Continuous estimation of rice (Oryza sativa (L.)) canopy transpiration realized by modifying the heat balance model

An estimation of canopy photosynthetic activities is needed in order to better understand biomass production of field crops. Thermal imaging techniques and the heat balance model enable us to estimate canopy diffusive conductance (g_c) and canopy transpiration rate (E) of crops under field conditions. However, because conventional methods are unstable when wind velocity is very low, it is difficult to apply this model directly to field-grown crops. In this study, we modified the conventional model by measuring aerodynamic resistance in rice (Oryza sativa (L.)) under windless conditions (r_a∗). The r_a∗ ranged from 9.50 to 35.40 s m⁻¹ among the genotypes. By introducing the concept of r_a∗ to the original heat balance model, the stability when wind velocity is under 3.0 m s⁻¹ improved greatly. Using seven rice genotypes, we evaluated genotypic differences in E with higher temporal resolution. The daily cumulative transpiration ranged from 2.32 kg m⁻² d⁻¹ to 10.29 kg m⁻² d⁻¹ depending on genotype and weather conditions. High-yielding cultivars consistently showed greater transpiration rates under various weather conditions. We confirmed the relationship between estimated E and final grain yield, especially during the daytime. Our modified model is useful as a monitoring tool for rice canopy transpiration.