Abstract: In this paper calculated the evapotranspiration of trees by climatological method during their growih season in loessial Plateau Gansu. The result showed that the calculated value was coincided with the real value. Based on the water consumption index of arbor during its growing season and the observed data of rainfall, water loss and soil erosion, the moisture condition of arbor in Loessial Plateau, Gansu is assessed in detail. Finally, the author suggests the concept of the "actual degree of aridity" and the "actual degree of wettability". According to the latter determining which kind of trees are suitable to plant in various kinds of climate-plantcover regions, and under various topographic features what arbor and bush should be selected.

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