Heat Exchange in a Paddy Field

In order to see characteristics of the heat exchange in paddy field, micro-meteorological observation was made in and above the field, and heat balance was estimated at the upper surface of plant layer and soil surface.Heat balance at the upper surface is represented by the equationR1=F1+l·E1+G+l·Z·dχ/dt+M·C·dθ/dt+λ·P(1)where R1 is net incoming radiation, G heat flux into the soil, F1 sensible heat flux, E1 water vapour flux, χ absolute humidity in the field, l latent heat of evaporation (590cal·g-1), Z height of crop (80cm), θ temperature in the field, M·C heat capacity of plant layer, and P CO2 flux (λ=2500cal·g-1). R1 was measured by use of Beckman net-flux radiomenter, and G was estimated with temperature in soil. Asuming a plausible value of P, inferred from a relationship between P and solar radiation given by MATSUSHIMA, F1 and E1 were calculated from equation (1) (refer to “Turbulent Transfer in The Lower Atmospher” by C.H.B. PRIETLEY, p. 91). In fig. 5, estimated values of the flux and the transfer coefficient at 120cm above the ground are shown, where transfer coefficient is calculated from F1 and a gradient of temperature at the level.The heat balance at the soil surface under plant layer was estimated in the same manner as above from equationR2=F2+l·E2+G (2)where net long wave radiation at soil surface was roughly calculated from equation (3) in terms of soil surface temperature T0 and temperature. T at the level 60cmR2l=0.63(σT4-σT04) (3)Estimated values of the flux in eq. (2) and transfer coefficient at about 7cm. above the soil surface are shown in Fig. 5. As shown in Fig. 6 the change of the flux and the transfer coefficient with respect of hour present a great contrast to those at the upper surface. From these results we may infer a dirnal variations of heat exchange in the plant layer.From eq. (1) and (2), the following equation is obtained(R1-R2)=(F1-F2)+l·(E1-E2)+l·Z·dχ/dt+M·C·dθ/dt+λ·P (4)where (R1-R2) is net radiation absorbed by the plant layer, and (E1-E2) is the transpiration. Thus, transpiration of the plant layer can be estimated through a principle of energy balance. Table 2 show the percentage of each term in eq. (4).By use of values shown in Fig. 5, a relationship between the evapotranspiration ∑l·E1 and total net radiation per day ∑R1 is obtained as∑l·E1=0.93∑R1 (5)which is in agreement with that obtained with a lysimeter by PRUITT·ANGUS (1961).