Evapotranspiration from Paddy Field

Abstract

During the growing season (June 26 to Oct. 6, 1965) of rice plants, as part of a wide-ranging study of problems on water balance in river basins, the N.I.A.S. Division of Meteorology made measurements of meteorological elements in the surface air layer over a paddy field at Konosu, Saitama, in order to estimate the evapotranspiration from a paddy field. The data are first summarized in hourly mean and hourly total values. The evapotranspiration was estimated by processing the data by heat balance, combination and gradient (bulk) methods. respectively.
The radiative dry index (=ΣS/Σlr) for the period of rice growing was found to be 0.89, implying that the climatic condition during the growing season was rather humid. The ratio of total evapotranspiration to total rain fall evaluated from the value of radiative dry index was 0.72 and was in accordance with the measured value (0.73). The total evapotranspiration from the paddy field was respectively estimated as 309mm by heat balance method, 347mm combination method and 317mm bulk method. These values were smaller than the total evaporation (379mm) measured by a small evaporimeter (diameter, 20cm) in a near enclosure. The net radiant equivalent (=ΣS/l, 378mm) was in good agreement with the pan evaporation.
Mean value of the ratio (=ΣE_T/ΣE_p; E_T, evapotranspiration by heat balance method; E_p, pan evaporation) was found to be 0.82 and to be somewhat small comparing with those (1.0-1.2) reported by several researchers. Our result indicates that, for the growing period of rice plants, (1+β) is constantly larger than 1.0 and that there is a some sensible heat loss. Dependence of evapotranspiration (E_T) on net radiant equivalent (S_*=S/l) was approximated by
E_T=0.82S_*.
The value of a proportional constant was in accordance with those obtained at irrigated ryegrass fields and paddy fields. This seems to indicate that the adverted heat does not contribute a significant part of the latent heat used in evapotranspiration from wide paddy fields under moist climatic conditions like summer in Japan. The following relation was obtained between E_c (evapotranspiration estimated by combination method) and E_T:
E_T=0.92E_C.
The difference between E_T and E_C may be due to not taking into account the effect of dryness of the plant canopy on evaportanspiration in the calculation.
It can be concluded from results presented above that the heat balance method and the combination method are suitable for estimating evapotranspiration from paddy fields. However, it is much desirable to compare evapotranspiration estimated by these methods with that determined directrly by lysimeters or an evapotron.