Introduction Directory UMM :Data Elmu:jurnal:A:Agricultural & Forest Meterology:Vol104Issue4Sept2000:

Agricultural and Forest Meteorology 104 2000 303–313 Dew formation on rice under varying durations of nocturnal radiative loss Weihong Luo a,b , Jan Goudriaan a,∗ a Department of Theoretical Production Ecology, Wageningen Agricultural University, P. O. Box 430, 6700 AK Wageningen, Netherlands b Department of Agronomy, Nanjing Agricultural University, Nanjing, Jiangsu 210095, China Received 9 September 1999; received in revised form 13 June 2000; accepted 14 June 2000 Abstract 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. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Shielding experiment; Nocturnal net radiative loss; Dew duration; Dew amount; Guttation

1. Introduction

Leaf wetness is an important factor in plant disease epidemics and also in the deposition of acidic air pol- lutants on plant surfaces. It provides the free water that is essential to the development of many foliar bac- ∗ Corresponding author. Tel.: +31-317-4839-87; fax: +31-317-4848-92. E-mail address: jan.goudriaanpp.dpw.wau.nl J. Goudriaan. terial and fungal plant pathogens Wallin, 1963 and may enhance the deposition of the pollutants Janssen and Romer, 1991; Hughes and Brimblecombe, 1994. For instance, leaf wetness is required by all models and systems that forecast rice blast epidemics one of the most severe diseases of rice crops Uehara et al., 1988; Manibhushanrao and Krishan, 1991; Kim and Kim, 1991; Horino, 1992. Dew, as a main contributor to leaf wetness, normally occurs during nighttime as a result of radiative loss 0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 6 8 - 4 304 W. Luo, J. Goudriaan Agricultural and Forest Meteorology 104 2000 303–313 of heat from the leaf surface, followed by condensa- tion of water vapour. Thus, nocturnal net radiative loss plays an important role in the dew formation process. Nocturnal net radiative loss is a direct or indirect in- put of many dew formation simulation models that use the energy balance approach e.g. Pedro and Gillespie, 1982a, b; Jacobs et al., 1990; Wittich, 1995; Wilson et al., 1999. Its effect on dew formation was theoreti- cally studied by comparing it with model outputs or by sensitivity analyses Scherm and van Bruggen, 1993. The amount of dew formed on a surface depends on how much nocturnal net radiation is balanced by latent heat. Simulated dew formation practically al- ways deviates from measured results. This deviation can be attributed to measurement errors, both of dew formation and of relevant weather data, but also to deviations in the model structure from reality. The objective of this study was to experimentally inves- tigate how the dew formation depends on nocturnal net radiative loss. The understanding of these factors is essential for accurate estimation of dew formation.

2. Materials and methods