A socio-hydrological understanding of the use of models in water allocation: the case of the Piancó-Piranhas Açu (PPA) Basin in Northeast Brazil

Numerical models play a critical role in socio-hydrology, influencing decisions that impact coupled human-water systems. For instance, policy decisions on the allocation of water increasingly rely on hydrological and hydro-economic models that structure the framing of scarcity, priority uses, and the acceptable trade-offs. This, in turn, has wider-reaching effects on both economic uses and hydrological systems as well as biophysical processes in the affected areas. In all of this, models are often seen as neutral entities providing objective information. However, subjectivity arises from assumptions of what is important to model, theories of human behaviour, technical model choices, and uncertainties. Understanding the socio-natural feedback between biophysical states, the assumptions that influence models, and the models’ roles in shaping water governance is crucial to improving transparency and, ultimately, in highlighting potentially more just and sustainable allocations of water that better meet the needs of people and the environment. This research examines the socio-hydrological role of models in the allocation of water resources by understanding models as socially constructed and therefore non-neutral tools that engender or favour certain types of outcomes over others. It takes as a case study the semi-arid Piancó-Piranhas Açu (PPA) Basin in Northeast Brazil, in which allocation of water is an ongoing, negotiated process. This process has been made more complex by a large-scale inter-basin transfer project bringing water to the PPA, resulting in overlapping authority (federal, state, basin committee) interacting to allocate and manage water. It also fundamentally alters hydrological processes in both basins. This study explores the development, characteristics, and uses of models in the PPA basin during the formation of allocation rules, and how these rules have impacted coupled human-water systems.  Through a social construction of technology (SCOT) framework, a grey literature analysis, and semi-structured interviews with modellers, water managers in key institutions at multiple geographic scales, and stakeholders, we reconstruct the development, configuration, and uses of key models. Challenging the notion of models as neutral decision-support tools by examining their implicit role in the framing of scarcity and the legitimisation of particular governance decisions on water allocation, the study aims to support more reflexive, transparent, and socially robust approaches to water allocation.