Southern Colorado river basins have suffered a setback in terms of the seasonal water supply outlook following a dry April, and dust-on-snow conditions have accelerated snowmelt in that region.
More northern basins, including the Upper Colorado River Basin in the state, are in better shape in terms of remaining snowpack but are still below average, as regional water managers brace for yet another year of below-normal streamflows impacting storage levels at reservoirs.
Statewide snowpack as of Friday was at 80% of normal, according to the federal Natural Resources Conservation Service. But while amounts in northern Colorado ranged from 93% in the Upper Colorado and Yampa/White river basins to 100% in the Laramie/North Platte basin, in southern Colorado the Gunnison River Basin is now at 79% of normal, and it gets worse from there elsewhere. The Arkansas River Basin is at 54%, the combined San Miguel/Dolores/Animas/San Juan rivers, 39%, and the Upper Rio Grande, just 25%.
“Although a couple small portions of the state are forecast to have near-median streamflow this year, most of the state is forecasted to have well-below median streamflow volumes,” the NRCS said in its latest monthly water supply outlook for Colorado. “Despite near median snowpacks for much of the winter, runoff during the spring and summer months will likely be shorter and less than anticipated without significantly increased precipitation in the coming weeks.”
But the federal Climate Prediction Center is predicting below-average precipitation in Colorado this month, the NRCS noted in its outlook.
Forecasts for total April-July streamflow from melting snow range from around 60% of normal for the far-southwest basins and the Upper Rio Grande; to close to 80% in basins including the Upper Colorado, Gunnison and Arkansas; to 90% and 101% for the Yampa and North Platte basins, respectively.
Grand Mesa stands out as a somewhat encouraging exception to the rule this year. Currently, snowpack amounts range from 70% to 109% of normal at Grand Mesa’s three NRCS measuring sites, and the NRCS is estimating that combined streamflows in the Surface and Kannah creek watersheds of Grand Mesa will average 106% of normal.
Grand Mesa snowmelt is relied on as a source of local agricultural and municipal water, just as Colorado’s overall mountain snowpack is crucial as a water supply both in the state and downstream. Seasonal snowpack accumulation typically peaks in early April before melting begins to outpace further additions from storms.
Besides experiencing drier and warmer weather, southern Colorado in the last month also was impacted by windstorms blowing dust and dirt onto mountain snow. That speeds up snowmelt due to the snow being warmed by what’s effectively a dark blanket on top of it that absorbs heat from the sun.
The NRCS says dust-on-snow storms contributed to rapid melting primarily in southern mountains and lower-altitude sites. In the San Juan Mountains, snow containing 10 inches of water melted in the Wolf Creek area last month. Lizard Head Pass outside Telluride lost 10.7 inches of snow water equivalent in April, melting out completely by Tuesday, three weeks earlier than normal, according to a recent update from the National Integrated Drought Information System.
Statewide reservoir storage currently is at 77% of median for this time of year. Reservoir storage has taken a hit in recent years due to ongoing drought, particularly at Blue Mesa Reservoir, which last year was drawn down further to help prop up fast-diminishing water levels downstream at Lake Powell. Blue Mesa currently is holding about 252,000 acre-feet of water, about half of normal for this time of year.
“This year’s water supply will need to be monitored closely and looked at basin-by-basin for the remainder of the water year,” NRCS hydrologist Karl Wetlaufer said in a news release. “With low streamflow forecasts in combination with low reservoir storage, Western, and particularly Southwest Colorado, have a difficult water supply management situation this year.”
NRCS notes that the U.S. Drought Monitor map for Colorado is little changed from a month ago. Much of the state is experiencing moderate drought.