摘要
Based on the conception of fluid mechanics, the paper propounds a model for monthly potential evapo-transpiration E<sub>Ti</sub> （mm）: where i is the number of a month, P<sub>i</sub> the mean monthly atmospheric pressure （hpa）, t<sub>i</sub> the mean monthlyair temperature （° C）, d<sub>i</sub> the number of days in the month, U<sub>i</sub> the mean monthly wind velocity measured atheight 10-12 m （m/s）, w<sub>oi</sub> the saturated water vapour pressure at t<sub>i</sub>（mmHg, 1 mmHg=133.322 Pa）, and h<sub>i</sub>the mean monthly relative humidity. The annual aridity K is:where ra is the mean annual precipitation （mm）. Based upon the data of 669 sites in China during 1951-1980, the relations among the soil moisture regime, the vegetation and the value of K may be illustrated asfollows:K Soil moisture regime VegetationK 【 1 Udic Forest Semi-udic Forest, forest-steppe and steppeSemi-aridic Arid steppe Aridic Desert-steppe Very aridic Desert
