Chalkiness degree (C.D.) is a commonly used indicator aimed at evaluating the appearance and the quality of milling of rice grains. This index in a sample is determined as the ratio of the total chalky area (pixels) of the 100-kernel set to the total area of the kernels, multiplied by 100 (Figure 9).

Figure 9. Kernels characterized by different chalkiness degree (Source: International Rice Research Institute - IRRI).

This modelling approach simulates the C.D. of rice as function of air temperature perceived by the crop during the R8 stage of the ripening period (Counce et al., 2009), when the main culm turns to brown (Lanning et al., 2011). In particular this HRY index is calculated using a quadratic function of the 95-th percentile of nighttime air temperature values observed during the above mentioned phenological phase.

The general equation developed for Japonica varieties (Lanning et al., 2011) is given below:

Where:

CD (%) is the simulated chalkiness degree value;

Tn95th (°C) is the 95th percentiles of nighttime air temperature frequencies that occurred during the R8 phase of rice ripening.

ai, bi, ci (unitless) are the coefficients of the equation, specific for medium grain Japonica varieties.

Basically, once a DVS equal to 2.5 is reached, the model for the simulation of HRY begins to receive as input the hourly values of air temperature. Thus, each day, the HRY model populate a list object by using the hourly temperature data included between Sunset and Sunrise hour. These latter are exogenous variables that vary depending on the latitude of the site where simulations are performed. At last, when physiological maturity stage (DVS = 3) is achieved, the values stored in the above mentioned list object are used to calculate the 95-th percentile of nighttime air temperature, which represents the input of  the equation listed above.

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