Global climate change, aridification, and reduced precipitation are causing serious problems for water supply management, water policy, and water law worldwide.  These problems are especially severe across the Iberian Peninsula, whose major rivers and reservoirs are governed by the Albufeira Convention between Portugal and Spain, and the Colorado and Rio Grande Rivers of the southwest USA, which are governed by interstate compacts within the USA and international treaties with Mexico. As water supplies contract to historic lows and their seasonal availability for agriculture, hydropower, and other uses change, the legitimacy and utility of long-established water laws and water-management policies are becoming vulnerable.  Yet many of these laws and policies are deeply entrenched and difficult to reform. This session surveys how precipitation science offers promising pathways to meeting water-supply and legal challenges in the shorter term and to reforming longstanding water policies and laws in the longer term. In the shorter term, precipitation science can assist water diplomacy by providing objective and forward-looking assessments of water availability at basin-wide scale, thereby helping to assist with legal compliance and the development of modern and more accurate water budgets.  The utility of this science has already been demonstrated in the “Lithium Triangle” of Argentina, Bolivia, and Chile.  In the longer term, precipitation science will be critical to rebutting archaic legal assumptions of stationarity, to enabling the development of conjunctive management mechanisms across trans-boundary basins, and to building more flexible and sustainable legal and regulatory structures for trans-boundary water management.