The territory of Castilla–La Mancha is divided into five principal watersheds. The Tagus, Guadiana, and Guadalquivir drain into the Atlantic Ocean and the Júcar and Segura into the Mediterranean Sea. The Tagus provides water for some 587,000 inhabitants in a watershed of 26,699 square kilometres (10,309 sq mi). It includes the entire province of Guadalajara and the greater part of the province of Toledo, including the two largest cities of the latter province: the capital, Toledo, as well as Talavera de la Reina.

The Guadiana watershed extends 26,646 square kilometres (10,288 sq mi) in Castilla–La Mancha, 37 percent of that river’s entire watershed, with a population of 583,259 inhabitants. It includes the southern part of the province of Toledo, nearly all of the province of Ciudad Real (except the very south), the southwest of the province of Cuenca and the northwest of the province of Albacete. The Guadalquivir watershed extends over 5.2 percent of the surface area of the autonomous community, extending 4,100 square kilometres (1,600 sq mi) through the southern parts of the provinces of Ciudad Real and Albacete, including such important population center as Puertollano.

The Júcar watershed had, in 2006, 397,000 inhabitants in an area of 15,737 square kilometres (6,076 sq mi), 19.9 percent of the Castillian-Manchegan territory and 36.6 percent of total of the Júcar watershed. It includes the eastern parts of the provinces of Cuenca and Albacete, including their respective capitals. Finally, the 34 municipalities of southeastern Albacete fall in the Segura watershed, with an extent of 4,713 square kilometres (1,820 sq mi).