Complex sentence: In Marrakech and the Haouz plain the qanats have been abandoned since the early 1970s as they've dried; in the Tafilaft area half of the 400 khettaras are still in use.
Split into simpler sentences: In Marrakech and the Haouz plain, the qanats have been abandoned since the early 1970s as they have dried. In the Tafilaft area, half of the 400 khettaras are still in use.