Studies on Crop-Weather Relationship Model in Rice

(1) Relation between Absorbed Solar Radiation by the Crop and the Dry Matter Production

Abstract

As the first step to develop a model to explain and predict the dynamic relationship between weather and rice crop growth and yield, we investigated the relation between absorbed solar radiation by the canopy and the drymatter production for three different rice cultivars grown in different seasons, years and radiation regimes (c. f. Table 1). The cultivars used were “Nipponbare” (Japonica rice), “IR-36” (Indica rice) and “Milyang 23” (Japonica-Indica cross variety). The canopy absorbed radiation was obtained by the continuous measurements of the incoming, the transmitting and the reflected radiations into/from the canopy, and the dry weight by the periodic harvesting.
The radiation transmittance and the reflectance of the canopy changed drastically with LAI (see Fig. 4), but the extinction coefficient of daily shortwave radiation, calculated from the transmittance, was fairly constant throughout the main growth period. The values of the coefficient are 0.45, 0.43 and 0.41 for “Nipponbare”, “IR-36” and “Milyang 23”, respectively. An equation was proposed to estimate the radiation absorptance from the LAI, the extinction coefficient and the crop optical parameters. The changes with time in the estimated radiation absorptance agreed well with the observation for almost the entire growth period except the very later stage (Fig. 5).
The crop dry weight increased linearly with the increase in the absorbed shortwave radiation or photosynthetically active radiation (PAR) as the crop grew until about 20 days after the heading and thereafter it increased curvilinearly, indicating that the conversion efficiency from the absorbed radiation to the dry weight is constant over the most growth period. In Fig. 7 are plotted the relationships between the absorbed radiation and the dry weight during the growth of “Nipponbare” rice grown under 10 different seasons and years, of which the growth curves in the dry weight and LAI in 1982 experiment are given in Figs. 1 and 2. From the results in Fig. 7 it may be concluded that the conversion efficiency from the absorbed radiation to the dry weight is independent of crop growth stage except for the later half of the ripening stage and also of the cropping seasons. The conversion efficiencies on the bases of the shortwave radiation (c_s) and PAR (c_p) are 1.90 and 2.88g/MJ in “Nipponbare”, which are equivalent to 2.9 and 4.5% in the energy conversion efficiency, respectively.
The constancy of the conversion efficiency of c_s or c_p over the growth stage and the cropping seasons is largely justified in Fig. 8, where comparisons are made between the observed growth curves and those re-constructed by multiplying the conversion efficiency to the absorbed radiation which in turn was obtained from LAI and radiation on each day. The constancy of the energy conversion efficiency was also validated by a model simulation in which the crop growth rate (CGR) is given as a function of the radiation intensity, LAI, canopy architecture and growth and maintenance respiration rates. It was made clear from the model simulation that CGR increases approximately linearly with the increase in the absorbed radiation and that the proportional constant, which is the conversion efficiency, is to a lager extent independent of the radiation regimes and LAI, but is affected by the radiation extinction coefficient (Fig. 11). The simulated conversion efficiency from the shortwave radiation to the dry weight c_s were 1.97 and 2.07g/MJ for the canopies with the extinction coefficient of 0.6 and 0.5, respectively, which are comparable with the experimental results.
The value of c_s of the high yielding rice cultivars of “IR-36” and “Milyang 23” was about 20% higher than that of “Nipponbare”