Brains
Recent storms brought much needed snow to ski areas in the western United States, but as the East Coast digs out from another blizzard and New England ski areas continue to enjoy record setting snow years, many Western ski areas continue to struggle to open and maintain terrain. With March rapidly approaching, this season has crossed the “bad start” threshold and is now solidly in “bad year” territory. The recent wave of storms brought much needed snow to many areas, with Palisades Tahoe reporting over 100 inches in the last 10 days, Alta reporting almost 60 inches, and Steamboat reporting almost 25 inches. Yet, most SNOTEL sites across the West still report below average snowpacks compared with twenty year averages.
For the almost 40 million people that live in the Colorado River Basin, which supplies water to seven states in the U.S. and two in Mexico, a low snow year can have dire consequences long after the ski season is over. Winter snowpacks store water up in the mountains, and their spring and summer melting feed streams and rivers throughout the summer. Between 85% and 95% of the water in the Colorado River Basin comes from mountain snowpacks. A system of dams on the Colorado River and its tributaries help store water, regulate spring runoff and prevent flooding, and generate hydroelectric power. Water supply requires active flows below dams with plenty of supply in reservoirs, flood mitigation requires keeping those flows below a certain level and having enough extra storage to handle all of the snowmelt, and hydroelectric power generation requires maintaining a certain flow out of the reservoir. All of these needs can often be in direct conflict with each other, and their balancing requires a detailed understanding of exactly where all of the water in the basin is.
The Colorado Basin River Forecast Center, or the CBRFC, part of the National Oceanic and Atmospheric Administration, is charged with keeping track of the water within the Colorado River Basin, and issues water supply forecasts to help water managers make decisions. Paul Miller, the Service Coordination Hydrologist for the CBRFC, told SnowBrains in a phone interview that water supply forecasts try and capture the current hydrological state of the forecast center, or to try and answer the question: where’s the water? Peering over the edge of the rim of the Grand Canyon might give you an idea of the current flow rate at that spot, but it is difficult to know if the river looks low because all of the dams in all of the tributaries are closed to let upper reservoirs fill, or if everything is wide open, but there’s no more snow to melt and feed water into the system. Water supply forecasts seek to describe how much water should be flowing through forecast points assuming no control measures are in place.
Miller told SnowBrains that the CBRFC uses an ensemble hydrological model to turn data from SNOTEL sites, weather stations, and stream gauges into forecasts for streamflows at 151 sites throughout the forecast area. Specific conditions vary from site to site, but with low snowpack across the mountains of the Colorado River Basin, the rest of the water year very likely will be a dry one. Miller said, “I think it’s fair to say that under these dry conditions and many reservoirs that are at relatively low levels, and streamflow conditions are expected to be below average, there are concerns that full water allocations will be difficult to fulfill, as well as meeting general environmental and hydropower targets.”
At the water supply briefing in early February, Hydrologist Brenda Alcorn described the impacts that the January drought had on the Colorado River Basin. Satellite data have shown the lowest snow coverage in the basin since data collection began in 2001, and many SNOTEL sites reported snowpacks well under half their median values. The lack of precipitation has caused water supply forecasts to drop sharply in some areas as much as 30% compared to the early January briefing. Every single site in the basin currently has a below average water supply forecast, with some sites in Utah, Arizona, and New Mexico sitting at less than 30% of the average.
One positive note that came out of the briefing was that some areas in the Colorado River Basin have above average soil moisture, thanks to a rainy fall. The San Juan Mountains and Central Arizona are both showing above average soil moisture, which can help runoff efficiency in the spring. However, most of Utah, the headwaters of the Colorado, and the Gunnison all are relatively dry this year.
With record low snowpacks, below average water supply forecasts, and dwindling reservoir levels in Lake Powell and Lake Mead, why not just close up all the dams and start saving every last drop of water possible? For one thing, uncertainties in the water supply forecast can still be relatively high in the end of February. Alcorn shared data that showed historical uncertainties in the forecast area range from 19% to 29%. She said most of this uncertainty is due to uncertainty in the weather. Miller shared one anecdote with SnowBrains about “Miracle May,” or May 2015, where heavy snowfall throughout the month boosted the water supply forecast from around 50% of average to just up over the average mark, essentially saving the water year. While many skiers are already hanging up their skis by then, there is still quite a bit of time for the snowpack to grow from a water supply standpoint.
The continuous demands on the Colorado river also complicate saving strategies. We use water continuously throughout the year, making it impossible to shut the gates completely. In fact, growing water demands and decreasing water supply has caused a multidecade decline in water levels in Lake Mead and Lake Powell, which now both sit at around 30% of their total capacity. Further upstream and closer to the mountains, the dynamics of spring runoff can be tricky to manage. Runoff dam operators typically like to have reservoirs almost completely empty at the start of the runoff season. Slow, continuous melting from May to July can be straightforward to manage, but a surprise spring or summer rainstorm could lead to huge spikes in runoff levels and overwhelm smaller reservoirs if they do not have enough available capacity. The CBRFC tracks spring runoff and issues Peak Flow Forecasts starting in March, and holds a special Spring Peak Flow Briefing in mid-March to help inform water managers.
Supporting vast metropolitan areas in the deserts of the Southwest means that every aspect of the water supply must be carefully accounted for. The team of hydrologists at the CBRFC and other river forecast centers, meteorologists, and researchers armed with data from hundreds of SNOTEL sites, weather stations, and satellite instruments is the level of detail that is required to forecast to the level of accuracy water managers, and water users, have come to rely on. Miller told SnowBrains in an email that “the Colorado River Basin is operated cooperatively between numerous federal and state agencies partnering with a wide variety of stakeholders. At the CBRFC we are striving to provide the best information to resource managers so that they can make the best decisions they possibly can to manage the basin efficiently and effectively.” The rest of this water year may present a challenge to the residents and water managers in the Colorado River Basin, but the CBRFC will continue working to provide the highest quality hydrological information that it can to help manage that challenge.
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