Streamflow Forecast

The first spring–summer streamflow forecast for the Southwest, issued on January 1, shows a 50 percent chance that flows in most basins in Arizona and New Mexico will be below average (Figure 12). There is a 50 percent chance that the Salt, Verde, and Gila rivers in Arizona will experience streamflows amounting to 65, 64, and 83 percent of the January–May average, respectively. Although widespread and copious rain and snow soaked many mountain regions during November and December, dry conditions have largely prevailed in the last month. These conditions are expected to continue for the most part because the La Niña event is forecast to persist into spring. The La Niña also is influencing forecasts in New Mexico. There is a 50 percent chance that the March–July flow in the Rio Grande, measured at Otowi Bridge, and the Gila River, measured near Virden, will be 88 and 78 percent of average, respectively. On the other hand, above-average flows are expected in the Mimbres and Pecos rivers. Streamflow forecasts are issued every month for New Mexico and every two weeks for Arizona and become progressively more accurate as the winter progresses.

Spring inflow to Lake Powell is forecast to be about 64 percent of the 1971–2000 average for April–July, or about 3.2 million acre-feet. The forecast also indicates a 30 and 10 percent chance that Lake Powell inflow will be 81 and 109 percent of average, providing an indicator that above-average flows are unlikely. Last winter’s exceptionally high streamflows, which increased combined storage in Lakes Mead and Powell by about 7 million acre-feet between April and July—or about 2 million more than average—will buffer below-average flows in the Colorado River this year.