At the National Water Research Institute’s Drought Response Workshop held in October of this year, Department of Water Resources climatologist Michael Anderson gave a presentation on some of the latest research regarding climate change and California’s water resources.
Michael Anderson began with the happy news of what we can expect from climate change: “The future climate in terms of water resources will mean increased air temperature, less snow and earlier runoff, precipitation continuing to fluctuate wildly but more of it’s going to fall as rain then snow, the ecosystem’s going to respond the way it will, and all of that’s going to change the timing and amount of water in the rivers; combine with a little sea level rise and you get a prescription for changing the way we operate our water resource systems.”
“California is different than the rest of the country,” he said, presenting a chart that shows the variation in precipitation across the country. “This is very helpful when you talk with your federal partners. Remember, the federal partner is back east in DC; look at their variability. One year looks like the next year looks like the next year … You see federal programs set up that reflect that behavior, and sometimes they have a hard time interfacing in an area where one year can look absolutely nothing like the next or as we found out this year, the year can start one direction, fake, and head the other direction in a hurry.”
He then presented a chart that showed the number of days of precipitation that it takes to get half of the annual precipitation for the last half of the 20th century. “You see out in the desert area, less than a week,” he said. “It doesn’t take long to get 2” of precipitation out there, but the rest of the state, we’re still under two weeks, so it’s a very short time window to accumulate precipitation that will turn into runoff and turn into water resources.”
He then presented a chart of the Northern Sierra 8-Station Index, an index that serves as an indicator of wetness of the Sacramento River hydrologic region. He noted that the index has an annual average of 50”, but it is highly variable, ranging from 88.5” in 1983 to just 17.1” in 1924. “We have had months that exceed what they had in 1924,” he said.
Over half of the annual total occurs in December, January and February, he said. “The biggest month was over 30” in December of 1955 which produced a very large flood that actually is the foundation for most of the Central Valley flood management system,” he said. “As for the minimum month, there have been 45 times where we have yet to find a drop of rain, which is expected in the summer time but it has happened in December. That gets to be a problem.”
He then presented a graph of showed the 8-Station Index for the period of record, as well as the last 30 years of the 20th century. He pointed out that precipitation in January, February, and March is increasing, while precipitation is decreasing in December. “Mother Nature’s take a left turn here,” Mr. Anderson said. “You can see where this can challenge trying to operate a system when you have rules that only tell you when you can do what with water.”
He then presented a graph that included the 8-Station Index for the 21st century. “In the 21st century we’re seeing a different pattern from that, so what we’re starting to see that it’s just not a monotonic march towards a future that is going to be different than today, but we can fluctuate quite a bit along the way,” he said.
He then presented a graph that plotted precipitation in Los Angeles, Sonoma and Mendocino counties, the Northern Sierra 8-Station Index and the San Joaquin 5-Station Index across for the period of record. “You see the large fluctuations and in some cases, a decadal scale variability of 10 to 15 inches up in the north,” said Mr. Anderson. “That’s a pretty sizeable fluctuation. I can tell you it fluctuates, but I can’t tell you why.”
“So one of the things we’re looking at is something called the Pacific Decadal Oscillation,” he said. “It’s an ocean temperature structure that has a warm phase and a cold phase. The warm phase tends to have a tropics that look more like El Nino; a negative phase tends to have a tropics that looks more like La Nina. We’re currently in a negative phase and as you see in a negative phase, December precipitation historically tends to be accentuated, so we have that happening on top of climate changing as well. Just to make it easy. “
Just a few storms are at the core of California’s water supplies, he said. He explained that there is another process called the Madden Julian Oscillation, which is a very large-scale organized convection that propagates across the tropics in the Eastern Pacific to create ‘atmospheric rivers.’ “This is concentrated moisture happening in the first 5000 feet of the atmosphere that goes right across the ocean and runs into California, and what do you suppose it runs into,” he said. “We have some hills that get in the way, and they’re actually very effective in making that water fall out of the sky, but again it depends on the dynamics of that winter storm system that are driving it. If it’s a warm system, there’s a lot of rain that can happen, and if it happens for a long enough duration, there is flooding. If it’s a colder storm, these can be absolutely fantastic for water supply. Entertainingly enough, we saw one of each this year. A little bit of everything.”
“For things to work in California we actually have to rely on the alignment of several processes happening across the Pacific, operating on multiple space and time scales; all of this is going on and all of this is important to climate change,” he said. “It’s really hard to tease this out of what you’re collecting in a bucket on a hillside in terms of precipitation. There’s a lot more going on, and that gets to some of the work that we’ve been doing.”
