As explained above, the water that flows out of river basin determines the volu me of po-tential water resources available to humankinds. The runoff ratio that determines the runoff volume changes significantly depending on the climatic c onditions of the river basin. Runoff ratio can be approximated by the following hydrological relation.

The above equation shows that runoff ratio decreases rapidly from values close to 1.0 in humid regions (RDI《 1.0) to values close to 0 in dry regions (RDI》 1.0).

Geographical distribution of annual RDI obtained from meteorological data from around the world is shown in Figure 5. Extremely dry regions with RDI 2.0 or mo re exist in the subtropical high zones around 20 degrees latitude of the both h emispheres. Humid zones of RDI < 1.0 exist in equatorial regions where intertro pical convergence zone de-velops, regions where extratropical cyclones develop and regions facing the Arctic Ocean. The fact that runoff ratio in these region s is estimated to exceed 0.6 clearly indicates that the area of humid regions i s small in the vast land.