The Bhumibol Dam, among others in the Chao Phraya basin, was constructed beginning in the 1950s to exploit the agricultural and hydroelectric potential of the basin. Construction on the dam began in 1958 and was finished in 1964. The reservoir was completely filled in 1970. The first two generators were commissioned in 1964. In 1972, the Sirikit Dam was completed on the Nan River, one of two major tributaries of the Chao Phraya including the Ping. The Bhumibol and Sirkit Dams control 22 percent of the Chao Phraya's annual runoff combined.[3] Both dams also help provide for the irrigation of 1,200,000 ha (4,633 sq mi) in the wet season and 480,000 ha (1,853 sq mi) in the dry season.[4]