Apr 082011

By Tammy Gray-Searles —

Despite a promising start to the winter, streamflow forecasts for Arizona call for low run-off and reservoir levels, including the Little Colorado River Basin.

“Well below normal streamflow levels are forecast for the basin,” a report by the U.S. Department of Agriculutre (USDA) Natural Resource Conservation Service (NRCS) reports.

At the Little Colorado River near Woodruff, streamflow is expected to be at only 18 percent of the median through May. The report for runoff at Lyman Lake is worse, with streamflow expected at only five percent of the median.

Streamflow is a result of snowpack accumulated over the winter months, and the report notes that few surprises are expected since the snow has already melted at most measurement sites across the state.

“Most measurement sites are now melted out, which is about a month earlier than normal,” the report states. “The statewide snowpack is also well below normal at 43 percent of average.”

According to the report, low precipitation levels in March also contributed to the reduced runoff amounts.

“The final forecast of the season considers several factors,” the report states, “including an exceptionally dry March, extremely poor snowpack conditions, current streamflows well below normal, and an expectation that these conditions will persist.”

In the Little Colorado River Basin, precipitation levels hovered between 70 and 80 percent of the average in October and November, spiked to more than 180 percent of the average in December, but then sunk to about 10 percent of average in January. February saw a recovery, with precipitation at about 80 percent of average, but in March it was around 30 percent of average.

The year-to-date precipitation for the basin through March is about 70 percent, thanks to large amounts of precipitation in December.

Snowpack, however, did not reach those levels. According to the NRCS, snowpack at the headwaters of the Little Colorado River was 16 percent of average. Snowpack in the central Mogollon Rim was 20 percent of average.

“The current conditions are in sharp contrast to what we saw a year ago at this time, when the snowpacks ranged from 219 to 246 percent of average (statewide),” NRCS state water supply specialist Dino DeSimone noted.

Reservoir water storage levels are also down across the state. At Lyman Lake, however, levels are up. Current storage is 17,700 acre-feet, while last year at this time the lake was at 11,500 acre-feet. The 30-year average is 17,200 acre-feet, while the lake has a capacity of 30,000 acre-feet.

Most other reservoirs across the state have not fared as well, with San Carlos Reservoir down to 95,000 acre-feet compared to 283,000 acre-feet last year. The Verde River system, Salt River system, Lake Pleasant, Lake Mead and Lake Powell are all down, too. Lake Havasu and Lake Mohave, however, are up slightly.

Streamflow predictions are used to help manage water supplies across the state, as well as to prepare for upcoming seasons.