Streamflow Forecast

Last winter’s La Niña event left a dry imprint on the Southwest. The spring–summer streamflow forecast issued by Natural Resources Conservation Service for the Southwest issued on April 1 predicts below-average flows for basins in the Mogollon Rim region of Arizona and New Mexico basins and near- to above-average flows for most of the Upper Colorado River Basin (Figure 12). These forecasts are based principally on snow accumulation in the mountains from which most of the water originates. Precipitation during March was well below normal throughout all basins in Arizona and New Mexico. In Arizona, snowpack levels were well below average in all of the basins as of April 1, with most measurement sites void of snow. A winter storm in early April boosted snowpack and precipitation totals in many basins in the Southwest, including the Upper Colorado River Basin, resulting in higher streamflow projections for the Colorado River.

The April 1 forecasts projects a 50 percent chance that inflow to Lake Powell will be about 120 percent of the 1971–2000 average for April–July, or 9.5 million acre-feet, an increase over last month’s forecast of 113 percent of average. Forecasts for the Salt and Upper Gila rivers, on the other hand, call for very low probabilities that flows will be near average.  Forecasts indicate a 50 percent chance that streamflow in the Salt and Verde rivers during the April–May period will be equal to or less than about 15 and 43 percent of average, respectively.

In New Mexico, the April 1 forecast shows the majority of the state on pace for well below-average runoff. Forecasts indicate a 50 percent chance that the Rio Grande streamflow at Otowi Bridge will be only 48 percent of average for the April–July period. Forecasts for the Upper Gila and Pecos rivers call for a 50 percent chance that streamflow will be at or below 23 and 35 percent, respectively, for the same period.