National Geographic : 1952 Aug
274 John Gutman, Iix Southern China's Bamboo "Ferris Wheels" Raise Water to Thirsty Rice Fields Drought and famine always at his heels, the Chinese farmer fights to conserve moisture. He and his wife and sons may spend long hours pedaling treadmills to lift water a few inches from canal to irrigation ditch. Brush dams repaired by these Yunnanese boatmen channel river current so that it turns the wheels. Bamboo scoops catch water and spill it into troughs 60 feet high. Africa's Sahara, synonymous with desert, forms part of the largest arid region in the world, a vast, nearly rainless belt that ex tends across Africa and continues through Saudi Arabia, Iran, Turkistan, and Mongolia almost to the Pacific shores of Asia (p. 281). Irrigation Turns Deserts Green Nearly a third of the earth's land surface receives 10 inches of precipitation or less an nually. For another third the usual amount is only 10 to 20 inches. In places that get less than 10 inches, and on much of the area re ceiving up to 20 inches, irrigation is required for raising crops. For maximum yield, even areas having 40 inches or more annually may need the magic touch of water sluiced through ditches if there are long dry periods during the growing season or if the climate is hot. In the United States east of the 100th me ridian, which divides the Nation almost ex- actly in half, there is generally enough rain fall.* Precipitation increases from 20 inches a year on the Great Plains to 50 or 60 on the Gulf coast. West of the meridian, comparatively speak ing, is where America's water troubles begin. Except in the rain-rich Pacific Northwest, much of the West is arid unless irrigated. Large parts of Utah and southern California could support only a sparse population if it were not for irrigation. Nearly 25 million acres are irrigated in the 17 western States. Why do places like Washington's Olympic Peninsula get so much rain, while our Great Plains get comparatively little? Washington, Oregon, and northern Cali fornia are visited by prevailing westerly winds heavy with moisture from the Pacific Ocean. When these wet winds reach the mountains, * See map, "The United States of America," a sup plement to the June, 1951, NATIONAL GEOGRAPHIC MAGAZINE.