A Simulation Model for Micrometeorological Environment in Rice Field

Abstract

A prediction model of canopy climate was developed to simulate micrometeorological environment including carbon dioxide and photosynthesis in relation to the physiological properties of leaves, canopy structure and external meteorological elements. The newly developed model (Soil-Water-Plant-Atmosphere system model) consists of four subset models, i.e. soil layer, water layer, canopy layer and surface air layer sub-models (Fig. 1). The results obtained by simulation of micrometeorological environment in rice canopy were compared with the measurements. The results are summarized as follows.1) It was found that the diurnal changes in temperatures of air, water and soil layers simulated by the model were in good agreement with those observed in a rice field (Fig. 2). Moreover, the rates of transpiration or evapototranspiration from a rice field simulated by our model agreed well with that reported in a previous paper (Inoue et al., 1984), with acceptable error (Fig. 3).2) The simulation results of stem temperature of rice plants, as reported previously (Toriyama and Inoue, 1984), were used to determine a main cause for the regional difference of the sterility of rice panicles due to cool summer. The chilliness [Σ(20-Tf)] calculated using simulated leaf temperature (Tf) was found to well explain the regional difference of the sterility. We can conclude from this results that solar radiation plays an important role in diminishing the sterility of rice panicles under cool summer conditions.3) Using the diurnal change in air temperature simulated by our model, effects of the depth of flood water on the temperature environment in rice canopy were studied in relation to the strength of air mixing within the canopy (Fig. 5). From the relationship between normalized temperature amplitude [ΔTf(z)/ΔTw] and normalized height [(z-Hw)/(h-Hw)], where suffixes f and w denote the quantities related to leaf and water, respectively, we can conclude that the temperature amplitude decreases very rapidly with increase of height from the ground surface and is well fitted by a hyperbolic function.4) Our model was also used to examine effects of the depth (Hw) of flood water on the temperature environment within rice canopy. The amplitude of diurnal change in leaf temperature decreases with increase of Hw. When the depth of flood water is 20cm, the average leaf temperature in the layer of 10 to 20cm, in which young panicles before the heading time are mainly distributed, was found to be 2°C higher than that in a rice field with the water depth of 5cm. This implies that the irrigation of about 20cm can protect young rice panicles before the heading time from dangerous cool summer (Fig. 7).