It must have been one of the largest in the Roman world.The aqueduct is thought to have been built during the Flavian dynasty, from the second half of the first century to the early 2nd century CE under the Emperors Vespasian and Trajan in order to carry the water of the River Acebeda to the city. http://www.spain.info/en_IN/que-quieres/arte/monumentos/segovia/acueducto_de_segovia.html