Modeling the effects of cool water on growth and grain yield in rice

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Water temperature (Tw) can limit rice yield via various growth processes. A simple simulation model was developed to evaluate the effects of Tw on rice growth and yield based on the field trials conducted under different Tw conditions (17-26℃). The model covers the effects of Tw on the developmental rate, leaf appearance, tillering and culm elongation. Leaf area estimates were derived from a product of leaf number on the main culm and tiller number. Radiation use efficiency was expressed as a function of developmental stage. Harvest index was determined as a function of spikelet fertility, which was estimated with the cooling degree-days based on either Tw or air temperature depending on the vertical position of the developing panicles. We tested the model at four locations (Pippu, Iwamizawa, Sapporo and Oono) in Hokkaido for 1990-1999. The estimated yields were generally in good agreement with the measured yields that ranged from 19 to 677 gm^<-2> with the root mean square deviation (RMSD) of 79gm^<-2> (n=33). The Tw estimation model based on the heat balance (Kuawgata et al. 2001) estimated those at the four locations with a RMSD of about 1℃ with no indicative trend in the residuals. Tw averaged for the past 10 years was estimated to be higher than Ta by 3.8℃ in Sapporo, the Tw difference being equivalent to about 300 gm^<-2> of grain yields indicating the marked "water blanket" effect on rice productivity in Hokkaido. The regional yield variation was largely attributed to solar radiation, but wind speed that affects Tw also had a significant effect. A simulation indicated that a rise in air temperature by 3℃ could result in a 1.3℃ increase in Tw and a 6 % increase in grain yield in Sapporo, if other climatic conditions were unchanged.