Hydro Digital Twins as Emerging Technologies: An alluvial mapping of objectives across marine, coastal, and freshwater applications

Water territories (coastal zones, river corridors, marine basins, and lagoons) face compounding climate pressures, yet the digital tools designed to support their governance remain poorly understood in terms of what they actually aim to achieve. While Hydro Digital Twins (HDTs) are increasingly framed as instruments for climate adaptation and environmental stewardship, systematic analyses of their governance purpose remain scarce. This paper examines 14 HDT applications spanning marine, coastal, river-lake-fjord, aquaculture, and flooding domains, published between 2020 and 2025. Rather than cataloguing technical features, the analysis traces how application contexts translate into governance functions and, in turn, into concrete operational targets. An alluvial diagram links five application themes (left) to six governancefunction categories: Forecasting, Modelling, Monitoring, and their combinations (centre)—and onward to 18 specific objectives (right). Three patterns emerge. First, monitoring, forecasting, and modelling functions overlap substantially across themes, revealing that HDTs are conceived as multi-functional instruments rather than singlepurpose tools. Second, coastal applications exhibit the broadest governance scope, spanning spatial planning, erosion management, and environmental assessment simultaneously. Third, several objectives (saltwater intrusion management, biological component integration, participatory governance) appear as isolated, single-flow connections, signalling emergent but not yet consolidated research fronts. The paper argues that these governance flows expose both the promise and the fragility of current HDT architectures: broad ambition coexists with narrow implementation, and the distance from stated objective to operational deployment remains the defining challenge. Governance ambition has outpaced delivery capacity; institutional and data-governance frameworks such as F.A.I.R., C.A.R.E., and T.R.U.S.T. must develop alongside technical infrastructure if HDTs are to serve as instruments of equitable water-territory management.