Measuring dew formation and its threshold value for net radiation loss on top leaves in a paddy rice crop by using the dewball: a new and simple instrument

In a field experiment on dew formation in tropical paddy rice, the threshol d value for the nocturnal net radiative loss required for dew formation was investigated during 23 nights from February to April 1994. The onset and c essation of dew were visually observed on both the top leaves of a rice cro p and on a glass sphere, the "dewball", installed in the field 1.0 m above the ground. The threshold value for the nocturnal net radiative loss (R-n,R -thresh) necessary for dew formation on the ball was deduced from the maxim um zenith angle on the ball surface reached by the dew formed. R-n,R-thresh was found to be linearly related to the nightly minimum vapour-pressure de ficit. This linear relationship as well as the Penman-Monteith equation tan energy-balance approach) were used to predict both the time of dew onset a nd the duration of dew on the ball. These predicted values agreed well with the observed results. Dew duration and time of dew onset on the ball were well correlated. During most nights, dew began to form on the top leaf surf aces at almost the same time as it reached the zenith angle of 60 degrees o n the ball. A linear relationship was found between the observed daily dura tion of dew on the top leaf surfaces of the rice crop and the estimated tim e when dew reached the zenith angle of 60 degrees on the ball. This relatio nship gave as accurate an estimation of dew duration on the top leaf surfac e of the rice crop as did the Penman-Monteith combination equation. This st udy showed that the dewball is a potentially useful device for observing de w formation.