The Tuolumne Utilities District is urging its water customers to conserve the supply through Oct. 27.
The primary source, the Main Canal from the Stanislaus River, will be shut down for regular maintenance by Pacific Gas & Electric Co.
TUD will have to get by on small reservoirs, storage tanks and wells during the weeklong maintenance, plus another week when the system recharges.
Customers are asked to limit outside watering, including turning off sprinkler timers. They should avoid washing cars and driveways, and fix water leaks promptly.
TUD is the largest water supplier for Tuolumne County residents, relying on a ditch system that dates to the Gold Rush.