Technical models are useful tool to address epistemic uncertainties but often fall short of attending to other types of uncertainty that characterize complex water challenges. It is unclear if and how they might be repositioned as a more deliberative tool to help deal with the many uncertainties related to problem framing, uncertain future conditions, and likely intervention effects at various scales. Through the case of a multi-stakeholder water quality project in East Java, Indonesia, this paper explores how technical systems modeling can be used to support consensus-building regarding the characterization of water pollution problems and adjacent policy goals, both in the use of outputs and in the process of model-making and attendant deliberation. The water quality model combines mapped terrestrial pollution source estimates with rainfall-runoff and pollution transport and fate process models to estimate localized, regional, and basin-wide impacts of various source-reduction scenarios on water quality. By visually identifying pollution source concentrations and illustrating estimated impacts of alternative strategies, the model offers a useful visual tool on which to anchor reframing discussions and scenario-building. In this way, the case demonstrates how modeling can be repositioned as an invitation for planners to simultaneously deliberate alternative problem structures alongside interventions to better deal with uncertainties inherent to water resources management.

Water managers and planners working within complex social-environmental systems are challenged with difficult choices when prioritizing interventions to manage water quality and reduce pollution from point and non-point sources. These choices are particularly important in low-resource environments where public funds must be carefully allocated. To support policy analysis for water quality management, a water quality modeling and policy consultation exercise was performed by Deltares, TU Delft, and government partners in the Brantas River basin, East Java, Indonesia. The modeling exercise combined mapped pollution source estimates for domestic wastewater and agricultural runoff with rainfall-runoff and pollution transport and fate models to demonstrate estimated impacts of various source-reduction scenarios on BOD loads along the main river. These outputs were used to inform deliberations regarding options to reduce water pollution and improve river health at a basin level. The model's ability to identify hotspots and assess the impact of targeted pollution reductions offers a powerful visual tool for policymakers to formulate geographically targeted interventions and identify the specific pollution source reductions that would yield the most substantial improvements. The case demonstrates the practical applications of scenario-building as an invitation for policy-makers to visually consider alternative interventions and focuses on lessons learned regarding capacities required to perform such activities, stakeholder engagement to build workable and meaningful model from an administrative perspective, and practical considerations for applying data-driven approaches to prioritization.