There is a pressing need to strengthen systemic integration and mutually supportive interpretation of norms and institutional coordination between the UNECE environmental agreements with a view to understanding their collective contribution to clarifying and further developing international water, environmental and general international law. This chapter identifies the contribution of the UNECE regime to established ‘central or cornerstone rules and principles’ of international water law, including the due diligence rule to avoid or mitigate transboundary harm, the duty to cooperate, the principle of equitable and reasonable use and transboundary environmental impact assessments (EIAs). Key findings demonstrate that these rules and principles are defined in greater detail in the UNECE treaties compared to global treaties, which fills gaps in international law. For example, the UNECE clarifies international law regarding transboundary EIAs by providing criteria to identify what level of risk triggers the obligation to conduct an EIA and compulsory minimum content of EIAs. The UNECE regime also clarifies the effect of applying general principles, like the precautionary and polluter pays principles to the ‘no harm’ rule which strengthens these obligations. The strengths and weaknesses of UNECE’s contribution to developing reporting mechanisms for the Sustainable Development Goals, especially SDG 6 on water, are discussed.

If all of the potential Pigouvian externalities were internalised, both the positive and negative, then growth could be sustainable development. One of the challenges discussed was the complexity of integrating preferences for future generations who remain unable to voice concerns to present-time policymakers. Law and Economics literature considers the difficulty for decision makers to evaluate the risk of needing to take action based on incomplete information; the transaction costs of implementing the precautionary principle are complex. The polluter pays principle appears based on Pigou’s theory of externalities and on legal notions of justice, that tortfeasors should render damages for the injuries that they caused. Coase’s observation that either side of an injury could efficiently avoid that result and Calabresi’s maxim to search for the ‘least cost avoider’ have made implementation of the polluter pays principle both less obvious and less compelling. The allocation of human cum environmental rights is at its core an act of Coasean re-allocation, to provide one group with ex ante rights to reduce the transaction costs to achieve an efficient outcome for all parties. Overall, the Law and Economics literature has been in broad alignment with many of the key principles of general environmental law.