風土としての日本の水―自然地理の視点から―

Abstract

This article summarizes the nature and the behavior of waters in Japan from the viewpoint of physical geography. Because Japan is located in a humid temperate zone lacking a severe dry season, a water deficit is not detected in normal years in terms of water balance analysis. Mean annual specific discharge is between 3 m3/s/100 km2 and 5 m3/s/100 km2 in many river basins, although it is more than 5 m3/s/100 km2 in rainy or snowy regions. Snow is an important source of rivers in Japan.Lakes are relatively abundant. Basin profiles are characterized by steep slopes, reflecting the general landform of Japan. Lakes of volcanic origins and coastal lakes are widely distributed. Strong thermal stratification is also a characteristic of lakes. There are about 210, 000 irrigation ponds, which is one of the special features in Japan.Heavy snowfalls in mountain ranges in northern Japan create perennial snow patches. As summer is very hot, the occurrence of snow patches needs an accumulation of snow of more than 20 m.Groundwater is also abundant, especially at flood plains, alluvial fans, and bases of large volcanoes. On plains where rice paddies are distributed, the groundwater level rises during the irrigation period due to seepage. Groundwater is also influenced by human activities as rivers and lakes.