Studies on the Windbreak Nets

Abstract

An observation was carried out at a bare ground field in winter and spring in order to make clear the effect on a micrometeorological modification and reduction of wind erosion. The main results were obtained as follows:(1) The minimum relative wind speed (ur) of horizontal variation at the height of 0.5H (height represented as a multiple of net height H=2.0m) was same 73%, and was recognized at the distances of 1H and 3H (distance expressed in net height and negative sign denoting windward and positive leeward) for a strong and oblique wind (12m/s at 6m height, incident angle 65°), and a weak and orthogonal wind (3m/s at 6m height, incident angle 0°), respectively. Under the condition of strong-oblique wind, both the reduction and recovery of wind speed, and appeared point of the minimum ur were faster than those of weak-orthogonal one. The lower the observation height was, the larger the tendencies of reduction, recovery and appearance mentioned above were. The ur was recovered by the distance of 20H for the strong-oblique wind, on the other hand, it was still reduced over 20H for the weak-orthogonal one (Fig. 1).(2) The ur of vertical profile was larger for the weak-orthogonal wind than that for the strongoblique wind at the lower level of the profile in the windward. After passing through the net, the reduction rate of ur was relatively high for the weak-orthogonal wind and its profile pattern was similar. At the height of 2H, both the winds took the similar value of 100% and those fluctuations were small (Fig. 2).(3) A surface soil temperature was increased from -5 to -1H and from 1 to 25H in the daytime on a clear-fine day and the maximum temperature 3.2°C at 2.5H. On the other hand from -0.5 to 0.4H, the temperature was lower than that at the reference point by reason of high wind speed, shade and increasing soil moisture.In the nighttime on a clear-fine day, increasing temperature or preventing low temperature was found at -2 to 4H. Increasing temperatures were about 2°C near 0H and about 2.5°C at 1 to 2H, because that soil solidity was appeared by wind erosion under the net and by tread, respectively and that soil freezing was also made by increasing of soil moisture. The low temperatures from 5 to 25H in the leeward and at a certain part in the windward were based on the radiation cooling by decreasing wind speed.Increasing temperature was found 0.2 to 0.3°C at -4 to 1H and 1 to 10H in night on a rainy day, and decreasing one slightly near the region under the net. It was recognized that the temperature was densely related with the roughness of soil surface and the dry or wet condition of soil (Fig. 3).(4) The temperatures of air, surface soil and soil layer down to -4cm depth were lower in the nighttime in the protected area. In the daytime, those of air, surface soil and soil layer except a certain region between 0 and -15cm depth were higher. All daily mean temperatures of air up to 1m height, surface soil and soil layer down to -30cm depth were higher, too (Fig. 4).(5) In the protected area, relative humidities in the daytime and nighttime were equal or higher up to 2%, and daily amount of transpiration was 3.4mm/day and 0.4mm/day smaller than that in the exposed area.(6) A wind erosion of soil was recognized at the bottom of the net, a projection area and both sides of hollow, and an accumulation of soil, at a hollow (12-13H). These actions consequently leveled the soil surface. The accumulations of soil by wind from 3 to 8H and at -2H were higher than that in the exposed area. At both areas of -10H and 20H, mulberry stocks and shoots reduced a little wind erosion. However, it is generally considered that the region