(1) Graduate School, Kasetsart University, Bangkok 10900
(2) Department of Conservation, Faculty of Forestry, Kasetsart University, Bangkok 10900
(4) Faculty of Agriculture, Tokyo University of Agriculture and Technology, Tokyo
Diurnal variation of latent heat (lE) in the paddy field in Sukhothai province, Thailand, during rice planting season in 1998 was estimated based on Bowen ratio method. Net radiation (Rn) was measured by a net radiometer while 3 soil heat flux plates observed soil heat flux (Gs). The fluctuation of water level (WL) in the paddy field was also measured. So that heat storage in the water (Gw) can be determined. The result indicates the lE value during rice planting season ranged from 8.1-12.6 MJ m-2 day-1 with in average 10.5 MJ m-2 day-1, or about 69.1 % of Rn. The positive value of lE start to occur at around 6.30 hrs. with a value about at 18.44 W m-2 and gradually increasing to the peak at around 13.00 hrs. with averaged value of 456.0 W m-2. It was then dropped continuously and reaching to negative value at about 18.30 and prevailed to about 21.00 hrs. While Rn negative value around 17.30 hrs. This phenomenon showed that the occurrence of lE after negative Rn value is not dependent on Rn but depending on Gw. The lE occurred after negative Rn was 14.5 % of daily lE or equivalent to Evapotranspiration (ET) at that time of about 3.01 % of daily ET. While the water temperature (Tw) when ET stopped was about 32.8 OC by averaged. There is no significant relationship between LE (or ET) and WL in the paddy field.
