USA: April showers may bring May flowers, but winter snow is water in the bank - USGS

By Suzanna Soileau, Catherine Puckett, Greg Clark

The type of precipitation falling from the sky matters, especially for delicate mountain ecosystems. It’s really all about snow – mountain ecosystems across the western United States are helped the most by precipitation in the form of snowfall. When snow falls on higher elevations it accumulates as snowpack and becomes a water “savings account” for lower elevation landscapes across the country for later in the year.

People living on the Mississippi River delta may not realize that one source of that great river is from water that began as snow in mountain ranges in Montana, Wyoming and Colorado.

While the east coast of the United States set records for snowfall this past winter, much of the western United States has entered into spring with substantially less mountain snowpack than previous years. In fact, the Cascade Mountains and the Sierra Nevada Mountains recorded the lowest snowpack ever measured in history, according to the U.S. Department of Agriculture. Through March 1 of this year, certain regions of Arizona, California, Idaho, Nevada, Oregon, and Washington have mountain snowpack at less than 25 percent of average from 1981-2010.

Though some western states received periodic rainfall events this winter, much of it came as heavy or even torrential rain that quickly ran off of the landscape without giving the soil a chance to soak up the much needed moisture. Additionally, in this very dry year the soils are so dry that a large proportion of the snowmelt is filling up the voids in the soils and reducing the runoff to the reservoirs. Unless that water is captured in reservoirs, much of it would escape as runoff and not be available as a water supply later in the year.

In drought-stricken California, torrential rain can cause significant flooding events, and USGS scientists are examining atmospheric rivers, which are the cause of many of the most extreme storms along the West Coast and a large majority of the floods in that region. In addition, USGS scientists are using hydrologic models that include estimates of soil moisture to assist the Department of Water Resources to understand how much of the melting snowpack will end up in the reservoirs. These tools are used to uncover evidence of climate connections in areas such as the Sierra Nevada Mountains, where there is less snow, and it is melting earlier in the year, leaving less water available for irrigation in the summer. The estimated percentage of snowpack in the Sierra Nevada Mountains that will end up as runoff to the reservoirs by mid-summer ranges from about 35% to 75%.

Mountains as a water savings account

When scientists are monitoring snowpack, they measure and record the Snow Water Equivalent (SWE), which is simply the amount of water contained within the snowpack. Because snow is so variable in density, SWE is just a convenient way of standardizing the water content. An inch of SWE (which is also an inch of rain) is what results when snow melts into water but the amount of snow that is required for that amount can vary from inches to feet. For instance drier, powdery snow – the kind skiers love – has very low water content and requires more depth to produce 1 inch of SWE. Heavier, wet snow would need less depth to produce that same 1 inch of SWE.

Meteorologists and hydrologists use SWE to forecast water availability and estimate potential flooding events. USGS scientists use SWE in a variety of research such as forecasting avalanche hazards and assisting Glacier National Park’s road crew in their spring plowing efforts of the popular Going-to-the-Sun Road. In the Colorado River Basin, scientists are studying snowpack sublimation, which is similar to evaporation from land or water bodies and which represent water loss from the snowpack that can be of particular concern during drought years.

The repercussions of snowpack declines

In the Rocky Mountains, runoff from winter snowpack accounts for 60 to 80 percent of the annual water supply for more than 70 million people living in the western United States, and is influenced by factors such as SWE and the timing of snowmelt. When snow melts affects not only when water is available for crop irrigation and energy production from hydroelectric dams, but also the risk of regional floods and wildfires. Earlier and faster snowmelt could have repercussions for water supply, risk management and ecosystem health in western watersheds.

Since the 1980s, there have been snowpack declines along the entire length of the Rocky Mountains, according to a USGS study published in 2013. The study showed increasing temperature as the major driving factor in snowpack declines over the past 30 years. In fact, warmer spring temperatures since 1980 have caused an estimated 20 percent loss of snow cover across the Rocky Mountains. Scientists identify much of the climate-related trends of river flow, winter air temperature, and snow pack are human caused, which may have significant repercussions for water supply for the western United States.

Ecosystems, of course, are dynamic. Consequently, what is observed over a period of time across a large geographic area may well have more localized, regional variations. One 2010 study found that in Idaho, snowmelt and peak streamflows are occurring almost 2 weeks earlier in the spring than they did in the mid-1900’s. The study also found that the yearly minimum streamflows during late summer had significantly decreased over the same time period.

The bottom line – top-down matters

The bottom line is that what happens at the top of our mountain ecosystems during winter months has long lasting impacts downstream.

Because alpine vegetation endures in the harshest mountain climates, researchers expect it to show early responses to these impacts. Trees and meadows in some areas are already advancing upslope, indicating that warming temperatures are facilitating higher elevations to accommodate these species, which could outcompete native alpine vegetation.

Similarly, many aquatic species are vulnerable to climate change because they are cold water dependent and confined to mountaintop streams immediately below melting glaciers and permanent snowfields. A recent USGS study found that the persistence of an already rare aquatic insect, the western glacier stonefly, is being threatened by the loss of glaciers and increased stream temperatures due to climate warming in mountain ecosystems.

USGS and others are currently studying climate change impacts on mountain species such as the American pika, denning animals such as wolverine and grizzly bears, amphibians, alpine berry species, and native fish to quantify potential impacts to habitat and species health. A 2014 USGS study documented stream temperature warming over the past several decades and decreases in spring flow over the same time period contributed to the spread of hybridization between native westslope cutthroat trout and introduced rainbow trout in Montana and British Columbia, Canada.

Loss of any one of these species – from a tiny and rare alpine insect to native trout – will cause changes from the top of the ecosystem down. For example, decreasing permanent snowfields and warming stream temperatures threaten alpine vegetation and aquatic ecosystems, which in turn affects the habitat and food resources for fish species – a key food for other mountain animals. Additionally, less snowpack and earlier spring runoff translates to less water availability downstream later in the year impacting agriculture, recreation and general quality of life for humans.