; purchase and erection or alteration or improvement of permanent and temporary buildings; and operation and maintenance of aircraft, $858,911,000, to remain available until September 30, 2019: Provided , That appropriations hereunder shall be available pursuant to 7 U.S.C. 2250 for construction and improvement of buildings and public improvements at plant materials centers, except that the cost of alterations and improvements to other buildings and other public improvements shall not exceed $250,000: Provided further , That when buildings or other structures are erected on non-Federal land, that the right to use such land is obtained as provided in 7 U.S.C. 2250a . Watershed and flood prevention operations For necessary expenses to carry out preventive measures, including but not limited to surveys and investigations, engineering operations, works of improvement, and changes in use of land, in accordance with the Watershed Protection and Flood Prevention Act ( 16 U.S.C. 1001–1005 and 1007–1009) and in accordance with the provisions of laws relating to the activities of the Department, $40,000,000, to remain available until expended: Provided , That of the amounts made available under this heading, $20,000,000 shall be allocated to projects and activities that can commence promptly following enactment; that address regional priorities for flood prevention, agricultural water management, inefficient irrigation systems, fish and wildlife habitat, or watershed protection