Massive expansion describes significant growth or increase in size, scope, or extent. In the context of this question, it refers to a substantial increase in irrigated land across certain regions in western United States during the 1900s.
Related terms
Boomtown: A rapidly growing town due to sudden economic prosperity, often associated with resource extraction.
Urbanization: The process of population growth and physical development leading to more urban areas.
Industrialization: The transition from an agrarian economy to one based on manufacturing and industry.