Abstract
Study region: The main canal of the Middle Route of the South-to-North Water Diversion Project, China, one of the world’s largest long-distance interbasin water transfer systems. Study focus: Winter low-temperature conditions and ice hazards constrain the winter water conveyance capacity of long-distance water transfer canals, yet the role of source reservoirs in regulating downstream thermal conditions remains unclear. This study developed and validated a winter water temperature model for the main canal, achieving RMSE values below 0.6°C and R² values above 0.9. Scenario experiments were conducted to assess how source-reservoir water-level rise and increased diversion discharge affect longitudinal heat transport and low-temperature constraints at the canal tail. New hydrological insights for the region: The Danjiangkou Reservoir acts as an active thermal regulation node rather than only a water-supply boundary. Increased diversion discharge enhanced heat transport efficiency, reduced the longitudinal temperature gradient by 19%, and shifted the 14°C isotherm 404.8 km northward. Reservoir water-level rise strengthened the headwork thermal boundary and mainly improved near-source thermal conditions. Under suitable discharge, reservoir-induced thermal gains could reach the canal tail about 1200 km downstream. Joint regulation reduced 0–4 °C low-temperature days at BJM by 12–24%, indicating its potential to alleviate low-temperature constraints and support further evaluation of winter water conveyance capacity.