Defining Reasonable Use in Transboundary Water Governance

Elfatih A. B. Eltahir, Yeon-Woo Choi, Shafiqul Islam

Ambiguity in the principle of “equitable and reasonable use,” central to Article 5 of the 1997 UN Watercourses Convention, has long hindered transboundary water negotiations. Without a clear definition, states invoke this principle to justify conflicting claims, prolonging transboundary water disputes across the world. We propose a sequenced interpretation: first define reasonable use quantitatively using physical science, then negotiate equitable use within those physically reasonable boundaries. Building on Budyko’s hydroclimatic framework and the distinction between a river’s natural drainage and irrigation services, we propose a definition for “reasonable use” of water resources in a river and show how to identify and quantify the physically sustainable envelope of river use across annual, seasonal, as well as spatial scales. Applied to the Nile and Ganges, this approach reframes disputes not as zero-sum volume allocations but as negotiations over complementary services provided by a shared river. By anchoring transboundary water negotiations in physical realism, this framework reduces contestation over hydroclimatic realities, improves transparency, and provides a quantifiable and replicable tool for resilient treaty design.