To improve its resilience to increasing climatic uncertainty, the City of Cape Town (the City) aims to become a water sensitive city by 2040. To undertake this challenge, a means to measure progress is needed that quantifies the urban water systems at a scale that enables a whole-of-system approach to water management. Using an urban water metabolism framework, we (1) provide a first city-scale quantification of the urban water cycle integrating its natural and anthropogenic flows, and (2) assess alternative water sources (indicated in the New Water Programme) and whether they support the City towards becoming water sensitive. We employ a spatially explicit method with particular consideration to apply this analysis to other African or Global South cities. At the time of study, centralised potable water demand by the City amounted to 325 gigalitres per annum, 99% of which was supplied externally from surface storage, and the remaining ~1% internally from groundwater storage (Atlantis aquifer). Within the City’s boundary, runoff, wastewater effluent and groundwater represent significant internal resources which could, in theory, improve supply efficiency and internalisation as well as hydrological performance. For the practical use of alternative resources throughout the urban landscape, spatially explicit insight is required regarding the seasonality of runoff, local groundwater storage capacity and the quality of water as it is conveyed through the complex urban landscape. We suggest further research to develop metrics of urban water resilience and equity, both of which are important in a Global South context.
 Significance:
 
 This research provides the initial groundwork of quantifying the magnitude of the urban water cycle of the City of Cape Town at an annual timescale, in relation to becoming a water sensitive city. The urban water metabolism framework used in this study provides important insight to assess whole-of-system urban water dynamics and to benchmark progress towards becoming water sensitive. By quantifying the magnitude of flows into and out of the urban system, this research sheds light on the opportunities to improve circularity in the urban water cycle. The spatial approach adopted here provides a platform to interrogate the urban landscape and its role in the urban water cycle. By using data products that are available via national data sets or remote sensing, this approach can be applied to other African or Global South where data is characteristically scarce. Further work is required to establish metrics that can adequately describe urban water resilience and equity.

The Cape Flats Aquifer (CFA) is a primary coastal aquifer that underlies the City of Cape Town (South Africa), identified as a major source of water by hydrogeologists since the 1970s. Scientific specialists called for its protection so that it could be used for bulk water supply to the City. Unfortunately, the resource was largely ignored by the municipality, resulting in the degradation of the underlying aquifer over time to the point where groundwater from some portions is non-usable due to contamination. In 2015–2017, Cape Town experienced its worst drought since 1904, driving the municipality to look at alternative sources of water such as desalination, re-use of treated effluent and various groundwater schemes of which the CFA was one. Given the time-constraints of an emergency response project, long-term testing and study of the system to support design and implementation have been significantly reduced. A no regrets approach was therefore adopted by the team to reduce the chance of any negative impact such as saline intrusion, reduction in water levels that would negatively affect existing users that rely on groundwater for their livelihood and sensitive ecosystems. We aim to present the geoethical question of developing an emergency scheme with limited data on its sustainability and costing versus not developing it and risking a City of ~4 million people running out of water.

The Atlantis Water Resource Management Scheme (AWRMS) in the Municipality of Cape Town (South Africa) is recognized as one of the first large-scale-managed aquifer recharge schemes in the world. It was implemented during 1970s and ran (apparently) successfully for almost 30 years. However, in the early 2000s, it was essentially abandoned and replaced with surface water supply. Most of the infrastructure was allowed to deteriorate and monitoring of the aquifer system was carried on without proper scientific purpose. Due to the recent drought crisis, in which Cape Town almost ran out of water to supply over 3 million people, the need for groundwater as an additional source of supply was once again recognized. As one of several projects to increase the City’s resilience to drought, the AWRMS was targeted for refurbishment and re-design. Although there have been many technical challenges, perhaps the most complex to solve has been coordinating the various disjointed, and often opposed, institutions and entities that affect the functioning of the scheme. The fundamental geoethical principles of scientific practice, communication, and the education of all those who affect and are affected by the scheme are paramount to ensure the long-term sustainability of the system.

The City of Cape Town initiated its “New Water Programme” in 2017 to diversify its bulk water supply, thereby improving long-term water security and resilience against future droughts—this includes bulk groundwater abstraction of potentially ~140–400 Ml/day from the major fractured Peninsula and Nardouw Aquifers of the Table Mountain Group (TMG; in the mountain catchments to the east of the city). Current TMG groundwater exploration and wellfield development are taking place in the vicinity of Steenbras Dam, in the form of the ~15–20 Ml/day Steenbras Wellfield scheme. The TMG aquifers are also essential in sustaining groundwater-dependent ecosystems associated with the Cape Floral Kingdom—a global biodiversity hotspot with exceptional endemic diversity, but also a global extinction hotspot. A strong geoethical, “no-regrets” approach is, therefore, required to develop TMG wellfield schemes for the City of Cape Town (and other towns/cities in the Western/Eastern Cape), in order to reduce the risk of any negative ecological and environmental impacts, while still enhancing the drought resilience of the city, providing water for future urban growth, and meeting Sustainable Development Goals 6 and 11.

