Abstract
An isopleth chart for P/PETt of the Eurasian continent was drawn to clarify general patterns and tendencies using climatic normals of monthly precipitation and temperature from 1961 to 1990. The P/PETt relates annual precipitation and annual sum of monthly potential evapotranspiration calculated by Thornthwait's formula (1948). The method may be dated but is still effective for estimating the climatic vegetation indexes, because that requires only annual precipitation and monthly air temperature. General patterns in the isoplethes of P/PETt agreed well with those in some other climatic charts, such as the rain factor by Lang (1920) and aridity index by Martonne (1926). A detailed isopleth chart of P/PETt was then redrawn for the steppe and surrounding areas of Inner Mongolia with 0.1 intervals of P/PETt, where the same data as in Fig.2 were used. Isopleth patterns in the detailed chart were similar to those in the chart drawn by Qion and Line (1965) based on Penman's formula for China. Finally, arid limits for the following vegetation zones were determined based on P/PETt by comparing the chart with the map of grass land types in Inner Mongolia (Li, 1991): forest 0.9, forest steppe 0.6, typical steppe 0.4, desert steppe 0.3, steppificated desert 0.2, and desert under 0.2. Here the three zones of P/PET<0.5 are the subdivisions of the desert.
