Average runoff ratio = Qr/P = 0.44 ・・・・・・・・・・・・・・・・・・・・・・・・・・・・・(1)

Whereas, Qr is annual runoff volume from the continent to the ocean and P Is annual precipitation amount on the continent

This value Is only 68% of the value for a pluvial country such as Japan (0.65). Forty four percent of all precipitation turns into maximum freshwater resource s that can be utilized ultimately (renewable fresh water).

Such low runoff ratio is brought about by the hydrological condition of the int erior re-gions of the Asian Continent, particularly the characteristics of geog raphic distribution of evaporation. Changes in latitudinal average of the ratio between potential evapotranspi-ration (Eo) and actual evapotranspiration (Ea) in the Asian Continent are as shown in Figure 4. The difference in Eo and Ea be tween high latitude and low latltude regions is small by reflecting the prevail ing wet climate and Ea/Eo ratio is higher than 0.7. However, the difference bet ween Eo and Ea becomes significantly large in reglons falling within 10 and 40 degrees north latitude because of the subtropical high region and dry inland cl imate and Eo/Ea ratio drops to below 0.4. Eo indicates the maximum evapo-transp iration (potentlal evapotranspiration) which is determined solely by climatic c ondi-tions of each region and reaches 1,800mm/year in subtropical high zone of the 20 degrees north latltude. Rapid decrease of Eo with increase in latitude i s attributable to the de-crease in global solar radiation and consequenty net s olar radiation.

Figure 4 Changes in latitudinal average of the ratio between potential evapotra nspiration (Eo) and actual evapotranspiration (Ea) on the Asian Continent