the availability of safe-to-use stormwater will be limited to fulfil the water demands of cities.

This paper critically evaluates the current practices in stormwater quality modelling to identify the changes needed to enhance stormwater quality prediction accuracy. Accordingly, current climate modelling approaches are critically reviewed to identify key aspects of dry and wet weather conditions that need to be accounted for in stormwater quality modelling. This review establishes the platform for climate impact assessment within the context of urban stormwater quality modelling.
