PhD dissertation “Planning circular urban water systems: a hydrosocial resource nexus framework to support systemic decision-making”, by Nicole Arnaud de Aguiar

Cities are hotspots of climate change, facing growing pressure from droughts, intense rainfall, rising temperatures and resource depletion, particularly in Mediterranean cities such as Girona. Within this context, urban water systems are key to urban environmental management: they move water, energy and nutrients, while being shaped by social, economic and governance conditions. Yet, current planning models still struggle to integrate circular resource flows, multifunctionality and socio-environmental justice, or to translate circular options into concrete structural design decisions.

To address these gaps, Nicole Arnaud de Aguiar’s doctoral thesis proposes a new Hydrosocial Resource Urban Nexus (HRUN) framework, which conceptualises urban water management as a multilayer resource nexus linking water, energy, food, ecosystems and society. Within the model, system infrastructure is defined by four design strategies: circularity, scale, greening and digitalisation. The framework sets out a planning cycle for circular interventions and assesses system performance through environmental and socioeconomic functionalities.

The research translates hydrosocial and socio-ecological theory into practical planning steps across four studies. First, it establishes the theoretical HRUN planning cycle. Second, it creates an assessment scheme to examine how combinations of the four design strategies affect socio-environmental resilience, including key synergies and trade-offs. Third, it analyses how decentralisation shapes the resilience of drainage, sewage and water-supply subsystems, identifying context-sensitive scales for different circularity purposes and spatial settings. Finally, it tests a spatial decision-support method in Girona, Spain, combining GIS-MCDA, graph-based clustering, risk indicators, socio-environmental criteria and stakeholder preferences. The method prioritises areas for circular water management units based on local vulnerability and appropriate de-centralisation scales.

In Girona, the methodology proposes a phased transition towards circularity. In the initial 15% uptake stage, it identifies nine neighbourhood sectors where interventions could be prioritised, mainly through water-cycle restoration measures (66%) and irrigation reuse schemes (25%). A governance-oriented filter further distinguishes parcels with greater potential for direct public action from those requiring incentives, partnerships, or negotiated uptake. In parallel, the reuse-clustering results indicate that a citywide distributed multipurpose reuse scenario could supply approximately 39% of current irrigation and toilet-flushing demand. When benchmarked against the estimated energy intensity of Girona’s current urban water cycle (0.68 kWh/m³), these de-centralised facilities perform very differently depending on the treatment configuration: wetland-based systems would reduce the total energy footprint, including treatment and transport, to 0.31 kWh/m³, whereas membrane bioreactor-based systems would increase it to 1.31 kWh/m³.

Altogether, the thesis provides a replicable spatial methodology for evaluating infrastructural synergies and trade-offs, linking hydrosocial theory, systems analysis and spatial planning in support of the transition to more just and resilient urban water systems.  This will contribute to improve urban water management at various levels and increase Mediterranean cities’ resilience to extreme events.  Nicole Arnaud’s doctoral thesis has been directed by Dr Manel Poch and carried out at the Laboratory of Chemical and Environmental Engineering, a multidisciplinary research group of the University of Girona renowned for its activity within the water field.

Main publication: Arnaud, N.; Poch, M.; Popartan, L.A.; Verdaguer, M.; Carrasco, F.; Pucher, B. A Systemic Approach for Assessing the Design of Circular Urban Water Systems: Merging Hydrosocial Concepts with the Water–Energy–Food–Ecosystem Nexus. Water 2026, 18, 233. https://doi.org/10.3390/w18020233