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米脂地处我国黄土高原腹部的丘陵沟壑区,是全国水土流失120多个重点县之一。该县水土流失面积占全县总面积98%,土壤侵蚀模数2万吨/平方公里/年,年降水量为451.6毫米。这样的降水量按理基本上足够一般林木生长的需要。然而,三十年来的造林实践告诉我们,在梁、峁、谷坡地上的成片林,大多出现“小老头树”,其中一个最主要的原因是,林地缺水,林木生长不良。这里的林地大部分都在30°以上的坡地上,降水损失较大。据测定,黄土丘陵区大于30°的坡面降水流失量为降水总量的1/2。米脂7～9月份的降水量占全年降水量的65%,而3～6月份林木生长期内的降水量仅占年降水量的25.5%。1978年8月29日一小时的降水量达45.1毫米,1961年9月27日日
Rice fat is located in the hilly and gully areas of the abdomen of the Loess Plateau in China, and is one of over 120 key counties in the country. The county’s total area of soil erosion and soil erosion accounted for 98% of the total area, soil erosion modulus 20,000 tons / km 2 / year, the annual precipitation is 451.6 mm. Such precipitation is basically sufficient for the general needs of forest growth. However, the practice of afforestation over the past 30 years tells us that most of the “small old trees” are found in the patches of wood on the Liang, Tuan and Gu Po slopes. One of the major reasons for this is the lack of water in the forests and the poor growth of trees. Most of the woodlands here are on slopes above 30 °, with a large loss of precipitation. It is determined that the amount of rainfall on slope above 30 ° in the loess hilly region is one half of the total precipitation. The precipitation of rice fat from July to September accounted for 65% of the annual precipitation, while the precipitation in the forest growth period from March to June accounted for only 25.5% of the annual precipitation. On August 29, 1978, the hourly precipitation reached 45.1 millimeters, on September 27, 1961