Effects of double line windbreaks on climatic elements, sand accumulation and crop are evaluated at arid or dry land of Turfan, Northwest China. Main results are as follows:
(1) The effects on wind speed, temperatures and humidity are accumulated by the increase of windbreak line.
(2) The increase and decrease of surface soil temperature are small by shade of sunshine in the daytime and by small radiation cooling in the nighttime based on windbreak, respectively.
(3) Relative humidity increased from 10 to 15% based on effects of transpiration from tree leaves and of evaporation from soil, particularly at weak wind night in summer. The effect is accumulated by increases of windbreak line and width.
(4) Variations of air temperature and humidity seem to be generally symmetrical patterns in particular, their characteristics and frequency are significant and high under the dry condition. The increasing and decreasing effects on air temperature by windbreak are based on decreasing wind and use of latent heat of transpiration, respectively, therefore, the effects are canceled each other and patterns are not changed in appearance.
(5) The climatic improvement by windbreak is very important at dry land. As windbreak trees seem to be streamline shape in relation to strong wind, dryness, sand accumulation, etc., the recover of wind is fast and the effective region is relatively small. The construction of ideal-shape windbreak needs earnest effort under the severe climatic condition.
(6) The fine-dense leaves and branches of tamarisk are effective on sand accumulation, and preventions of wind erosion and drifting sand. As the elongation of tamarisk is large, the tree can grow even if it is covered by sand. Tamarisk is suitable for windbreaks at dry land.
(7) Though the maximum effect on crop height is found from 2 to 5 H (maltiple distance of windbreak height) and the effect destroys relatively fast under the severe dry condition, windbreaks are indispensable to crop cultivation in dry land.
