High ambient air and water temperatures can cause high-temperature damage to ripening rice grains after rice heading and has recently become a major issue of concern. Spill-over irrigation (also referred to as continuous irrigation with running water) is an effective countermeasure that involves simultaneous irrigation and drainage to reduce the ponded water temperature in paddy fields. However, it is necessary to understand the impact of this technique on downstream water temperatures. In this study, the discharge and water temperature variations in a dual-purpose canal (also referred to as irrigationcum-drainage canal) were measured along the main canal of the Tedori-gawa Shichika canal system in Japan. The discharge and water temperature variations in a branch drainage canal were also monitored to analyse the influence of the return flow on the water temperature in the main canal. Based on these observations, a new numerical model was developed to simulate the water temperature variations in the main canal considering meteorological conditions, heat transfer of water in the open canal, and return flow from upstream paddy fields. Finally, the model was applied to examine possible global warming scenarios and discuss the effectiveness of paddy water management in mitigating high-temperature damage in downstream paddy fields. K E Y W O R D S climate change, heat balance, high-temperature damage to rice grains, repeated use of water, spillover irrigation and paddy water management