The Dolores River runoff forecast into McPhee Reservoir is expected to be better than last year’s historic low but will likely fall far short of filling the Southwest Colorado lake relied on by farmers, officials said.
As of April 14, snowpack in the basin was at 70% of the average snow-water equivalent, and the runoff has started into the Dolores River and McPhee.
It is too early to tell exactly how much will make it to reservoir, said Ken Curtis, general manager for the Dolores Water Conservancy District.
“We think we are better off than last year, we’re not sure yet by how much,” he said. “The low-elevation snow below 9,000 feet is gone.”
A more definitive runoff forecast is expected in the coming weeks. A special meeting will be held May 5 at 7 p.m. at the DWCD office, 60 S. Cactus, St., Cortez, to provide an update on McPhee irrigation supply.
McPhee has 37,300 acre-feet of active supply, and is filling at a rate of 3 inches per day. The reservoir has a capacity of 229,000 acre-feet active supply.
Most of the current active supply in McPhee will be used to fill up adjacent Narraguinnep Reservoir, of the Montezuma Valley Irrigation District, which has started delivering water.
The Dolores Water Conservancy District has begun delivering water south via the Towaoc Highline Canal to the Ute Mountain Ute Farm and Ranch operation. Water delivery in the Dove Creek Canal to northern farms is expected in early May.
Last year at this time, the snowpack in the basin was at 32% average for snow water equivalent. Farmers received only 1 to 2 inches per acre, or 5% to 10% of the 22 inches per acre during normal snowpack years.
Last year’s water supply was the lowest since the reservoir first filled in the late 1980s.
This year, according to the Colorado River Basin Forecast Center, the April 1 runoff for the Dolores Basin ranged from 106,000 acre-feet to 230,000 acre-feet.
DWCD’s early predictions are on the conservative side, with a 90% probability of 2.5 inches per acre and a 70% probability of 6 inches per acre, still far below average.
“We have farmers thinking about the bottom end for supply, with the hope to raise it a bit,” Curtis said.
Many factors make predicting runoff difficult.
How much of the snowmelt will make it to the reservoir depends on the amount absorbed into the soil and how much evaporates during high winds and warm temperatures.
Dust on snow is also a factor because the darker layer increases absorption of heat, melting and evaporation. It also contributed to runoff happening a week early in the Dolores River.
There were five dust-on-snow events this winter in the San Juan Mountains, according to the Center for Snow and Avalanche Studies.
Dust layers are in the upper third of the snowpack, and the latest layer was deposited Monday, before Tuesday’s snowstorm, said Executive Director Jeff Derry.
As the snow melts off, the layers will merge and “further darken the snow surface,” he said in an April 14 report. “When all of these layers combine, we will see a snow surface darker than what we have seen in a good number of years, at least in the southern basins.”
To aid in the forecast prediction, on Friday a flyover of the Dolores Basin was planned by Airborne Snow Observatories Inc. to measure snowpack depth.
The plane uses technology that scans the elevation of the snow cover, then compares it with the bare ground elevation to determine snowpack.
The Dolores River at Dolores rose to 650 cubic feet per second on April 10, and dropped to 310 cfs on April 15, according to the U.S. Geological Survey. At Rico, the river was running at 65 cfs.