The United States and Mexico have reached an agreement on how to share the pain of future droughts on the Colorado River.

Under the five-year agreement, signed by U.S. and Mexican officials yesterday in San Diego, Mexico will siphon less water from the river during dry periods but will be permitted to store water in Lake Mead to use when there is a surplus of water in the system. Currently, Mexico has very little storage capacity.

Furthermore, the three lower basin states -- Arizona, California and Nevada -- will buy about 100,000 acre-feet of water from Mexico, which will provide enough water for 200,000 homes for one year. In all, Mexico will receive $21 million under the deal, which will go toward much-needed repairs to irrigation canals and other infrastructure damaged by an earthquake in 2010, allowing agricultural production to resume on thousands of acres of farmland that relies on river water.

At Mexico's insistence, the United States also pledged to purchase additional water to help restore the Colorado River Delta, parts of which have dried up over the decades due to diversions upstream.

Interior Secretary Ken Salazar said the agreement is the most significant Colorado River accord since the 1944 treaty that partitioned the river's water among the seven U.S. Colorado River Basin states and Mexico. Under the 1944 Colorado River Compact, which the agreement amends, Mexico receives 1.5 million acre-feet a year from the United States.

"We have signed a historic agreement," Salazar said during a news conference following the signing ceremony. "The U.S. and Mexico are connected by our reliance on the Colorado River. [The agreement] demonstrates that we're strengthening our relationships based on our common need for water."

Signatories included Salazar, Bureau of Reclamation Commissioner Mike Connor, International Boundary Waters Commission U.S. Commissioner Edward Drusina and Mexican Commissioner Roberto Salmon, and officials from the seven Colorado River Basin states' water organizations.

The agreement will expire in five years, at which point officials will review how well it is working and decide whether to revise it.

The new binational agreement follows a similarly tricky accord hammered out among the lower basin states in 2007, during a multi-year drought. The agreement laid out how much each state should give up during shortages on the river. At the time, officials in the United States and Mexico promised to figure out a way to deal with such shortages at the binational level, as well.

One glaring omission from the list of signatories, however, is California's Imperial Irrigation District. IID officials held a special meeting late Friday afternoon to discuss this new agreement and decided not to sign onto it. But Connor said he expects the district will change its mind.

"The door is still open for bringing IID into this process," he said, adding that the irrigation district will soon have a new board that may be more amenable to joining the agreement.

"This is such a historic agreement that I think, at end of day, it will be hard for IID not to sign on," Salazar added.

Conservation groups that have fought long and hard to get water to the delta and restore it to at least some of its former vigor commended officials on both sides of the border for coming together to help resolve one of the Colorado River system's thorniest problems.

"For the first time, the U.S., Mexico and NGOs will be dedicating water for the mainstem Colorado River in Mexico," said Francisco Zamora, who works on Colorado River Delta issues for the Sonoran Institute. "Even if it's only a short period of time, it will reconnect the river with its estuary."

By the time the 1,450-mile Colorado River reaches the U.S.-Mexico border, it is reduced to a small stream. Where it historically emptied into the Gulf of California, it rarely flows at all. The river supplies water for 33 million people in seven U.S. Colorado River Basin states and two Mexican states, irrigates 4 million acres of farmland, provides hydropower and supports an important recreation industry, ranging from rafting to fishing.

The agreement will restore areas of the Colorado River below the border that have been dry for years, Zamora said.

"It will help to not only to protect habitat, it will create new habitat, and that will benefit many species," he said. "It will also help communities that live along the river."

People along the river in Mexico soon will be able to swim, fish and canoe -- activities that have not been possible for many years, he said.

"Right now, most of the river is dry -- there is no water in the river," he said. "So the agreement will put water in the river, and people will be able to go and enjoy the river."

The binational agreement is a good starting point for resolving the long-standing issue of how to ensure that Mexico receives its fair share of water while also providing reliable water supplies for the southwestern United States -- and it goes a long way in restoring flows to the beleaguered delta, as well, said Jennifer Pitt, Colorado River Project director for the Environmental Defense Fund, which participated in the negotiations leading to the agreement.

"It is a remarkable commitment, and while the NGOs have established the Colorado River Delta Trust, and it's been functioning for four years, acquiring water rights for restoration, there is no way that the trust alone, or Mexico alone for that matter, could provide the kind of flows to be used for environmental purposes," she said. "And it's really only in partnership between the U.S. and Mexico, using all the tools available including reservoir storage to store water over time that that water can be delivered over time."

The flexibility built into the agreement -- particularly balancing the distribution of Colorado River water between the two countries based on conditions -- will also help water managers deal with the vagaries of climate change, Pitt added.

According to an assessment of climate change impacts on the Colorado River issued by the Bureau of Reclamation last year, flows will likely decline by 8.7 percent by 2060.

The agreement also provides a framework for future discussions about the delta and other issues, she said.

Negotiations on the agreement, which took four years to craft, were "delicate," officials said.

"I think even in the last 20 days, there were times when it seemed like it was Humpty Dumpty falling off the wall, but we kept Humpty Dumpty on the wall," Salazar said during the news conference.

"We still have a lot of work to do between the U.S. and Mexico," he added.

Reese writes from Santa Fe, N.M.