Temporal and Spatial Variation Characteristics of Water Temperature in the Main Canal of South-to-North Water Diversion Middle Route Project

To characterize the spatiotemporal variability of water temperature in the main canal of the South-to-North Water Diversion Middle Route Project and ensure its safe and stable operation, thirteen check gates along the open-channel segment of the main canal were selected as monitoring sites. Based on hourly water temperature data collected from March 2015 to April 2025, temporal and spatial statistical analyses were conducted to comprehensively investigate the annual distribution patterns, longitudinal variation trends, and principal factors influencing water temperature. The results showed that: (1) The annual water temperature exhibited a pronounced unimodal pattern, with monthly average temperatures ranging between 2°C and 30°C. The maximum temperature was observed in August (mean 28.5°C), while the minimum occurred during January and February (mean 4.2°C), indicating significant seasonal variability. Temperature fluctuations were most pronounced during spring and autumn (March–May, October–November), with deviations from the monthly mean reaching up to 4.38°C below the maximum and 5.41°C above the minimum; conversely, winter months displayed relatively stable temperatures, with variations generally below 2.1°C. (2) A clear seasonal reversal in longitudinal water temperature gradients was identified: from spring through summer (March–August), temperatures increased from south to north, with the gradient diminishing notably north of the Anyang River Check Gate; this pattern inverted during autumn and winter (September–February), exhibiting a south-to-north decreasing trend. Notably, between December and February, water temperature declined by 0.48 to 0.75°C per 100 km along the canal, with the segment between Gangtou and Beijuma River Check Gates experiencing the steepest decrease of 1.32°C per 100 km. This region constitutes a critical zone susceptible to ice formation, as water temperatures approach the freezing threshold of 0°C. These insights furnish a scientific foundation for precise winter operational scheduling and ice regime prevention and management of the project.