The water helps sustain the region’s half-million residents and ensures that Turpan’s family farms can grow the grapes that have shaped the city’s identity for centuries. Grape arbors grace nearly every home here, and the rural landscape is dotted with imposing brick-and-mud drying towers, where the grapes are turned into raisins.

Standing beneath a tangle of grape vines, Mijiti Saludin, 32, said he and his wife were forced to buy water from the municipal government after the karez across from their home ran dry several years ago. “We used to get it for free, but now we have to pay for our water and it isn’t very clean,” he said.

He led a visitor to an opening in the chalky gray earth, and into a tunnel just tall enough to crouch inside. When he was a child, he recalled, the entire community would turn out each spring to clean out the karez, using buckets to remove silt that impeded the flow of water. “Every family would send a young man, but now it’s hard to get people to work for free,” he said.

The Chinese government recognizes the threat to the region’s karez, and in recent years it has sought to ban the drilling of new wells that have contributed to a steady drop of the water table. In 2008, the regional government announced a $182 million project, funded in part by a loan from the World Bank, to protect and rehabilitate the system.

According to government estimates, the aquifer beneath the Turpan Basin shrinks by about three million cubic meters a year, much of it because of oil drilling and agriculture.