How did farmers supply water to their crops in central asia

Supply of water to crops in Central Asia by farmers:

The major portion of the agricultural land in central Asia is desert areas. The water supply is carried out from the rivers and the melting of glaciers.Syr Darya and Amu Darya Rivers is the two most important rivers for the farming in central Asia along with the mountain ranges Pamir, Hindu Kush and Tien Shan.The snow melts and the glacier's melted water from these ranges is supplied to the crops in central Asian region of farming.

Ibelieve the answer would be a large-scale natural disaster