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.