Development of water temperature control method and field water management software for rice paddies using weather information and canopy micrometeorological model

Abstract The water temperature of paddy fields is an important factor affecting the growth and yield of paddy rice. In this study, the optimum timing of irrigation to control (increase or decrease) water temperature was simulated based on the canopy micro-meteorological model under two different climate conditions, i.e., high and low temperature conditions, assuming the need to prevent high and low temperature injury of paddy rice. Under both conditions, the optimum timing of irrigation to increase daily minimum water temperature was during nighttime from 0:00 to 4:00, whereas the optimum timing to decrease daily maximum water temperature was during daytime from 11:00 to 14:00. Under the low temperature condition, the daily mean water temperature increased with increasing water depth. The main reason is that roughness length decreases at deeper water surfaces, which suggests that practicing deep-water management in cold regions would have the effect of keeping water temperature higher. Simulations to determine the optimal timing of drainage in addition to irrigation showed that daytime flooding (irrigation in the morning and drainage in the evening) is effective for decreasing water temperature. As a result of field experiments of three years, a significant difference in average water temperature of 0.67℃ was observed between the daytime-flooded plot and nighttime-flooded plot during the ripening period. We developed a field water management software to optimize irrigation time to increase or decrease water temperature based on simulation using the canopy micrometeorological model with weather information. Using this software, water management to prevent cold damage was conducted in actual paddy fields. The water temperature in the software-controlled plot was on average 0.85℃ higher than the water temperature in the conventional plot. The developed software can be used as a countermeasure for low and high temperature injury in rice cultivation for adaptation to climate change.