Global warming could result in severe drought conditions in parts of Washington state one out of every two years by the middle of this century.

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YAKIMA — Global warming could result in severe drought conditions in parts of Washington state one out of every two years by the middle of this century, according to a new study.

The decadelong study focused on water availability in south-central Washington’s Yakima Valley, where 370,000 irrigated acres of orchards, vineyards and other crops cover 6,150 square miles.

According to the study, the amount of water available for irrigation in the Yakima Valley will fall an average of 20 to 40 percent in a typical year at midcentury, due to global warming.

The resulting losses to agriculture in the region during the next several decades would be between $92 million a year when the temperature rises 2 degrees Celsius and $163 million a year when the temperature rises 4 degrees Celsius, the study said.

“Under global warming, the normal year looks much more like an El Niño year, and the odds of having very low runoff available to agriculture go up dramatically,” said Michael Scott, a staff scientist at the Pacific Northwest National Laboratory in Richland and author of the study.

“The odds go up to almost one year in two that you wouldn’t have enough water,” he said.

The findings of the study were to be presented today at the American Association for the Advancement of Science annual meeting in Washington, D.C., during a session on adapting to climate change.

El Niño, a warming of Pacific waters that can alter the flow of the jet-stream winds overhead and affect global weather and climate, occurs roughly every three to seven years.

Severe droughts on average occur roughly one year in every seven, but in El Niño years, the average frequency for severe drought rises to one year in three. In the non-El Niño years, the odds are only about one year in 50.

By conservative estimates, global warming will result in El Niño-severe drought conditions one of every two years, Scott said.

Irrigators in the Yakima Valley draw water from five reservoirs and snowpack in the Cascades. The study applied data from bad drought seasons in the past 80 years to computer projections of diminishing snowpack this century to determine the effects of global warming on the region.

Runoff for agriculture in the Yakima Valley in a typical year is about 2.7 million acre-feet. If it comes in the form of winter rain instead of snow, or the snow falls too early in the season and melts, it travels downriver anyway and is unavailable for irrigation when farmers most need it during the growing season, he said.

“The problem with climate change is that even if the total runoff is the same, it’s timed wrong. It’s too soon, and therefore it’s not available for irrigation. It’s as if it were a drought, because it is, for agriculture,” Scott said.

The study wasn’t all bad news, though. Policy-makers and farmers can adjust practices for the future, such as increasing water storage, improving water conservation, moving up planting and harvesting dates and planting less-thirsty crops, he said.

Philip Mote, state climatologist and University of Washington research scientist, said the findings conform with other research.

“I think this year is a good indication of the Cascades’ sensitivity to temperature,” he said, referring to this year’s low snow pack.

Drought or no drought, the strategy remains the same, said Don Schramm, assistant manager of operations for the Sunnyside Valley Irrigation District, which provides irrigation water for about 100,000 acres.

“You get as efficient as you can with the water that you have,” Schramm said.