Dew formation on rice under varying durations of nocturnal radiative loss

In order to understand how nocturnal net radiative loss affects dew formation, a shielding experiment was carried out in a rice field at the International Rice Research Institute during 16 nights in February, March, and April 1994. Four plots (with an area of 4×5 m for each) in the field were used to measure the dew amount and duration in the rice crops. During each night, two of the four plots were covered using black plastic sheets from sunset (about 18:00) till 03:00 or 04:00 or 05:00 of the next day. The other two plots were used as a control (without cover). The results showed that in the dry season in the tropics, daily total dew duration on top leaves mainly depended on night time dew duration and the latent heat released by dew formation on the top leaves was about 20% of the nocturnal net radiative loss. The measurement error in nocturnal net radiative loss caused by dew formation on the dome of the net radiometer was typically about 7 W m −2, with 18 W m −2 as an extreme value in the dry season in the tropics during heavy dew nights. Both dew amount and dew duration on the top leaves could be reasonably predicted ( r 2=0.8) by a physically based simulation model, although a standard error remained of about 0.03 mm for dew amount and 2.3 h for dew duration, respectively. Measurement errors in weather variables, especially in net radiative loss and in water vapour pressure deficit, are probably the main causes of these deviations. Dew amount was more strongly affected by the nightly minimum value of water vapour pressure deficit than by its mean value. Guttation by the rice plants themselves supplied as much water to the leaf surface of the paddy rice crops as dewfall did. Unexpectedly, shielding from nocturnal net radiative loss provided a feasible method to measure guttation water intercepted by different layers inside the paddy rice crops.