Southern California is imposing mandatory water cutbacks as the state tries to cope with the driest conditions it has faced in recorded history. Starting Wednesday, about 6 million people in parts of Los Angeles, San Bernardino and Ventura counties are limited to watering outdoor plants once a week — an unprecedented move for the region.

The Metropolitan Water District of Southern California, which supplies water to about 19 million people, declared a water shortage emergency in April and voted unanimously to curtail water use, either by restricting outdoor watering or by other means.

“Metropolitan has never before employed this type of restriction on outdoor water use. But we are facing unprecedented reductions in our Northern California supplies, and we have to respond with unprecedented measures,” Adel Hagekhalil, the district’s general manager, said in a statement. “We’re adapting to climate change in real time.”