Among the synthetical methods of representation of climate, there is an attempt to represent it by the water balance problem on each region, that is, to represent climate by the aridity of the area. We can understand that on considerably extensive area, the amount of water flows in is equal to that of flow out. Then the regional aridity can be considered as the difference of amount of precipitation and evaporation in that area. This amount is termed the effective precipitation. In these.cases, the amount of precipitation is measurable anyway, but the amount of evaporation from a total area can not be measured exactly in ordinary way. Therefore the latter is only estimated from other various amounts easily measurable. There are following ways to estimate the regional evaporation amounts:
1. Evaporation amount from evaporimeter…… Transeau (1905)
2. Deficit of saturation……Meyer (1926)
3. Air temperature……Lang (1920) ; Rain factor, Koppen (1918-1931); Arid boundary, de Martonne (1926) ; Aridity index, Thorn-thwaite 1931) Preipitation effectiveness index (precipitation-e vaporation index or P-E index), Ångström (1936); Coefficient of humidity, Hythergraph.
4. Air temperature, vapor tension and atmospheric pressure…… Szymkiewicz (1925)
5. Amounts of discharge in river……Wallén (1927) Using some of these indexes, distribution maps of precipitation effectiveness in Japan have been drawn (Figs. 1-4). But the types of distribution are similar to each other. This is why, these indexes come from the same back ground of idea, that is, regional water balance or degrees of regional aridity and humidity. Up to the present, these indexes have been considered very important in climatology as purely climatological meaures, and it has been understood to calculate these indexes and to discuss on their geog-raphical distribution is the climatology. But from above saying, it will be clear -that these considerations are not true.

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