Comparison of the Evapotranspirations among a Paddy Field, Cassava Plantation and Teak Plantation in Thailand

Evapotranspiration (ET) is an essential factor for estimating crop water use. It is also one of the factors in soil water storage and water resource in a region. This study is to compare the trend of actual evapotranspiration (AET) and crop factor (Kc) among three measurement sites in Thailand that are a rain-fed paddy field, cassava plantation and teak plantation. Bowen ratio energy balance technique (BREB) was applied in this investigation to measure the AET and heat flux between ground surface and atmosphere. The Penman-Monteith equation recommended by FAO was used to calculate crop coefficients. Measurements were carried out during 1999-2003 in the paddy field, 1999-2002 in the teak plantation and 2002-2003 in the cassava plantation. Results showed that the amount of daytime AET during the rainy season in the paddy field and cassava plantation varied between 1 and 7 mm/daytime and in the teak plantation between 2 and 6 mm/daytime. The average amount of daytime AET in the rainy season was about 4 mm in all sites although the variations of AET were different. In the dry season, AET of the teak plantation was around 3.2mm/daytime, slightly higher than those of other sites. During the growing season, the Kc factor varied in the paddy between 0.4 and 1.2 and in the cassava between 0.3 and 1.2. Variation of the Kc factor in the teak plantation was smaller than those of the others. In the rainy season, the LE/Rn ratio in the cassava plantation and the paddy field was around 70% but in the teak plantation it was about 77%. The Kc factor in the paddy field was more affected by soil moisture than at other sites.