The Colorado River, along with its tributaries, is a crucial source of water for hydropower, agriculture, and drinking supplies across seven U.S. states and Mexico. Much of this water is derived from winter snowpacks that melt each spring. Each April, water managers rely on these snowpacks to gauge the water availability for the coming year.
The Colorado River and its tributaries provide water for hydropower, irrigation, and drinking water in seven U.S. states and Mexico. Much of this water comes from snow accumulating over winter and melting in spring. Every April, water managers utilize the snowpack data to forecast the water supply for the next year.
Since 2000, however, these forecasts have often been incorrect, with actual river flows falling short of what was predicted. This puzzling issue has left water managers and scientists questioning where the water is actually going.
Recent research from the University of Washington indicates that the key issue is the reduced rainfall during springtime. The findings suggest that warmer and drier springs account for nearly 70% of the difference between predicted and actual streamflow. With less rainfall, local plants become more dependent on melting snow for hydration, which results in less water flowing into streams. Furthermore, sunnier weather encourages greater evaporation from the soil and boosts plant growth.
The study’s results were published on August 16 in Geophysical Research Letters.
“The concerns about missing water coincided with the decline in spring precipitation, marking the start of the ‘Millennium drought’ in 2000, which persists today,” explained lead author Daniel Hogan, a doctoral student at UW’s civil and environmental engineering department. “Our research emphasizes the cascading effects of reduced spring rain, leading to fewer clouds and thus more sunshine. This excess sun encourages plants to take full advantage of the recently melted snow, leading to rapid growth. Our findings underscore the need to examine the entire snow season rather than just focusing on when snowpack is at its thickest.”
Hogan and senior author Jessica Lundquist, a professor of civil and environmental engineering at UW, examined this issue as part of a broader investigation to understand the mystery of water shortages. Initially, they speculated whether sublimation — the process of snow turning directly into vapor — was causing the drop in water levels. However, they later found that sublimation accounted for only about 10% of the water loss, indicating that another factor was responsible.
“With limited possible explanations, I began comparing significant factors,” Hogan noted. “We discovered that changes in spring conditions were much more pronounced than in any other season. There’s a dramatic transition from thick snow to blooming flowers in a short time frame. In the absence of spring rain, plants — from wildflowers to trees — behave like giant straws, extracting water from the snowpack.”
The researchers analyzed changes during spring across 26 headwater basins at various elevations within the Upper Colorado River Basin. To better understand these changes over time, they utilized numerous datasets, including streamflow and precipitation records dating back to 1964. This allowed them to model how much water the vegetation in each basin would likely absorb.
“An important assumption of our study is that we consider that plants can access unlimited water from snowmelt, even with below-average rainfall,” Hogan stated.
All the basins they examined exhibited a decline in streamflow due to the lack of spring rain, with lower-elevation basins experiencing even more significant declines. This occurs because snow in these areas typically melts earlier, providing plants with more opportunity to utilize the snowmelt.
Having identified spring rain as the primary factor impacting water flow, the researchers are now working to deepen their understanding of the dynamics during this season. For instance, one ongoing project is exploring the role of residual snow patches acting as mini-reservoirs supplying a consistent water flow to plants.
As the Millennium drought continues, these findings will increasingly influence water forecast assessments conducted each April.
“April is crucial for determining the water content in the snowpack,” Lundquist said. “However, this poses a challenge since spring has not fully unfolded by that time. With the recognition that spring precipitation is more pivotal than rain during other seasons, we need to enhance our predictive capabilities regarding rainfall to improve the accuracy of our April forecasts.”
This research was supported by the National Science Foundation, the Sublimation of Snow Project, and the Department of Energy Environmental System Science Division (the Seasonal Cycles Unravel Mysteries of Missing Mountain Water project).
