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Coming decades could see delays in summer monsoon rains in the U.S. Southwest and northwestern Mexico that water crops for 20 million people, researchers say.

The North American monsoon delivers as much as 70 percent of the region's annual rainfall, but much of the arid southwestern United States is expected to get even drier as winter precipitation declines under climate change, scientists as Columbia University report.

While models predict summer rain levels will stay constant over much of the region, what will shift is the arrival of the heaviest rains, from around July and August to September and October, they said.

"There still will be a healthy monsoon which is good news for agriculture in the southern U.S. and northwestern Mexico -- the timing is the problem here," study co-author Richard Seager, a climate scientist Columbia's Lamont-Doherty Earth Observatory.

A delayed monsoon could potentially lower crop yields, the researchers said, as rains could come later in the growing season when the days are getting shorter.

That would have considerable have economic impact, whether in northwestern Mexico or the U.S. Southwest, where agriculture and range lands are mostly rain-fed, the scientists said.

"We hope this information can be used with other studies to build realistic expectations for water resource availability in the future," said study lead author Benjamin Cook, a Columbia climate scientist with joint appointments at Columbia and at the NASA Goddard Institute for Space Studies.