 About 82 percent, , of its basin is in Spain, while about 17 percent, is in Portugal.The exact source of the river in Castilla-La Mancha is disputed, but it is generally believed to spring in the Ojos del Guadiana, Villarrubia de los Ojos municipal term, Ciudad Real Province, Castile–La Mancha, about in elevation.A classic theory introduced by Pliny the Elder, was that the river originated from the Lagunas de Ruidera and divided into two branches