Mexico ships televisions, cars, sugar and medical equipment to the United States. Soon, it may be sending water north.

Western states are looking south of the border for water to fill drinking glasses, flush toilets and sprinkle lawns, as four major U.S. water districts help plan one of two huge desalination plant proposals in Playas de Rosarito, about 15 miles south of San Diego. Combined, they would produce 150 million gallons a day, enough to supply more than 300,000 homes on both sides of the border.

The plants are one strategy by both countries to wean themselves from the drought-prone Colorado River, which flows 1,450 miles from the Rocky Mountains to the Sea of Cortez. Decades of friction over the Colorado, in fact, are said to be a hurdle to current desalination negotiations.

The proposed plants have also sparked concerns that American water interests looking to Mexico are simply trying to dodge U.S. environmental reviews and legal challenges.

Desalination plants can blight coastal landscapes, sucking in and killing fish eggs and larvae. They require massive amounts of electricity and dump millions of gallons of brine back into the ocean that can, if not properly disposed, also be harmful to fish.

But desalination has helped quench demand in Australia, Saudi Arabia and other countries lacking fresh water.

Dozens of proposals are on the drawing board in the United States to address water scarcity but the only big project to recently win regulators' blessings would produce 50 million gallons a day in Carlsbad, near San Diego. A smaller plant was approved last year in Monterey, some 110 miles south of San Francisco.

Mexico is a relative newcomer to desalination. Its largest plant supplies 5 million gallons a day in the Baja California resort town of Cabo San Lucas, with a smattering of tiny ones on the Baja peninsula. Skeptics already question the two proposed plants in Playas de Rosarito — known as Rosarito Beach to American expatriates and visiting college spring-breakers.

"It raises all kinds of red flags," said Joe Geever, California policy coordinator for the Surfrider Foundation, an environmentalist group that has fought the Carlsbad plant for years in court, saying it will kill marine life and require too much electricity.

Water agencies that supply much of Southern California, Phoenix, Las Vegas and Tijuana, Mexico, are pursuing the plant that would produce 50 million gallons a day in Rosarito near an existing electricity plant. They commissioned a study last year that found no fatal flaws and ordered another one that will include a cost estimate, with an eye toward starting operations in three to five years.

Potential disagreements between the two countries include how the new water stores will be used.

The U.S. agencies want to consider helping pay for the plant and letting Mexico keep the water for booming areas of Tijuana and Rosarito. In exchange, Mexico would surrender some of its allotment from the Colorado River, sparing the cost of laying pipes from the plant to California.

Mexico would never give up water from the Colorado, which feeds seven western U.S. states and northwest Mexico, said Jose Gutierrez, assistant director for binational affairs at Mexico's National Water Commission. Mexico's rights are enshrined in a 1944 treaty.

"The treaty carries great significance in our country. We have to protect it fiercely," Gutierrez said.

Rick Van Schoik, director of Arizona State University's North American Center for Transborder Studies, said laying a pipeline across the border would be too costly.

"It's expensive enough to desalinate. I just don't see how it calculates out," he said.

The other big plant proposal joins Consolidated Water Co., a Cayman Islands company, with Mexican investors. Their proposal would send much of its 100 million gallons a day from Rosarito to the United States via a new pipeline, with operations beginning in 2014.

Mexico isn't likely to approve both plants, said Gutierrez, whose government is sponsoring the 50-million-gallon-a-day plant with the Metropolitan Water District of Southern California, the San Diego County Water Authority, the Central Arizona Water Conservation District and the Southern Nevada Water Authority.

A key question is whether Mexico will allow water first used at the neighboring electric plant to be desalinated — a giant potential savings. California recently adopted rules that prohibit the state's electric plants from sucking in vast amounts of seawater to cool their machinery.

The Carlsbad plant illustrates how difficult it can be to build a plant in California. Poseidon Resources Corp., based in Stamford, Conn., has survived about a decade of legal challenges and regulatory review.

The company, which plans to begin major construction when it secures financing, was required to restore 66 acres of wetlands and take other measures to offset carbon emission from the electricity it consumes.

The San Diego County Water Authority is also considering a plant at Southern California's Camp Pendleton that would produce up to 150 million gallons a day. Poseidon wants to build one in Huntington Beach, near Los Angeles, that would churn out 50 million gallons a day. Those ideas face significant challenges.

"The planets will never be in alignment like they were in Carlsbad," said Tom Pankrantz, editor of Water Desalination Report. "They had the right project, at the right place, at the right time."

The San Diego agency wants to get 10 percent of the region's water from desalination by 2020 as a way to lessen its dependence on the Colorado River, which is connected by aqueduct about 200 miles away. Tijuana also wants to rely less on the river, a priority that gained urgency after a 2010 earthquake knocked out its aqueduct for about three weeks.

The U.S. and Mexico can save money by joining forces, achieving economies of scale, said Halla Razak, the San Diego agency's Colorado River program manager. At least half of the plant's water would stay in Mexico, she said.

"Mexico is the entity that is driving the project, even more than the United States," she said.

U.S. and Mexican officials say they expect the new plants will adhere to the same standards as California, including water quality, but that Mexico's regulators may act faster and shield sponsors from legal challenges.

"The Mexicans will ask all the same questions that we ask here, but it's not endless lawsuits," said Mark Watton, general manager of Otay Water District, which would buy about 20 million gallons a day from Consolidated's Mexico plant for its San Diego-area customers. "You get an answer quicker."