Simulation and measurement of leaf wetness formation in paddy rice crops

The study described in this thesis focuses on a quantification of leaf wetness formation in paddy rice crops based on insight in the physical processes of the formation of leaf wetness. For this purpose, experimental research was conducted in a tropical paddy rice field.

A shielding (nocturnal net radiative loss) experiment was designed to investigate the dependence of dew formation on nocturnal net radiative loss. A simple method was developed to estimate dew amount and duration using the nightly total net radiative loss. The shielding experiment unexpectedly provided a feasible method for experimentally distinguishing dew from guttation. For a better understanding of the characteristics of the formation of leaf wetness in paddy rice crops, warm water flooding both in the paddy rice field and in a phytotron was conducted to investigate the effects of water temperature on dew formation and guttation. The experimental data indicated an optimum paddy water temperature of 30°C for rice leaf guttation. The effect of water temperature on dew formation depended on the weather condition above the canopy, especially the thermal stratification condition, net radiation and vapour pressure deficit. A simple device, the dewball, was designed to observe dew formation and to determine its threshold value of nocturnal net radiative loss from top leaves. This was shown to be a simple and accurate method for estimating daily dew duration on top leaves. The possibility of using the mean diurnal patterns of weather variables to estimate dew formation was analysed by comparing the dew estimated using a simulation model to the measured results. This helped to understand to what extent the diurnal patterns will give as good an estimate of dew formation as when the observed hourly weather data are used. Based on the observation results on the dewball and the results of the shielding experiment, a simple approach to estimate daily dew duration on top leaves of the paddy rice crops was formulated and its potential applications to plant protection were discussed.