Numerical model to predict water temperature distribution in a paddy rice field

Abstract

Irrigating water into a paddy field inevitably changes the water temperature and produces a temperature distribution in the field. In this study, a numerical model for predicting the horizontal water temperature distribution in a paddy field was developed. In addition to considering the heat exchange between the atmosphere, paddy rice, water, and soil, the model also considers the horizontal heat convection within the paddy field due to the inflow of irrigation water and outflow of paddy field water. The horizontal water flow in the field is modeled by the water balance equation of paddy water. The model was validated using field measurements collected in a paddy field where the continuous irrigation with running water (CIRW) management method was applied. The calculated changes in water temperatures over time and distance showed good agreement with the observed values. Further, the root mean square error (RMSE) of the water temperature during the entire validation period was 0.98 °C, and the RMSE during the nighttime CIRW irrigation was 0.55 °C. These results indicate that the model satisfactorily expresses the features of paddy water temperature distribution under the effect of irrigation water inflow. Moreover, a comparison of the calculation results of the proposed model to those of the conventional methods that do not consider the water temperature distribution demonstrated that the prediction accuracy significantly improves when the water temperature distribution is considered. Our results suggest that the prediction of water temperature distribution is useful for a more accurate prediction of paddy heat environment and various phenomena affected by water temperature in a paddy field.