Evaluating integrated model structural uncertainty of aggregated urban sewershed models

Aggregated sewershed models provide simulations of sewer system behaviour with relatively low computational cost and limited structural complexity. In contrast, fully distributed models, such as SWMM and Infoworks, provide detailed representations of flows within individual pipes and network infrastructure, but require extensive data for model construction and substantially greater computational resources. Sewersheds are also embedded within wider integrated water systems, interacting with hydrological processes through both inputs and outputs. Capturing these interactions introduces additional modelling complexity and motivates the use of integrated modelling approaches. Across all model types, uncertainty is inherent. In particular, structural uncertainty (arising from choices related to model formulation, process representation, and model component configuration) remains a significant challenge and is especially difficult to quantify. In this study, we examine three urban case studies in the Greater Manchester region, developing aggregated integrated models using the WSIMOD integrated modelling framework. WSIMOD enables the simultaneous representation of urban infrastructure, hydrological processes, and land use within a flexible model structure, making it well suited to integrated sewershed modelling and the exploration of alternative model configurations. We introduce and apply a novel framework to systematically explore the impacts of key modelling decisions on both model behaviour (simulated system dynamics) and performance (simulation-observation metrics). Alternative model structures were constructed to represent different modelling choices, and a series of sensitivity analyses was conducted to assess parameter sensitivities and to group model processes according to their influence on model behaviour and performance. The results provide insights into how structural modelling decisions affect aggregated sewershed model outcomes and highlight implications for integrated urban water system modelling.