According to the THORNTHWAITE's idea. on the potential evapotranspiration, it has been calculated total amounts of actual evaporation and transpiration from extensive land areas on various localities in Japan. Distribution of annual amounts is shown in the Fig. 1. From this figure, we can see that regions where evaporate and transpire more than 850mm annually, are found on the South Seaside. And isopleth of 800mm runs across the Honshu nearly along the Parallel of 35 N. Amounts of the Sanin nd the Kwanto Basin are 750-800mm, and those of the Central Highland and the Tohoku are less than 700mm.
If we compare these amounts with those of ordinal evaporation which are measured by atomometer at weather stations, ratios of actual to observed evaporation become mostly about 60-70%, and regional differences in this ratio can not be clearly distinguished each other. Frequencies of the ratios in each class are shown in the Tab. 1. Seasonal variation of the evapotranspiration is fairly systematic. The maximum amounts occur in summer and the minimum can be seen in winter in general., as is shown in the Tab. 2.
Geographical distribution of the evapotranspiration at various months are shown in the Figs. 2-4. Fig. 2 indicates the January situation. The larger amounts of this month are only about 10mm which are observed at the South Seaside region. Various places in Central Highland. and the Tohoku District present very` little amounts less than 0mm. The same pattern of distribution can be seen also October-April. Fig. 3 shows the situation of May. Fig. 4. shows the July situation. This is the summer type of distribution. The absolute values of evapotranspiration amounts are very large. We can observe more than 100mm almost all places. The larger amounts more than 160mm can be observed at the Kuyushu and the Setouchi Districts.
Then monthly ratios of the actual evaporation at several. places are shown in the Tab. 3. In winter their ratios are very small, but in summer they become very large. And the maximum values, of this ratios mostly observe in July or September. In these months on some localities, amounts of evaporation from extensive land areas (calculated) exceed those of evaporation from water surface being measuered by a atom-ometer.
