What extensive flatlands cover most of nevada and are nearly surrounded by mountains?

The answer is The Great Basin. For the most part, its eastern limit is the Wasatch Mountains of Utah and the western edge is framed by the Sierra Nevada and Cascade Mountains that make a rain shadow over a great part of the Great Basin, keeping numerous Pacific tempests from achieving the district. 
The Great Basin Desert, the biggest U. S. leave, covers a dry span of around 190,000 square miles and is circumscribed by the Sierra Nevada Range on the west and the Rocky Mountains on the east, the Columbia Plateau toward the north and the Mojave and Sonoran betrays toward the south.

Answerequatorial guinea,gabon, and western sahara;

Answer the use of flood plains and irrigation;