Since 1951, wind profiles over the cultivated field were observed, and now some empirical results are obtained from these data.
Six or ten small Robinson anemometers, and six aspiration type thermometers and psychrometers were used for the measurement of profiles. These instruments were installed between crop height and about 6m above the ground.
Among many data, those obtained in September are selected.
The wind profile over the paddy fields under the nearly neutral condition can be represented by
U(z)=u*/k(lnz-d/z0+βz-d-z0/L) (1)
where d is zero displacement, L stability length and β a constant (Ogura 1952, Monin and Obukhov 1954). It is known that d and z0 vary with wind speed U (Deacon 1949, Rider 1954). At first the observations made under almost neutral stability condition are selected, and d and z0 are determinded graphically. The relationship between wind speed at 1.5m above the ground and d is shown in Figure 1. It is divided into three regions as follows:
A. (0-2m/s at z=1.5m)d decreases and z0 increases, because light wind bends only the top of leaves and ears and can not penetrate into the space under the ear layer.
B. (2-4.5m/s) d increases and z0 decreases. Slightly stronger wind shakes leaves and stems of crop. The prevailing period of the fluctuating wind is close to the period of vibration of crop, and crops obstruct the penetration of wind.
C. (above 4.5m/s) When the very strong wind blows, the crop bends deeply and effective height of crop decreases.
The fact pointed by Deacon and Rider seems to corresponding to “B” region.
β in equation (1) was given 0.6 by Monin and Obukhov. However, these observations give 5.30 as a mean value. (Table 1).
Taking wind speed U, temperature θ, and absolute humidity χ at three height Z1=z1-d, Z2=z2-d, and Z4=z4-d, and 4Z1=2Z2=Z4, then β and L in equation (1) are eliminated and sensible heat flux F and vapour flux E can be obtained by following equations:
F=-cpρk2/(ln2)2{2(U2-U1)-(U4-U2)}{2(θ2-θ1)-(θ4-θ2)} and E=-k2/(ln2)2{2(U2-U1)-(U4-U2)}{2(χ2-χ1)-(χ4-χ2)} The results calculated with these equations are shown in Figure. 2.
