摘要
Based on the conception of fluid mechanics,the paper propounds a model for monthly potential evapotranspiration ETi(mm):ETi=22di(1.6+Ui^1/2)woi(1-hi)/Pi^1/2(273.2+ti)^1/4where i is the number of a month,Pi the mean monthly atmospheric pressure (hPa),ti the mean monthly air temperature(℃),di the number of days in the month,Ui the mean monthly wind velocity measured at height 10-12m(m/s),woi the saturated water vapour pressure at ti(mmHg,1mmHg=133.322 Pa),and hi the mean monthly relative humidity.The annual aridity K is:K=12∑i=1 ETi/ra 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 as follows:K Soil moisture regime Vegetation K<1 Udic FOrest 1≤K<2 semi-udic Froest,froest-steppe and steppe 2≤K<3.5 Semi-aridic Arid steppe 3.5≤K<11 Aridec Desert-steppe K≥11 Very aridic Desert.
