Water resources management (WRM) models have traditionally distributing water based on system capacities, economic efficiency, and demand (Loucks and Van Beek 2017). However, these efficiency-oriented models often fail to address water distribution inequities (Savenije and Van Der Zaag 2002, Hanemann 2005, Garrick et al 2020). As challenges around water scarcity and distribution inequity intensify, the need for models integrating justice principles to ensure fair and equitable resource allocation is evident.

Ideas of morality have long been central to human thought, shaping essential debates about equity and justice in modern political philosophy. Prominent philosophers like John Rawls and Amartya Sen have significantly contributed to these discussions, offering influential frameworks for understanding these concepts (Rawls 1958, Sen 2008). They argue for the inseparability of justice and fairness, that individuals should have not only equal opportunities (justice) but also equal chances to utilize those opportunities (fairness). Equity is thus achieved when justice and fairness are consistently applied to all.

Justice considerations in WRM broadly encompass distributive justice, focusing on fair resource allocation, and procedural justice, emphasizing transparency in decision-making. Water resources equity may concern distribution between upstream and downstream states in a transboundary basin (Zeitoun et al 2014, Yalew et al 2021), rights and access to clean water in communities (Syme et al 1999) or governance issues addressing multi-sectoral water demands, such as irrigation water demands in the agriculture sector (Gross 2014, Neal et al 2014).

Distributive justice addresses the questions of 'what' (what to distribute), 'to whom' (to whom to distribute), and 'how' (how to distribute) of allocation of common pool resources. This aligns with the principle of 'equitable and reasonable use' of water resources outlined in the United Nations Watercourses Convention (United Nations 1997). It is also reflected in the United Nations Sustainable Development Goal 6 (SDG6), which aims to 'Ensure access to water and sanitation for all' (United Nations 2015). Despite some attempts to incorporate equity aspects in water resources assessments (Dore et al 2012), a significant gap remains in effectively integrating justice principles into WRM models. Addressing these challenges requires the operationalization of specific fairness and justice principles within WRM models. By incorporating insights from socio-economic and philosophical theories, such as welfare economics and Rawlsian justice, into water resource assessments, hydro-economic models could be significantly improved to deliver operational and policy alternatives that balance efficiency and equity.@en