#ColoradoRiver Basin’s #snowpack season earns low grades: Bad news for water in the West #COriver #aridification

From The Washington Post (Becky Bollinger):

The lack of water worsens drought conditions, fire potential and could impact agriculture

The snowpack season is ending in the Colorado River Basin as the spring melt is underway. If we take stock of the water supply over this vast basin, a critical resource for millions of people in the West, the news is not good.

The snowpack season, so important for the storage of water that can be tapped during the dry summer months, fell well short of expectations. The consequences of the shortfall for the basin, encompassing Arizona and parts of six other states, from Wyoming to California, are major.

1. There is an increased risk for large wildfires that can devastate state and national forests, reduce summer recreation activities, compromise air quality for large areas of the country and put populations near the urban-forest intersections in danger.

2. The reduced water supply affects municipal and agricultural water users not only within the basin’s 246,000 square miles, but also outside it, including Denver, Salt Lake City and Los Angeles.

3. Prolonged drought could ultimately affect food supply, causing reductions in crop yields and livestock herds.

To put this season in perspective, I’ve made a report card of the various indicators of snowpack to illustrate why the low grades are so serious.

The snowpack itself: C

The snowpack picture seemed promising at times in recent months, especially in February when several storms unloaded hefty snows. Even now, some late season snow in the northern part of the basin is working in some extra credit. But it’s not enough.

At the headwaters of the Colorado River, the snowpack peaked on April 2, about 10 days ahead of average. Since then, more than two inches of water have melted. In fact, since the beginning of April, the majority of stations in the upper Colorado River basin have seen melt rates between 2 and 6 inches. When the snowpack peaks and melts early it often portends a lower water supply during the dry season.

The progression of snow water equivalent, an indicator of snowpack, in 2021 in the Colorado River Basin compared to other years. (Natural Resources Conservation Service)

Indeed, throughout the entire Colorado River Basin, snowpack values peaked at levels well below average. From the Upper Green Basin in Wyoming and south through Utah and Colorado, many locations peaked in the bottom 25th percentile.

Snow water equivalent percentiles, an indicator of snowpack, as of early April (Natural Resources Conservation Service)

Soils: F

Soils have been the problem child since the very beginning of the water season, when the summer-fall monsoon was essentially a no-show.

If the monsoon had provided the needed moisture in June-September to the lower part of the basin and the southern portion of the upper basin, healthy soils would have been locked in during the cold season. But without the monsoon moisture, the basin went into the snowy part of the season with dry soil, essentially saddling the water supply with a debt that is far from being repaid.

Map credit: USGS

Streams: D

Stream flow data doesn’t look too bad at the moment. According to the U.S. Geological Survey, the percent of the upper part of the basin observing near normal flow conditions has actually increased from 21 percent to 42 percent. But don’t let that deceive you.

Late in the water season, streams often appear to be doing better than they actually are. So, what’s happening?

(U. S. Geological Survey)

Check out the hydrograph (above) from the Colorado River at the Colorado-Utah state line. The black line shows the average flow in recent months, compared with historical values (indicated by the colored shading).

Back at the beginning of March, flows were in the brown shading, ranking in the bottom 10th percentile. More recently, you can see that flows have bumped up to the yellow category, slightly improved from the brown. But this bump is mainly due to an early rise toward the peak. That early rise has been kicked off by early melting of the snow. The “improvement” is only an artifact of the early snow melt and will not be sustained.

Reservoirs: D

The water stored in reservoirs is akin to the output of a group project, contingent on the performance of its contributors. Since snow quantities, soil moisture and streams underachieved, reservoirs also end up with a low grade.

Water Supply Outlook April 1, 2021 via the Colorado Basin River Forecast Center

According to the April 1 water supply forecast, published by the Colorado Basin River Forecast Center, all of the Colorado River Basin will experience below-average water supply.

Across the Lower Colorado River, water supply accumulations began in January and most of the snow has completely melted. For the Lower Basin and southern half of the Upper Basin, water supplies are expected to be below 50 percent of average. Lake Powell inflows are forecast at 38 percent of average, a deficit of almost 4 million acre-feet! For perspective, current levels are already 6 million acre-feet below what they should be right now.

Further north, the forecast is marginally better, with water supply expected to be between 50 and 70 percent.

These low forecasts are largely based on less than stellar snowpack conditions, but dry soil moisture conditions at the beginning of the season are also considered. According to Cody Moser of the Colorado Basin River Forecast Center, antecedent soil moisture conditions can make a 5 to 10 percent difference in predicted runoff.

Lake Powell water supply 1999 to 2019.

Lake Powell, which represents the majority of the Upper Colorado River Basin’s water supply, has still not recovered from the drought in the early 2000s. It takes more hits from each new drought. The system had a nice recovery from the 2018 drought, but still hasn’t made up lost ground from another drought in 2012-2013. Unfortunately, we’ll put 2021 down as another year to further deplete this system.

Becky Bolinger is the assistant state climatologist for Colorado and a research scientist at Colorado State University.

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