Drought-stricken states in the western United States could face severe flash flooding later in the year due to the combination of an intense wildfire season in the summer and a strong El Niño event the following winter.
An El Niño is a complex weather event that drives warm ocean water from the Asian-Pacific east, raising ocean temperatures along the west coast of the Americas, invigorating regional weather systems and increasing the amount of precipitation in the American West. They occur about every four years, but the United States hasn't seen a strong El Niño since 1998.
A moderate El Niño occurred in 2010, leading to the warmest year on record in the United States, and Pacific Ocean temperatures increased at most 3 degrees Celsius, according to Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research. Scientists have already observed a 6 C increase in some areas of the Pacific, which hasn't occurred since the 1998 El Niño, Trenberth said.
"The question is will it be a weak or moderate event, or will it become a large event, and certainly this one has started off with the potential to be a larger event -- but it's still not fully clear," Trenberth said.
Ocean temperatures off the coast of Peru and Ecuador just jumped 2.5 C above normal, he added, and if the warm water moves farther north up the coastline toward the United States and Mexico, it's possible it could lead to increased rainfall, especially at the end of the year when the ocean and the atmosphere have the strongest interaction. A simulation from the National Oceanic and Atmospheric Administration shows how the 1998 El Niño developed along the North and South American coastlines.
For Western states currently mired in devastating drought and likely to be beset by wildfires through the summer, rainfall intensified by El Niño may not bring relief. Instead of canceling each other out, the two extreme seasons could dovetail with major implications for people living near scorched areas.
When forest fire debris can fill reservoirs
The Mescalero Apache Tribe in south-central New Mexico is in its fourth consecutive year of exceptional or extreme drought and has experienced three of its four largest ever wildfires in the last four years.
The largest came last year, burning away the forest above the reservation's fish hatchery that would usually absorb rainfall. With the trees gone, Mike Montoya, director of the tribe's fisheries department, described what happened the next time it rained on the reservation.
"Any regular rainstorm would become a flood event, and ashes would come into the water," Montoya said during a Department of Energy Web-based seminar in April. The ash would contaminate the hatchery water, leading to significant fish mortality (ClimateWire, April 11).
In burned areas, the soil chemistry is changed to such an extent that it becomes much more water-repellant. The roots and vegetation that typically absorbed rainfall have been burned away, and rainwater then runs straight downhill, often carrying whatever ash and debris is left on the surface with it.
"We really ought to be prepared for this and ought to have appropriate water management systems in place," Trenberth said.
Lester Snow, head of the nonprofit California Water Foundation and former leader of the state Department of Water Resources under Gov. Arnold Schwarzenegger (R), said the wildfire and El Niño combination this year could shorten the life span of some California reservoirs by building up sediment and decreasing the volume of water they can hold. At worst, he said, the debris and contaminant runoff from post-fire flooding could severely damage ecosystems and threaten properties.
"The state's going to see a great burden on it," he said, "and probably a significant impact on ecosystems from the fire, and then flood and debris impacts if in fact El Niño hits in a classic fashion."
There are numerous post-fire treatments that state and federal agencies already use to mitigate stormwater runoff in burn areas -- including installing water bars that divert storm water and laying down straw, mulch and seeds that can absorb water. With all of California now in a state of drought, Snow said the potentially widespread wildfire season and intensified winter rainfall could stretch state resources to an unprecedented degree.
"There's a chance we're going to see requirements for post-fire treatments that surpass anything we've had to do in the past," he said.
Early warning may be key
NOAA's Climate Prediction Center estimates there is a 65 percent chance of an El Niño occurring this summer. The summer has also already seen a number of large wildfires, including large fires in Alaska and Arizona and two in California near San Diego and Camp Pendleton, a U.S. Marine Corps base (Greenwire, May 15).
But fire treatment officials in the West believe they are equipped to handle this year's fire-flood cycle.
Janet Upton, deputy director of communications with the California Department of Forestry and Fire Protection, said that even with the precipitation from a potential El Niño year, they "wouldn't have to be reinventing the wheel." The agency is usually able to predict heavy rainfall events 48 to 72 hours in advance, which would give it enough time to take extra precautions if heavier rains were coming in.
"We would just employ the resources and mitigations we already have in place," Upton said. "We would do our normal suppression repair and then keep an eye on it, monitor it and respond accordingly."
The only additional threat an El Niño event could bring compared with a normal winter is an increase in intense rainfall events. Jill Oropeza, a watershed specialist with the City of Fort Collins Utilities in Colorado, has been helping treat forests in the area after the June 2012 High Park fire burned more than 87,000 acres. Oropeza said the agency only saw major debris flows after intense rainfall.
"It would certainly be a concern to us if an El Niño produced higher intensity rainfall events, but at this point I think we would have to rely on the early warning alert systems that we have in place to kind of manage the situation. I don't think we would change what we're doing on the ground," Oropeza said.
Penny Luehring, leader of the National Forest Service's Burn Area Emergency Response (BAER) program, said that an El Niño event -- even if it does increase precipitation -- doesn't necessarily mean an increase in intense rainfall events.
"Gentle precipitation coming more often is not going to be something we're extremely worried about," Luehring said.