San Diego — The United States and Mexico are rewriting rules on how to share water from the Colorado River, capping a five-year effort to form a united front against future drought in their western states.

The far-reaching agreement to be signed Tuesday gives Mexico rights to put some of its river water in Lake Mead, which stretches across Nevada and Arizona, giving it badly needed storage capacity. Mexico will forfeit some of its share of the river during shortages, bringing itself in line with western US states that already have agreed how much they will surrender in years when waters recede.

Water agencies in California, Arizona. and Nevada also will buy water from Mexico, which will use some of the money to upgrade its infrastructure.

The agreement, coming in the final days of the administration of Mexican President Felipe Calderon, is a major amendment to a 1944 treaty that is considered sacred by many south of the border. The treaty grants Mexico 1.5 million acre-feet of river water of water each year — enough to supply about 3 million homes — making it the lifeblood of Tijuana and other cities in northwest Mexico.

Mexico will surrender some of its allotment when the water level in Lake Mead drops to 1,075 feet and reap some of the surplus when it rises to 1,145 feet, according to a summary of the agreement prepared by the Metropolitan Water District of Southern California, which will buy some of Mexico's water.

The agreement expires in five years and is being billed as a trial run, potentially making it more palatable in Mexico.

"These are big political steps for Mexico to take," said Jeffrey Kightlinger, Metropolitan's general manager. "Chances are we won't have a surplus and we won't have a shortage but, if we do, we'll have the guidelines in place on how we're going to handle the situations as they come about."

In 2007, facing an eight-year drought, California, Arizona, and Nevada agreed on how much each state should sacrifice during shortages on the 1,450-mile river that flows from the Rocky Mountains to Mexico. That same year, the US and Mexico promised to work on ways to jointly address shortages.

The negotiations gained a sense of urgency for Mexico in 2010 after a magnitude-7.2 earthquake damaged canals and other infrastructure, forcing it to store water temporarily in Lake Mead, the body of water held by the Hoover dam

"They have some storage but it's not enough for drought and emergencies," said Halla Razak, Colorado River program director at the San Diego County Water Authority.

Roberto Salmon, Mexico's representative to the International Boundary and Water Commission, was scheduled to attend a signing ceremony in Coronado, near San Diego.