Streamflow Forecast

The May 1 spring-summer streamflow forecast for the Southwest shows a 50 percent chance that all basins in the Colorado River, Rio Grande, and Arkansas watersheds will be below average (Figure 13).

In Arizona, the last streamflow forecast was made on April 1 because the vast majority of the rain and snow that contributes to spring streamflows has already fallen by this date. On April 1, there was a 50 percent chance that the Salt, Verde, and Gila rivers would have streamflows equal to or less than 28, 41, and 7 percent of the February–May average, respectively. In New Mexico, the last streamflow forecast, issued on May 1, suggests a 50 percent chance that streamflow in the Rio Grande, measured at Otowi Bridge, will be 45 percent of average and a 50 percent chance that inflow into Navajo Reservoir will be only 44 percent of average.

In the Upper Colorado River Basin, all snow-monitoring stations are reporting below-average snowpacks, with most measuring no snow. Many stations have been persistently below average all winter. As a result, the most recent forecast issued on April 1 projected that the April–July inflow into Lake Powell would be only about 3.5 million acre-feet (MAF), or 44 percent of the 1971–2000 April–July average. In snow-dominated basins like those in the upper Colorado River, April 1 forecasts are usually 80–90 percent accurate. Also, projections for the entire 2012 water year, which extends from October 1, 2011 through September 30, 2012, call for inflow into Lake Powell to be around 5.57 million acre-feet (MAF). If this holds true, inflow will rank as the fourth lowest on record since the closure of the Glen Canyon Dam in 1963. Precipitation in coming months could increase or decrease, with the likely inflow range falling between 4.9 MAF (45 percent of average) and 6.5 MAF (60 percent of average).