More Snow Pack Will Not Translate To More Runoff

By Tammy Gray-Searles
   Although snow pack was higher than average in Northern Arizona this winter, the U.S. Department of Agriculture (USDA) is predicting that spring runoff will be lower than average.
   “Our snow pack peaked early in the winter, but our March 1 survey shows that much of the snow pack has melted,” noted USDA Natural Resource Conservation Service (NRCS) water supply specialist Dino DeSimone.
   A press release from NRCS explained, “Historically, the beginning of March shows the maximum snow accumulation for the year. Although the statewide snow pack is currently at 111 percent of the long-term average, several sites have no snow, and the aerial extent of the snow pack is limited in the uppermost reaches of the basins.”
   “What snow remains is rapidly melting, and the outlook for more precipitation in the coming months is poor,” DeSimone also noted.
   According to NRCS, the flow rate of the Little Colorado River near Woodruff is expected to be nearly 66 percent below average during the spring runoff period from March to May. Flows in other rivers across the state are also expected to be below average.
   “Long-range predictions call for well below median runoff for the March-May forecast period,” the NRCS press release notes. “Flows over these three months are expected to be 63 percent of median in the Salt River and 54 percent in the Verde River. Forecasts call for 45 percent of median flows in the San Francisco at Clifton and the Gila River is forecast to run 33 percent of median at Safford.”
   Runoff for the Little Colorado River above Lyman Lake is expected to be about 79 percent of the average, even though precipitation levels in the same area were above average through the end of February.
   The difference between snow pack and precipitation levels and runoff levels is explained by many factors, including the water content of the snow, the amount of moisture absorbed by the soil, changes in air temperature and the melting rate of the snow pack.
   Even though runoff is predicted to be lower than average, snow pack as of March 1 was 116 percent of the 30-year average on the Mogollon Rim, 121 percent on the San Francisco Peaks and 117 percent on areas that flow to the Little Colorado River. The Grand Canyon, on the other hand, only had about 58 percent of the average 30-year snow pack, and the San Francisco-Upper Gila River test site, only 54 percent.
   Literature on the difference between snow pack and runoff provided by the NRCS explains, “The stage is set for the snow-water year even before the first snowflakes fall. The amount of moisture that accumulates in the soil early in winter, before the snow pack develops, will affect runoff the following spring. Dry soils tend to absorb more of the meltwater than wet soils. The amount of moisture that is absorbed depends on soil characteristics as well as precipitation. Wind, air temperature, storm frequency and the amount of moisture in the atmosphere determines the accumulation of the snow pack. How the snow pack accumulates affects its density, the amount of water per unit volume of snow, and texture, its crystalline structure.”
   According to NRCS, not all snow contains the same amount of water. Light powdery snow, such as that preferred by skiers, contains less water per foot than heavy, sticky snow.
   The runoff predictions are provided by the NRCS to help both farmers and ranchers plan for upcoming seasons, and to allow cities, towns and other agencies that supply water to predict incoming water supplies. Water storage in many of the state’s large reservoirs is already below last year’s levels and are predicted to continue to stay far below the 30-year averages.