Streamflow Forecast

The first spring–summer streamflow forecast for the Southwest, issued on January 1 by the National Resources Conservation Service (NRCS), shows a 50-percent chance that flows in most basins in Arizona and New Mexico will be below average (Figure 12). Based on the accumulated precipitation through January 1, there is only a 50-percent likelihood that the Salt River, measured near Roosevelt Lake, and the Gila River, measured at the inflow to San Carlos Reservoir, will exceed 65 and 44 percent of the January–May average, respectively. In these probabilistic forecasts, lower likelihoods are accompanied by higher percent of average streamflows, and vice versa. For example, the Salt River has only a 30 percent likelihood that flows will be near average. The Verde River is the only watershed in Arizona where spring streamflows have an equal chance of being above average. For Lake Powell, there is only a 50-percent chance that spring inflow will be above 56 percent of the 1971–2000 average for April–July, or about 4.0 million acre-feet. The forecast also indicates a 30 and 10 percent chance that Lake Powell inflow will above 72 and 98 percent of average, respectively, providing an indicator that above-average flows are very unlikely.

In New Mexico, there is a 50-percent chance that the March–July flow in the Rio Grande, measured at Otowi Bridge, will be 47 percent of average. If this occurs, irrigators in the Elephant Butte Irrigation District could experience another season with below-average allotments. As of January 1, Elephant Butte Reservoir contains only 7 percent of its full storage. Also, the projected inflow into the El Vado Reservoir, on the Rio Grande north of Otowi Bridge, is only 64 percent of average; El Vado contains only 5 percent of average storage.

The winter is still early and a large fraction of the winter precipitation typically falls during the January–March period. As the season advances, therefore, streamflow forecasts become progressively more accurate.