Predict the Peak: Spring Runoff 2021
It’s time for my annual San Juan River spring runoff prediction! The 2021 water year is half over and spring runoff on the San Juan River has begun. Over the last 2 weeks, there have been above-average flows on the San Juan River and daily peaks have eclipsed 700cfs on 3 occasions. Unfortunately, that means we’ve lost 20% of our snowpack during a time when we normally cache away a few more inches of water. Historically, the Upper San Juan SNOTEL Site (USJSS) reaches its peak snowpack on April 15th. The good news is the thundersnow that’s currently falling outside my window may replenish some of our early losses.
I use the snowpack at the USJSS to forecast the timing and magnitude of the peak spring runoff event because its elevation it’s closer to the mean elevation of the watershed. The site is located at 10,200 feet on the west side of Wolf Creek Pass and it has a historical snowpack record that dates back to 1935. The San Juan River streamflow records also date back to 1935.
The 2020-2021 winter snowpack at the USJSS peaked at 93% of normal on April 2nd at 30 inches of snow water equivalent (SWE), which is two weeks earlier than normal. The Wolf Creek Summit Snotel Site also peaked on April 2 (a month earlier the normal) with 31.8 inches of SWE, which is 90 percent of normal.
Unfortunately, the soil remembers the dry summer and fall of 2020 and will likely absorb more runoff than usual. Based on observations in my neighborhood, the low elevation snowpack didn’t top off Lake Hatcher as I expected, and the dry soil conditions may have been the culprit. Currently, Lake Hatcher is at full capacity mainly due to the inflow of ditch water from Fourmile Creek all winter long.
In 2013, I created a class project in which I taught my students how to correlate snowpack with spring runoff. We’ve continued to add to that database and use the equation that we generated to predict spring runoff each year. Our spring runoff equation predicts 2550 cfs will crest in downtown Pagosa Springs on May 23rd. Unfortunately, I don’t think our snowpack will last that long.
Another method of forecasting spring runoff involves multiplying the median peak runoff by this year’s peak snowpack percentage. As far as timing goes, the peak snowpack occurred two weeks early, which should shift the peak runoff earlier. This alternative method suggests a peak of 2300 cfs on May 9th, which I think is more reasonable. Last year, the river peaked at 1930 cfs on May 2nd, but temperatures in Pagosa Springs reached lows 80s by the end of April.
The long-term weather outlook shows above-average temperatures and below-average precipitation, which also suggests the river will peak early again this year. Even though I love sunshine, I hope a few more storms sneak into our area and keep our forests moist so they can resist beetle attacks and escape any major summer wildfires.
Meanwhile, we'll wait expectedly for 2300 cfs on May 9th. I'm sure I'm wrong, but can you do better? Leave your prediction in the comments!