Variable April Precipitation Widens Differences in Streamflow Forecasts — NRCS #runoff #snowpack

Here’s the release from the NRCS (Brian Domonkos):

The month of April brought below average precipitation across all of Colorado with extremely dry conditions in southern Colorado in particular. All major basins in the southern half of the state received less than 50 percent of average precipitation with the Rio Grande at the bottom of the list with a meager 16 percent of average. Most SNOTEL sites in the San Juan and Sangre de Cristo Mountains recorded the lowest or second lowest April precipitation on record. The basins of northern Colorado all received between 77 and 84 percent of average precipitation. “The precipitation differences across the state have caused notable changes to streamflow forecasts across the state since the beginning of April. Considerable declines in streamflow forecasts were observed across southern Colorado over the last month,” states NRCS Hydrologist Karl Wetlaufer. On average streamflow forecasts dropped 10-15 percent in the Rio Grande and combined San Miguel, Dolores, Animas, and San Juan basins and more in some southern tributaries to the Arkansas.

In addition to the past month the 2020 water supply situation is further complicated by conditions leading into the winter season. The late summer and fall of 2019 was also extremely dry leading to very low soil moisture conditions going into winter which can have an effect on the efficiency of snowmelt runoff. Wetlaufer notes that “Dry soils underlying the snowpack can absorb much of the snowmelt in the spring which has the potential to substantially decrease the amount of water that actually makes it to a stream channel to contribute to runoff”. Because of this effect it is anticipated that streamflows will be less than are commonly observed in other years with a similar snowpack peak and particularly in the southern half of the state. Statewide snowpack peaked on April 4th at 103 percent of normal but snowpack accumulation and melt patterns varied widely across the state after that point.

Reservoir storage has not varied widely this water year and statewide storage remains near normal at 104 percent of average. The largest monthly change was observed in the combined San Miguel, Dolores, Animas, and San Juan basins with a 9 percent drop bringing it down to 95 percent of average storage. Recent warm temperatures have been driving accelerated snowmelt which has been helping increase reservoir storage over the last two weeks but that could also lead to an early streamflow peak making that storage more important later in the summer.

So far water year 2020 has been a roller coaster of conditions through time and across the state and the last month has been no different. Given the unique nature of water supply conditions across the state and future weather uncertainty, it will be worth keeping a close eye on changes over the coming months.

Leave a Reply