Dew formation on rice under varying durations of nocturnal radiative loss

In order to understand how nocturnal net radiative loss affects dew formati on, a shielding experiment was carried out in a rice field at the Internati onal Rice Research Institute during 16 nights in February, March, and April 1994. Four plots (with an area of 4x5 m for each) in the field were used t o 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 p lots 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 d epended on night time dew duration and the latent heat released by dew form ation on the top leaves was about 20% of the nocturnal net radiative loss. The measurement error in nocturnal net radiative loss caused by dew formati on 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 rea sonably predicted (r(2)=0.8) by a physically based simulation model, althou gh 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, especi ally in net radiative loss and in water vapour pressure deficit, are probab ly the main causes of these deviations. Dew amount was more strongly affect ed by the nightly minimum value of water vapour pressure deficit than by it s mean value. Guttation by the rice plants themselves supplied as much wate r 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 ri ce crops. (C) 2000 Elsevier Science B.V. All rights reserved.