The Cape Flats Aquifer (CFA) is a primary coastal aquifer under the low-lying eastern suburbs and townships in the City of Cape Town (South Africa). It was identified as a major groundwater source since the 1970s but only intermittently explored and monitored until 2018, when Cape Town initiated an emergency programme in response to a prolonged drought. The objectives in this programme—still in progress—include detailed mapping of aquifer extent and properties, establishment and maintenance of a monitoring programme, and design and implementation of a Managed Aquifer Recharge abstraction scheme. Sustaining the potential of the CFA requires a holistic approach, considerate of the social, cultural and economic context. Support from the local communities is essential if this natural resource is to be harnessed for the benefit of all, as a supply and a means to store water for reuse from wastewater treatment works situated above the aquifer. CFA development can potentially help heal many of the historic societal wounds arising from the apartheid system of segregation and discrimination. The practices of transformative art may provide a practical and useful way to consider and structure the necessary “edges of engagement” in the socio-hydrogeological and environmental context.

In early 2018, Cape Town faced “Day Zero,” the date when it was expected to run out of water, and when all municipal supply would be rerouted to emergency collection points. A three-year drought, considered ~1-in-400-year hydrological event, had brought the level of its largest storage reservoir to around 11% of full capacity. Day Zero was averted by relatively good rainfall early in 2018. Two years on, water-security remains precarious and uncertain in the face of rapid urban expansion, slow environmental degradation, and long-term climate change. The wider catchment region contains two important subsurface resources: the Palaeozoic Table Mountain Group (TMG) Aquifer System, and the Cenozoic Cape Flats Aquifer (CFA). The development of these groundwater options is confronted by challenges related to environmental and societal impacts. In the case of the TMG resource, which underlies mountain biosphere reserves of the extraordinary Cape Floral Kingdom, geoethical concerns arise in the context of uncertainties related to anticipated impacts on stream flow and floral biodiversity. In the CFA case, geoethical issues revolve around uncertainties related to impacts on the coastal environment, the groundwater–seawater interface, and the power-generation costs of groundwater management and treatment under constraints imposed by South Africa’s fossil-fuel dependence.

Management of groundwater resources is increasingly reliant on numerical simulation. Unfortunately, decision-support modelling is often conducted under the premise that predictive reliability increases with modelling complexity. In truth, while modelling complexity can support quantification of predictive uncertainty, the latter is a function of data availability. Excessive complexity can often erode, rather than enhance, a model’s ability to quantify and reduce the uncertainties of decision-critical predictions by reducing its capacity to assimilate prediction-salient information. We submit that a groundwater model is more productively viewed as a data assimilator for decision-pertinent information than as simulating subsurface processes, even though the latter role (though imperfect) underpins the former. Assimilated data may, or may not, allow rejection of the hypothesis that a certain course of management action will have adverse consequences. Either way, the decision-making process requires that this hypothesis be tested. In the following document, we outline how decision-support environmental modelling can be implemented with the scientific method, and discuss how uncertainties of decision-salient predictions can be addressed with appropriate model complexity so that stakeholder expectations are better aligned with what models can and cannot deliver to the decision-making process.

Fluctuations in groundwater level in the Ria Formosa coastal aquifers, southern Portugal, owe 80% of the variability to climate-induced oscillations. Wavelet coherences computed between hydraulic heads and the North Atlantic Oscillation (NAO) and East Atlantic (EA) atmospheric teleconnections show nonstationary and spatially varying relationships. The NAO is the most important teleconnection and the main driver of long-term variability, inducing cycle periods of 6–10 years. The NAO fingerprint is ubiquitous and it accounts for nearly 50% of the total variance of groundwater levels. The influence of EA emerges coupled to NAO and is mainly associated with oscillations in the 2–4-year band. These cycles contribute to less than 5% of the variance in groundwater levels and are more evident further from the coast, in the northern part of the system near the main recharge area. Inversely, the power of the annual cycle increases towards the shoreline. The weight of the annual cycle (related to direct recharge) is greatest in the Campina de Faro aquifer, where it is responsible for 20–50% of the variance of piezometric levels. There, signals linked to atmospheric teleconnections (related to regional recharge) are low-pass filtered and have periods >8 years. This behavior (lack of power in the 2–8-year band) emphasizes the vulnerability of coastal groundwater levels to multi-year droughts, particularly in the already stressed Quinta do Lago region, where hydraulic heads are persistently below sea level.