U.S. officials for the first time on Monday issued an official water shortage declaration for the massive Western reservoir of Lake Mead, triggering supply cuts to parts of the drought-stricken Southwest.

The shortage will reduce water apportionments to Arizona, Nevada and Mexico for the year beginning in October, the U.S. Bureau of Reclamation said in a statement.

Arizona will lose 18% of its annual apportionment, while Nevada will see cuts of 7%. Apportionments to Mexico, which are required under a 1944 treaty, will be cut by 5%.