Water often falls from the sky and is stored in the mountains of the United States as snow before melting and flowing to urban and rural communities. Knowing what factors influence when and how much of this snowmelt ends up reaching streams, rivers and reservoirs is crucial for water managers trying to make the most of limited water resources. A new study by researchers from the University of Nevada, Reno and the Desert Research Institute (DRI) published in Environmental Research Letters identifies three major factors that influence snowmelt water supply and identifies regions where the he mountain water supply responds differently to climate change. The study used data from 537 watersheds across the United States
Building on more than 30 years of previous research compiled over more than two years, the research found that three factors – the amount of total winter snow available at the end of winter, the rate of snowmelt and the timing snowmelt – can be used to better predict the impact of climate change on critical snowmelt water supplies. And, the research team found major differences in the influence of each of these factors on different watersheds across the country.
“Particularly in the western United States, snow is really the backbone of our water supply systems,” said Beatrice Gordon, the lead author and doctoral student at the University of Nevada, Reno Graduate Program of Hydrologic Sciences and the Department of Natural Resources & Environmental. Science. “But the challenge is that mountain water supplies react differently to changes in snow depending on where you are in the United States. Given this challenge, our goal was to provide d other scientists and water managers a simple yet powerful framework that can be used to improve predictions about the timing and amount of flow as climate change accelerates.
The first step in building this framework was to take stock of what was known, and perhaps more importantly, what was not known, about how snow shapes runoff. waterways. To do this, researchers from the University of Nevada, Reno, and DRI teamed up with researchers from the University of Utah, Boise State University, and the University of Concepción in Chile to review more than three decades of prior research. This review included over 150 studies of snow and river flow from around the world. Based on this synthesis, they found that the amount of flow, and to a lesser extent when spring flow occurs, varies in response to changes in snowfall across the United States.
“It looks like we should have very accurate systems to predict water supply to the mountains, but the dirty secret is that there’s a lot of uncertainty caused by climate change and changes in the snowpack,” he said. researcher and co-author Adrian Harpold, associate professor at the University of Nevada, Reno and head of the University’s Nevada Mountain Ecohydrology Lab. As part of the University’s Experimental Station, which is a unit of the College of Agriculture, Biotechnology, and Natural Resources, the laboratory performs field and remote sensing observations, as well as modeling, to help respond to some of the most pressing issues related to mountain ecohydrology. in the face of changing environmental conditions.
Lack of certainty about when and how much water will come from winter snow accumulations is a serious problem for water managers tasked with meeting the needs of a growing population and increased food demand. Particularly in the western United States, which is characterized by wetter winters and drier summers, the majority of surface water comes from snowfall in the mountains, with more than two-thirds of reservoir inflow coming from snow. For context, approximately 40 million people, and much of the most agriculturally productive land in the western United States, are at least partially dependent on snowmelt-derived flow stored in two of these reservoirs. .
“At the best of times, water managers in western North America face the difficult task of ensuring a reliable supply of water to agricultural, urban and recreational users,” said the co-author. Paul Brooks, professor at the University of Utah. “They’ve done such a good job that many residents rarely think about their own water usage. Increasingly, these managers are caught between a rock and a hard place, as climate change and population growth increase demand for water, while the amount and timing of water availability become more variable.
A framework for navigating uncharted waters
The researchers’ review highlighted the great challenge of managing mountain water supplies given all the uncertainty and variability in regional responses to climate change. Drawing on their review of the literature and decades of experience from senior researchers, the authors have developed a simple framework, centered on three factors, designed to help water managers better meet the challenges posed by a future more complex and uncertain.
- Water vapor flow during the snow season. Despite the complicated name, the function of this mechanism is simple. It controls the amount of snow available at the end of winter. Throughout the winter, water stored in snowpacks is “lost” to the atmosphere through evaporation (when water turns into gas) and sublimation (when snow turns into gas). As winters warm, these losses are likely to increase, which research suggests can reduce the amount of snow available to be released as streamflow.
- Intensity of liquid water inputs. This mechanism relates to the rate at which water from melting snow and/or rain reaches the ground. The rate at which precipitation reaches the land surface has an impact on the final destination of the water. Warmer winters mean more intense winter precipitation, which research shows can lead to more runoff.
- Synchronization between water and energy inputs. This mechanism deals with when water from melting snow and/or rain is available versus when we need it most. Unlike rain, snow stays on the ground after it has fallen, acting as a large temporary (and free) reservoir for towns and farms. Snow accumulations have always released water into streams in late spring and early summer, when water demand is highest. Less snow means less storage, which research shows will increase the gap between when water is supplied and when it is most needed.
Using publicly available data from 537 sites across the United States, the researchers then used this framework to determine where each of these factors matter most and least. The relative importance of these mechanisms to mountain water supply, which the authors summarize as “how much, how fast and when,” varies geographically, depending on the study results.
Streamflow in the Great Basin, for example, is particularly sensitive to changes in all three mechanisms, making water management predictions an acute challenge in this region. However, nearby, across Donner Pass in the Sierra Nevada Mountains, research shows that the rate at which water reaches the ground surface and when water is available will be more important than the amount of snow lost to the atmosphere through evaporation and sublimation. .
More work to do
The framework is only the first step in answering the big questions about the fate of the water supply for the team. Harpold and Gordon, along with DRI’s Gabrielle Boisrame and Rosemary Carroll, are participating in a USDA-sponsored project, “SNOWPACs,” led by the University of Nevada, Reno, in collaboration with other universities.
“This study arose out of a need to characterize the variable flow changes that occur in Intermountain West and the difficult management of reservoirs that are critical to agricultural water supply,” Harpold said.
Complementing the USDA project, Brooks worked with the Salt Lake City Utilities Department, the Salt River Project, the Weber Basin Water Conservation District, and other managers in the western United States to identify and plan for changes in water supply in a warming climate.
The team is focusing its next efforts on understanding the impact of these flow changes on agriculture-dominated basins, including the Walker River Basin, which provides water for agriculture in Nevada.
“As someone who grew up farming in the western United States, I know how important snow is to our farming and ranching communities,” Gordon said. “And I’m thrilled to see how the work of SNOWPACs is providing critical information to decision makers who are currently grappling with these changes to our mountain water supplies.”
Support for research recently published in Environmental Research Letters was provided by USDA NIFA (Project #NEVW-2017-08812) and the Lincoln Institute’s Babbitt Dissertation Fellowship Program.