57 a conventional SAG-mill-based concentrator using froth flotation and accounts explicitly for water inputs, internal recirculation, and losses across mining and processing stages. The system boundary is limited to on-site operations up to the production of a marketable copper concentrate, corresponding to the point at which material is transported to off-site smelters. Indirect water use associated with energy supply, infrastructure, or downstream processing is not included. Water requirements are computed as a function of deposit-specific parameters, including ore tonnage, ore grade, production rate, processing configuration, tailings storage assumptions, and internal recycling efficiency. The production rate is estimated using the empirical copper porphyry production rate rule presented in Northey et al. (2025). Tailing-related water losses, which largely determine water losses, are represented through a conceptual tailings storage facility (TSF) water balance consistent with the framework proposed by Wells and Robertson (2003). Evaporation losses from tailings storage facilities and other exposed water surfaces are explicitly represented and are calculated using daily pan evaporation data derived from UNdata (2010). These losses constitute the dominant component of consumptive water use in arid and semi-arid mining regions. 2.4 Mining Water Stress Ratio and Coupling of Water Supply and Demand To enable comparison with extractable water availability, deposit-level water requirements are aggregated spatially to the grid resolution of the hydrological indicators. When multiple deposits scheduled for production fall within the same grid cell at the same time, their water requirements are summed. A fixed fraction of 10% of total extractable water is allocated to mining to represent sectoral water access at the grid scale, with further work planned to assess more realistic allocation schemes. A mining water stress indicator is then defined as the ratio of total water required for copper production within a grid cell at a monthly timestep to the extractable water available for mining in that cell in that month, considering combined contributions from rivers, lakes and reservoirs, and aquifers. This ratio provides a dimensionless measure of the extent to which copper production water requirements exceed, or remain within, physically extractable water volumes under projected climate conditions. This coupling enables the identification of locations and periods where water availability may act as a binding constraint on copper production. 3. PROOF OF CONCEPT: COPPER PRODUCTION IN SOUTH AMERICA Copper production in South America is used as a proof-of-concept application, allowing the framework to be demonstrated without implying site-specific predictions. 3.1 Copper Demand Scenario and Regional Focus Future copper production requirements are derived from demand projections published by the International Energy Agency (IEA). Three IEA scenarios are considered: the Stated Policies Scenario (STEPS), reflecting currently implemented energy and climate policies; the
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