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Published April 24, 2013
Streamflow Forecast(for spring and summer)
Data Source(s): National Water and Climate Center
The spring–summer streamflow forecast for the Southwest, issued on April 1 by the Natural Resources Conservation Service (NRCS), calls for well-below-average flows in all river basins in Arizona and New Mexico and the Upper Colorado River and Rio Grande basins (Figure 12). Projected streamflows for the April–May period in the Southwest have nearly all decreased since last month—the result of dry conditions in March. Moreover, the below-average forecasts reflect in large part the scant precipitation that has fallen this winter throughout the region. In Arizona, the April 1 forecasts call for only a 50 percent chance that the Salt River, measured near Roosevelt Lake, and the Gila River, measured at the inflow of San Carlos Reservoir, will exceed 32 and 5 percent of the April–May average, respectively. The 50 percent likelihood can be considered the best estimate. In these probabilistic forecasts, lower likelihoods are accompanied by a higher percent of average streamflows, and vice versa. For example, the Salt River has only a 10 percent chance of exceeding 57 percent of average flows.
For Lake Powell, there is only a 50 percent chance that spring inflow will exceed 34 percent of the 1981–2010 average for April–July, or about 2.4 million acre-feet. The forecast also indicates only a 10 percent chance that Lake Powell inflow will be more than 56 percent of average, providing an indicator that above-average flows are extremely unlikely. These forecasts represent a decrease from one month ago. If the April to July runoff into Lake Powell is around 34 percent of average, it would be the fourth-lowest total since Lake Powell became operational, says NOAA’s Colorado River Basin Forecast Center.
In New Mexico, streamflow forecasts are all below average. For the Rio Grande, the best estimates suggest a 50-percent chance hat the Rio Grande will experience less than 50 percent of average flow for the April–July period. This is a slight decrease from estimates made one month ago. Snowpack conditions in the upper Rio Grande headwaters in Colorado are less than 50 percent of the historical average. Total reservoir storage in the state is also well below average and stands at about 20 percent of total capacity.
Water supply forecasts for the Southwest are coordinated between the National Water and Climate Center, part of the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS), and the Colorado Basin River Forecast Center (CBRFC), part of NOAA. The forecast information provided in Figure 12 is updated monthly by the NWCC. Unless otherwise specified, all streamflow forecasts are for streamflow volumes that would occur naturally without any upstream influences, such as reservoirs and diversions. The coordinated forecasts by NRCS and NOAA are only produces for Arizona between January and May, and for New Mexico between January and May.
The NRCS provides a range of forecasts expressed in terms of percent of average streamflow for various exceedance levels. The forecast presented here is for the 50 percent exceedance level, and is referred to as the most probable streamflow. This means there is at least a 50 percent chance that streamflow will occur at the percent of average shown in Figure 12. The CBRFC provides a range of streamflow forecasts in the Colorado Basin ranging from short fused flood forecasts to longer range water supply forecasts. The water supply forecasts are coordinated monthly with NWCC.
For state river basin streamflow probability charts, visit:
For information on interpreting streamflow forecasts, visit:
For western U.S. water supply outlooks, visit:
Southwest Climate Outlook Staff
- Michael Crimmins, UA Extension Specialist
- Stephanie Doster, Institute of the Environment Editor
- Gregg Garfin, Founding Editor, Institute of the Environment
- Zack Guido, Managing Editor, CLIMAS Associate Staff Scientist
- Nancy J. Selover, Arizona State Climatologist
- Jessica Dollin, CLIMAS Publications Assistant
- Dave Dubious, New Mexico State Climatologist
Please direct your Southwest Climate Outlook comments and suggestions to Zack Guido.
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