Observations on dew formation in the rice canopy and its simulation using a multilayer microclimate model

Dew formation in the rice canopy was directly observed to assess its actual nature and was simulated using a multilayer microclimate model to understand the relationship between the dew amount and atmospheric conditions. Observations were made on four nights during August and October in a paddy field in Kumamoto, Japan under warm temperate climate. The vertical profile of the dew amount in the canopy was measured every 2 h at 0.2 m intervals. The maximum dew amount was recorded in the early morning on all observation days. The total dew amount for the whole canopy at 6:00 on August 19, 21, and 24, and on October 1 was 0.10, 0.07, 0.15, and 0.23 mm, respectively. The dew amount determined from the simulation showed a good agreement with the actual observations. The dew amount per unit leaf area was larger in the upper layers than in the lower layers of the canopy on all days. This vertical difference in the dew amount was explained by the difference in effective radiation (net longwave radiation). The vertical gradient of the dew amount was steep on August 19 and October 1, whereas it was gentle on August 21 and 24. This difference in gradient was explained by the difference in paddy water temperature. When the water temperature was higher than the air temperature, the vertical gradient of the effective radiation and dew amount became steeper. The total amount and source of dew also varied with water temperature. The dominant source of dew was dewfall (water flux from the atmosphere) at lower water temperature, whereas it was distillation (water flux from the ground surface) at higher water temperature. From these results, we conclude that water temperature plays an important role in dew formation in the rice canopy.