This bill proposes an amendment to the Colorado River Basin Project Act, specifically targeting water distribution among the lower basin states during periods of scarcity. It mandates that whenever the Secretary determines a shortage or reduction in mainstream Colorado River water for the Central Arizona Project , all consumptive diversions by Arizona, California, and Nevada must be reduced. These reductions are to be implemented on a pro-rata basis , directly proportional to each state's base annual apportionments. A significant change introduced by this amendment is the explicit removal of any preference for present perfected rights during these shortage declarations, ensuring that all diversions are treated equally under the pro-rata reduction scheme.