Final answer:
The main water sources in Northwest China are rivers and lakes, primarily the Yellow River and the Yangtze River.
Step-by-step explanation:
The main water sources in Northwest China are rivers and lakes. The Yellow River and the Yangtze River are the major river systems that provide fresh water to the agricultural regions of the central part of China Proper. Dams, canals, and irrigation projects along these rivers supply water for extensive agricultural operations, including the cultivation of wheat, sorghum, corn, soybeans, vegetables, fruits, and tobacco.