In Korea, there are five large rivers: Nakdonggang, Hangang, Geumgang, Seomjingang, and Yeongsangang (“ang”is the Korean word for rivers). Several mid- to small-scale rivers are also found in the country, including Anseongcheon (“heon”is the Korean word for mid to small-scale rivers), Sapgyochun, Mangyeonggang, Dongj ingang, and Hyeongsangang. In order to systematically manage river and water resources, the rivers have been divided into 117 sub-basins. Hangang has the largest drainage area of 35,770 km2 (including its portion in North Korea). It also has an annual runoff volume of 16 billion cubic meters, which constitutes 35.1% of the nation’ total runoff volume. The longest river in Korea is Nakdonggang, with a length of 510 km.
Soil texture is determined by the relative proportion of three kinds of soil mineral particles; sand (0.05 ? 2 mm), silt (0.002 ? 0.05 mm), and clay (particles smaller than 0.002 mm). Soil texture is often considered as one of most important attributes that controls the physical and chemical characteristics of soil.
On average, the soil in the paddy/farm areas of Korea is composed of 41.7% sand, 41.5% silt, and 16.8% clay. Major soil textures are moderate coarse sandy loam (44.5%) and netextured clayey loam (34.1%), and these soils cover up to 7.8% of the total land area. Gravelly soils are also observed in 5.9% of the total area. Paddies have the lowest percentage of sand in soil, followed by farms and forests. Soils with higher sand content are distributed throughout the mountainous regions from southern Gyeo- nggi-do to Chungcheongnam-do and Chungc- heongbuk-do.]