Characteristics of the water temperature of a paddy field were investigated in order to estimate the influence of canopy density on the water temperature. The water temperature of a paddy field was compared with that of a non-vegetated water surface under various weather conditions. The results are summarized as follows:
Sunny weather conditions:
(1) The daily mean water temperature depended on the wind speed and the canopy density (leaf area index including leaf sheaths, LAI) under sunny weather conditions. Under the strong wind conditions (daily mean value > 1.7 ms-1 at a height of 2.5 m above ground), the water temperature of the paddy field (WTP) was 0 - 1°C higher than that of a non-vegetated water surface (WTNP) for 0 < LAI < 2.0, while WTP was lower for LAI > 2.0. Under weak wind conditions ( < 1.7 ms-1 ), WTP monotonously decreases with increasing LAI for all LAI ranges.
(2) The daily minimum WTP was about 1°C higher than WTNP for LAI > 0.6 regardless of the strength of wind. This is mainly due to nocturnal radiational cooling from the water surface being reduced by the greenhouse effect of the plant canopy.
(3) The plant canopy has two main effects on the daily maximum WTP.Firstly, solar radiation at the water surface is reduced by the canopy, resulting in the decrease in WTP. Secondly, turbulent transfer between the water surface and air is depressed in the canopy, which has an opposite effect. Under the strong wind conditions, turbulent transfer is reduced to some extent in spite of smaller reduction in incident solar radiation when LAI < 1.4, which leads to higher WTP compared to WTPN. Under weak wind conditions, the reduction of turbulent transfer does not occur since turbulent transfer is almost independent of the wind speed. Thus, WTP is lower than WTPN for all LAI ranges under such conditions.
Cloudy weather conditions:
(4) The temperature difference between the water surface and air was smaller under the cloudy weather. There were no significant differences between the WTP and WTNP for all LAI ranges.