When a Summer Drought Begins in the Winter: Investigating Snow Drought

From thirsty agricultural crops to whitewater rafters contemplating a low river, a lack of water is the most obvious in the summertime. Its impact is particularly clear when many people rely on the same source of water. What happens in the Colorado River's East River Watershed affects 40 million people from seven U.S. states as well as Mexico. Around the world, similar mountainous areas provide the water that helps feed one to two billion people. In fact, scientists call these regions "the world's water towers."

But problems with these watersheds don't start in the summer or even the spring. In fact, they begin in the winter, when snow isn't building up in the Rocky Mountains and similar areas as it once did. The snow that falls - or doesn't fall - in the mountains has huge effects on what's available for the rest of the year. Future climate change may cause less and less snow to fall in these areas and reliably convert to water downstream.

Researchers supported by the Department of Energy's (DOE) Office of Science are working to understand the role of snow drought, how to measure it in the future, and how to use such data to inform decision-making.

Droughts in the winter

In a regular drought - also called a meteorological drought - there's a lack of precipitation. It often has immediate and obvious effects. In contrast, a snow drought's effects are delayed. When snow falls in the winter, it builds up as snowpack. In the spring, much of this snowpack melts and moves through the watershed as runoff. It ends up in rivers that provide water to people far beyond the mountains.

But if there's less snow than usual in a single winter, or if less snow fails to transition to water downstream, there's less spring runoff. The lack of snow can change both the amount and the timing of the runoff. The situation gets even worse when there are multiple years of low snowfall, as the snowpack further decreases each year.

Snow drought can happen for three reasons. When temperatures are exceptionally warm, precipitation can fall as rain instead of snow. When overall precipitation is low, there's less rain and snow. Lastly, when temperatures are warm and precipitation is low, areas end up with less precipitation and a smaller proportion of it as snow.

Providing water in a warmer future

While the mighty Colorado River often has huge amounts of water, that water comes from many small streams and rivers in the form of snowmelt. In fact, almost three-quarters of the Colorado River's water comes from runoff from snowfall. Snow drought can take a tremendous toll.

Poor water management can leave communities struggling to have enough water throughout the year. Unfortunately, the unpredictability from snow drought can make it hard for water managers to know how much and when water will be available.

Future climate change is very likely to make this predicament worse. Higher elevations are already warming faster than lower ones. The hotter temperatures from climate change will result in less snowpack over time. Scientists are already seeing an increase in snow droughts across the world from the late 1980s to the present.

To understand how much and where these shifts in snow droughts will occur, Marianne Cowherd, a researcher at the University of California Berkeley (UC Berkeley), worked with researchers at DOE's Pacific Northwest National Laboratory to run scenarios on a number of climate models. Climate models provide a computer simulation of past, present, and future climate and Earth systems. Because each model has its strengths and gaps, the team compared the results from nine different models.

They conducted two different scenarios. The medium emissions scenario assumed that greenhouse gas emissions will stay at the same level and start to decrease in 2050. The higher-emissions scenario assumes there will be no decrease in 2050 and that the emissions trends will continue.

The news wasn't good. Both scenarios predicted snow drought increasing in most areas of the world. In particular, all snowy regions of the Northern hemisphere and the Andes were modeled to have less snow than they do now. Not surprisingly, the higher-emissions scenario was worse. In addition to snow droughts becoming more frequent, both scenarios predicted they will become more severe.

About two-thirds of the decrease would be from higher temperatures alone, with the rest a combination of higher temperatures and decreased precipitation. This is a major shift from the past, when snow droughts were mainly caused by low precipitation. This split from meteorological droughts will make it even more difficult for water managers to predict and accommodate snow droughts.

Measuring a changing world

On top of all of that, it's likely that the tools water managers rely on are likely to become less accurate due to climate change.

Measuring snow drought is already harder than measuring regular drought. Scientists use a combination of climate models and real-time measurements taken in the field to understand what will happen in the future. While climate models can make big-picture estimates, most are not yet precise enough to provide year-to-year predictions. For example, several models represent mountaintop temperatures as cooler than they are in real life.

That leaves most of the short-term predictions up to field measurements. Fortunately, there's already an extensive network of sites around the world. Unfortunately, these sites weren't designed for a changing climate. Over time, they will become less accurate as the snow line shifts to higher elevations. In particular, these changes will have major effects on the Lower Colorado River Basin and Nevada.

Cowherd collaborated with scientists from DOE's Lawrence Berkeley National Laboratory and the University of California Los Angeles (UCLA) to study and solve this challenge. They determined that we can still use present snow-measurement networks, albeit with a few changes. They also found that it will be important to have additional information about the relationship between temperature, snow, and geographic space. In addition, climate models that are flexible enough to handle new information will be important for understanding these year-to-year differences.

SAILing towards solutions

It's clear that more information about snowfall in mountain terrain is essential to ensuring people in the American West can have access to the water they need. Thankfully, DOE is helping fill that gap.

The Surface Atmosphere Integrated Field Laboratory (SAIL) campaign was a 21-month effort to collect a massive amount and variety of data about the conditions above, at, and under the surface of the East River Watershed in Colorado. Scientists used more than 50 instruments from the DOE Office of Science's Atmospheric Radiation Measurement user facility, as well as guest and existing regional instruments to collect data on how and when water moves through this landscape.

Researchers from SAIL and nearby field campaigns that ran simultaneously observed atmospheric, surface, and subsurface changes from season to season. In the winter, they slogged through deep snow. In the summer, they looked out on forested mountains and green valleys.

SAIL finished collecting data in June 2023. However, the work of SAIL and its related campaigns is far from over. SAIL collected a far more comprehensive, detailed set of data than any previous mountain hydrology campaign had collected. Printing out all the data would generate 15 billion pages of documents.

Right now, researchers are analyzing the data and considering how to use them to make climate models more accurate and precise. They're closely collaborating with scientists funded by DOE Office of Science's Earth and Environmental Systems modeling program.

There are already useful results. Both the study on snowpack measurement sites and the one describing how models underestimate temperatures on mountaintops used data from SAIL.

Scientists will continue to dig into data from SAIL and related campaigns. This data will help us better understand the "world's water towers" and how they will change over time. The resulting improvements to climate models will help water managers and others better predict snowpack in the years to come. From the data collected in the past to the present, our scientists are helping us face a future with a changing climate.