Water supply and wildfire concerns grow for the dry season
By Becky Bolinger and Andrew FreedmanMarch 3, 2021 at 12:07 p.m. MSTAdd to list
The year 2020 is going to be remembered for a lot of things, many of them not so good. Included in the not-so-good list is the drought that has plagued the West, lasting into 2021. According to the U.S. Drought Monitor, which has published weekly maps since 2000, the 2020 drought is the worst, in terms of its geographical scope, in more than 20 years.
Almost 80 percent of the Western U.S. is in drought, with nearly 42 percent of the region in “extreme” or “exceptional” drought.
Much of the region experienced developing drought in the summer, following a warm and dry spring. Since then, conditions have deteriorated, and the precipitation deficits continue to build. At its maximum extent in January 2021, 47 percent of the West was in extreme drought or worse. Nearly a quarter of the area was in the worst drought category, an event with a probability frequency of once every 50 to 100 years.
February did bring an active weather pattern with it. The Pacific Northwest received more than 10 inches of precipitation last month. Much of the interior Rockies through Idaho, Montana, Wyoming, Utah, and Colorado received between 1 and 5 inches of moisture for the month. The Sierra Nevada in California received between 2 and 6 inches, much of that in the form of snow.
However, despite the precipitation, some areas are still struggling. Blue outlines in the map below show where snowpack increased last month. The Southwest was much drier in February.
Where the purple outlines overlap on this map, these areas are above average for snowpack now. Outside of the purple outlines, snowpack is still largely below average. Areas outlined in orange experienced a decline in the percentage of average snowpack since the beginning of February.
And red outlines show the areas where snowpack is extremely low compared to normal. The evidence is clear — February was beneficial for many, but it was not a drought buster, and drought continues to maintain its stranglehold on the West.ADhttps://548e45179891f91d487d0534a7ca47d2.safeframe.googlesyndication.com/safeframe/1-0-37/html/container.html
So, what would it take to get out of drought? To answer that, we first need to know the magnitude of these deficits. It’s not as simple as comparing the past year’s precipitation to normal and making up that difference.
Water in the West relies on a complicated relationship between what’s in the ground, what’s stored on the surface, what accumulates in the winter over the mountains, and what trickles down in the spring.
The deficit started in the spring of 2020. Snow water equivalent (commonly referred to as snowpack, this is the amount of water in the snow that’s accumulated) in the Cascades and Sierra Nevadas peaked well below average.
Across the interior Rockies, snowpack usually reaches its peak in late March/early April and begins its slow melt — adding water to the rivers and eventually filling the reservoirs. While 2020 snowpack peaked around the time we’d expect, it melted out too fast, thanks to anomalously warm temperatures and no new snowstorms.
Does how it melts make a difference? You bet! Check out this water supply forecast for Lake Powell from the Colorado Basin River Forecast Center.
Forecasts in the blue shading started out a bit below average (the green line is the average supply into Lake Powell). With each passing month, that forecast got a bit lower. And what actually happened was at the very low end of what was forecast (the orange line is the observed supply into Lake Powell for 2020). The actual inflow into Lake Powell was 3.4 million acre feet below average.
An acre foot, which is a measurement commonly used by water resource managers, is equivalent to more than 300,000 gallons of water. That’s quite a big deficit to start things off!
Deficits for Lakes Powell and Mead are significant. Both are connected to the Colorado River Basin and supply water for millions throughout the West. Long-term deficits have been building since the turn of the century, and each drought exacerbates the situation.
Fast forward to a hot and dry summer. With the exception of a couple of isolated locations in the Pacific Northwest, Idaho, and Montana, most of the West experienced much above average temperatures and below average to record low precipitation for June-September last year.
In the Southwest, July-September typically ranks as the wettest time of year, which is largely a result of the North American Monsoon. Monsoon moisture in the late summer is key for replenishing soil moisture. Without an active monsoon, soils dry out just before the beginning of snow season. And unfortunately, that happened in the fall of 2020.
Modeled soil moisture at the end of September shows the extremely dry soils in the West. As we entered the cold season, this soil moisture was “locked in.”
The high elevation ground freezes, and that is the state the soil moisture will be at when the thaw begins in the spring. Start the season with dry soils, and that is the first “bucket” that needs to be filled when the snow starts melting.
The Colorado Basin River Forecast Center estimates that much of the Colorado River Basin needs 10 inches or more of precipitation for soil saturation. Averaged over watershed basins, normal snowpack peaks around at around 20-25 inches.
But to get the snowpack needed and cover the soil moisture deficits, these basins would potentially need 120-150 percent of average snowfall for the season. Can we expect that much snowpack this season? Unfortunately, no.
Climate change is playing a significant role in influencing water supplies in the West, with early spring snowmelt, hotter and drier summers and warming winters all acting to exacerbate drought conditions.
In fact, a study published last year found that a vast region of the western United States, extending from California, Arizona and New Mexico north to Oregon and Idaho, is in the grips of the first climate change-induced megadrought observed in the past 1,200 years. A megadrought is loosely defined as a severe drought that occurs across a broad region for a long duration, typically multiple decades.
The study found that warming temperatures and increasing evaporation, along with earlier spring snowmelt, have pushed the Southwest into its second-worst drought this millennium. The drought analyzed in the study dates back to the year 2000.
Within the context of this larger scale megadrought, there is still climate variability. There have been wetter winters, such as the heavy winter rains in 2017 that resulted in the failure of California’s Oroville Dam, or the snow season of 2019 that resulted in Lake Powell inflows reaching 145 percent of average that summer.
But the dry years become more frequent. And, even by 21st century standards, this drought is particularly severe and widespread.
This map shows all the stations in the west that measure snowpack. As of Feb. 28, stations colored orange and red have below average snowpack for this time of year. The Sierra Nevadas in California are well below average for snowpack. With a typical peak date of April 1, there is only one month left to add to that snow cover. And considering they receive almost no moisture during the warm season, this month is extra critical.
For the mountain ranges throughout Utah, Wyoming, and Colorado, snowpack is also below average. For the Colorado Headwaters region, there are roughly 45 days until normal peak snowpack, but the likelihood of a normal snowpack is decreasing by the day.
For the areas that struggled this winter, the Climate Prediction Center’s outlookis discouraging, with increased chances of below average precipitation and above average temperatures for the March-April-May time period.
The good news is that recent February moisture and a decent spring forecast have helped alleviate drought conditions in the Cascades and Pacific Northwest.
The bad news is that it’s increasingly likely severe drought will continue in other parts of the West as we head toward summer. Agriculture, water supplies, and forests are likely to be impacted. Expect crop losses and selling of livestock; watering restrictions may begin as temperatures warm, and the risk of large wildfires will return again this summer.
Beyond that, let’s hope for ample spring and summer soakers, a strong monsoon, and a fresh start to a better snowpack next fall!
Becky Bolinger is the assistant state climatologist for Colorado and a research scientist at Colorado State University.