Theoretical analysis of the effects of irrigation rate and paddy water depth on water and leaf temperatures in a paddy field continuously irrigated with running water

Water management techniques such as continuous irrigation with running water (CIRW) have been used by Japanese farmers to control the thermal environment for rice cropping. In this study, to optimize this method to control the thermal environment in paddy fields, theoretical equations for predicting water and vegetation temperatures in a paddy field were obtained. First, the equations to calculate water and vegetation temperatures in a paddy field were obtained by solving the heat balance equations of the paddy water and vegetation, taking into account the effect of horizontal heat convection driven by irrigation. The equations were validated by comparisons with observed water temperatures in a conventional paddy field under CIRW. The calculated changes in water temperatures over time and distance showed good agreement with observed values, with a root mean square error of 0.39 °C. This result indicated that the equations satisfactorily expressed the features of paddy water temperature under CIRW. Next, these equations were used to determine the effects of irrigation rate, paddy water depth and wind speed on water and rice plant temperatures. The following results were obtained. 1) The area cooled by CIRW was positively related to irrigation rate and negatively related to water depth. 2) Low water depths were preferable for application of CIRW during the nighttime. 3) Slower wind speeds and latent heat flux from vegetation strengthened the effects of water management on vegetation temperature. The proposed equations and analyses represent the common features of the effect of irrigation on water and vegetation temperatures in a paddy field, and provide quantitative information about the effects of water management techniques on the thermal environment for rice cropping.