NEWS

United States Department of Agriculture
Natural Resources Conservation Service
9173 W. Barnes Dr., Suite C
Boise, Idaho 83709
 
For More Information:

Ron Abramovich, 208/378-5741
Phil Morrisey, 208/685-6983

Current Snowpack and Water Supply Needs for 2008

BOISE, Idaho, November 16, 2007 -- The outlook for the new water year and next year's water supply is optimistic as the entire mountain SNOTEL (SNOw TELemetry) Network in the Pacific Northwest has received above average precipitation.  Some basins received double their normal October allotment.  After one of the driest summers on record, the vegetation and soils greatly needed the rain to increase soil moisture before the onset of the winter snowpack.  However, not all central Idaho streams increased in response to the precipitation. Still, this is a good sign, since often several storms are needed to saturate the soils after a typical dry Idaho summer.  Saturated soils are efficient at producing more streamflow when the snow melts.

Each watershed responds differently to snowpack amounts. Historically, an above average snowpack does not always equal above average streamflow.  Spring and summer streamflow is dependent upon many factors such as spring and fall precipitation, soil moisture, groundwater and spring levels, geology, spring air temperatures, previous long-term climatic effects, etc… 

Water users can find data on reservoir storage, current precipitation and streamflow levels by individual basins on the USDA Natural Resources Conservation Service (NRCS) snow website at http://www.id.nrcs.usda.gov/snow/.  The current data, listed under Fall 2007-Snow Survey News, communicates to water users the current water supply and shows how much precipitation is needed during the water year to meet demands for water users and managers.  For example, fall 2007 numbers currently show the Boise reservoir storage at 310,000 acre-feet.  Based on projections from past years, the estimated storage levels for March 31 will be 460,000 acre-feet.  The irrigation demand for the basin, which is the combination of reservoir storage and streamflow, averages 1.5 million acre-feet per year.  This means that the seasonal streamflow volume will need to be 1.04 million acre-feet, or 68 percent of average to meet water demands.  Based on historic snowpack and streamflow relationships, runoff of 68 percent or better happens when April 1 snowpack is greater than 85 percent of average.  Data for all of Idaho’s major irrigation basins are available on the NRCS snow website under ‘2008 Snow & Streamflow Needed’ report.

In some years, spring precipitation provides supplemental moisture to saturated soils, allowing excess water to flow into reservoirs.  Too often producers and decision-makers cannot wait until spring to see if precipitation levels will increase or decrease snowmelt streamflow forecasts.  Over the next five months, NRCS will inventory the snow and make projections of next year's water supply as water managers, hydropower producers, river runners and others base their decisions on the snow.

The bottom line is that April 1 snow levels need to range from 80 to 120 percent of average, depending on the basin.  The earlier it is known how this winter’s snow levels compare to the April 1 snow average, the better for decision makers.  If a good snowpack does not accumulate in the mountains this year, then Idaho's numerous water users will need abundant spring precipitation. In May 2005, precipitation ranged from 200 to 300 percent of average across central Idaho. Overnight, the dismal water supply outlook changed and the rain provided much needed runoff for water users, hydropower producers and river runners.

The National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center has confirmed La Nina conditions in the equatorial Pacific, which is usually associated with a wet, cold pattern in the Pacific Northwest.  The key word is “usually.”  This year’s La Nina conditions are stronger and look more promising than past years, but it is too early to count on the precipitation to meet the water demands of the Pacific Northwest until it falls.

To stay current on this year's precipitation levels, snowpack amounts and streamflow forecasts, visit the following links for daily or monthly updates:

Daily snow levels: http://www.id.nrcs.usda.gov/snow/data/current.html
Daily and monthly streamflow forecasts: http://www.id.nrcs.usda.gov/snow/watersupply/

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