He then presented a satellite picture of an atmospheric river storm that occurred in January of 2008. Over two days, the storm dumped six to ten feet of snow in the mountains, and Half Moon Bay had about a foot of rain and yet the storm system was centered by Washington and British Columbia, more than 500 miles away. “This is something that doesn’t look like anything like what the East Coast is used to dealing with when they deal with their major storms,” he said. “Their major storms and hurricanes – everything’s wrapped up in a nice little package that rolls, and out here we just have things that are looking different. So we have a lot of impacts that are associated with landfalls of these atmospheric river elements. Yet operationally we’re still just learning how to take a look at these. Like I said, there’s more to it than just catching the water that falls out of the sky.”
NOAA storm systems research lab has a program called the Hydrometeorological Test Bed, or HMT, which spent a decade here in California working on how best to monitor atmospheric river storms. The Department of Water Resources has invested in four coastal observatories; the Bodega Bay observatory is currently operational, he said. He explained that satellite imagery can determine how much water vapor is in the air as long as the lower boundary is water, but now, new technology will be able to provide that information over land. This information will be combined with the wind profiler at the coastal observatories to determine how much water vapor flux is coming in, or more simply put, “It tells us how big that fire hose of moisture is coming ashore.”
There are now vertically pointing radar that will tell us when rain turns to snow – how warm is that event, which is important to determining water supply building or flood management, he explained. Sensors will monitor the soil moisture, important because after going without precipitation for a few months, the soils dry out and it takes awhile for the watershed to wet up before it starts producing runoff. “If you go through a dry year, it takes even longer. So we’re actually trying track that rather than wait until the river responds and say, okay the watershed’s wet now, which is what we have done historically.”
“Back in November , right before the American Geophysical Union’s fall meeting in San Francisco, we were doing a press conference to announce this network, and lo and behold, Mother Nature was kind enough to give us an event to demonstrate,” he said. He presented a map that displayed the water vapor sensors during the storm. “That sensor is looking up and if you took all the water vapor above that sensor, squeeze it out, that’s how much is over that, so you take that inch and multiply that by the miles and you realize all of that’s being driven up against a mountain range in a much smaller space, you’re converging that water and creating a very effective way to create precipitation and runoff,” he said.
“Here we can actually see that,” he said, displaying a graph of the 8-Station Index for last year. “During the first Atmospheric River Event, 15 ½” fell on the 8-station index in 8 days – that turned out to be a third of the year’s total,” he said. “Eight days – there was your big precipitation event, and that provided a good boost of runoff into the reservoirs and proved to be really helpful to keep things from being really interesting. Then, a couple of weeks later, we get a second blast, but here, a smaller jump – this was a colder storm that provided a six inch increase in snow water equivalent that happened in 5 days, that proved to be almost a third of the northern Sierra snowpack in that one event. So these events are big pulses that come to us.”
“So if you think about the way climate change is done where we look at averages and how averages change slowly over time, when we deal with water supply and flood management here, we’re dealing with event processes that rely on physics that span across the entire Pacific, so for California, so we’ve got a little more to work on than just trying to follow a trend,” he said.
We don’t have a scale to say how big these storms are yet, so Michael Dettinger and Marty Ralph started working on it by taking 3-day totals of precipitation and categorizing them by size, just to see how do these storms rate, compared to the rest of the country, he explained. “You can see that California’s big events here rival the Gulf Coast hurricane belt quite well, so just as big, but you can imagine we rely on them. Those are atmospheric river events.”
We’re working to make advancements in monitoring these storms and conditions, and then being able to document their change. “It’s great we found out about them, but now we need to start documenting that so we can start following along and looking at how a warmer atmosphere that might hold more water, how that might relate in terms of its sudden delivery to California,” he said.
“To be able to talk about change, you have to have climatology or characteristics, so looking at characteristics of past, present, and future atmospheric rivers and how that might relate to water resources in California,” he said. “This isn’t a monotonic process. We have to map that onto the background of decadal variability to understand how those decadal scale fluctuations that are on the order of 20% by themselves map on, and if we can understand that better, that might get to the desire to actually know what’s happening next.”
There’s a lot of work to do, Mr. Anderson said. We are working with our federal partners to develop targeted opportunities for monitoring, and to develop tools and indicators that track change and facilitate planning efforts. We are working to improve our understanding the variability across time scales. “We are working to understand what happens when you start with a really wet fall that suddenly left turns and then goes completely dry, understanding how that can happen year to year, decade to decade and how that might change as we warm the planet,” he said. “And then figure out how everything works together so that you can actually execute that in your water management plans in a changing climate.”