Drought deepens in the West as window for rain, snow closes

The year 2020 is going to be remembered for a lot of things, many of them not so good. Included in the not-sogood list is the drought that has plagued the West, lasting into 2021. According to the U.S. Drought Monitor, which has published weekly maps on drought since 2000, the 2020 drought is the worst, in terms of its geographical scope, in over 20 years.

Almost 80% of the Western U.S. is in drought, with nearly 42% of the region in “extreme” or “exceptional” drought.

Much of the region saw 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% 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 only once every 50 to 100 years.

February did bring an active weather pattern with it, however. 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 also received between 2 and 6 inches, much of that in the form of snow.

However, despite the precipitation, some areas are still struggling. The Southwest was much drier in February.

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 is 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 high temperatures and no new snowstorms.

Does how it melts make a difference? You bet!

For example, water supply forecast for Lake Powell from the Colorado Basin River Forecast Center started out a bit below average. With each passing month, that forecast got a bit lower. And what actually happened was at the very low end of what was forecast, and 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 through 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 about 20 to 25 inches.

But to get the snowpack needed and cover the soil moisture deficits, these basins would potentially need 120% to 150% 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.