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Idaho Water Supply Outlook Report March 1, 2020

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Idaho Water Supply Outlook Report March 1, 2020 Natural Resources Conservation Service Idaho Water Supply Outlook Report March 1, 2020 Boise Foothills near Bogus Basin Road Snow Course, February 28, 2020 Photo courtesy of Melissa Ghergich The Bogus Basin Road snow course (5,568 ft), visible above in the sunlit aspect of the image foreground, serves as an important barometer for transition zone snowpack. The March 1 snow measurement yielded 8.6” of snow water equivalent (SWE), which is 130% of normal. Several other snow measurement sites in the Payette and neighboring Boise River basin depict a similar story: above normal snowpack in the ‘lower’ elevation (~5,000 to 6,000 ft) zones. Interestingly, in and near the Boise River basin, all measurement sites below 5,700 ft are reporting above normal SWE, while all stations above 5,700 ft are reporting below normal SWE. Continue reading the full report for snowpack and water outlook details throughout Idaho. Water Supply Outlook Report Federal - State – Private Cooperative Snow Surveys For more water supply and resource management information: Contact: Your local county Natural Resources Conservation Service Office Internet Web Address: http://www.id.nrcs.usda.gov/snow/ Natural Resources Conservation Service Snow Surveys 9173 West Barnes Drive, Suite C Boise, Idaho 83709-1574 (208) 378-5700 ext. 5 To join a free email subscription list contact us by email at: [email protected] How forecasts are made Most of the annual streamflow in the western United States originates as snowfall that has accumulated in the mountains during the winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff that will occur when the snow melts. Measurements of snow water equivalent at selected manual snow courses and automated SNOTEL sites, along with precipitation, antecedent streamflow, and indices of the El Niño / Southern Oscillation are used in computerized statistical and simulation models to produce runoff forecasts. Unless otherwise specified, all forecasts are for flows that would occur naturally without any upstream influences. Forecasts of any kind are not perfect. Streamflow forecast uncertainty arises from three primary sources: (1) uncertain knowledge of future weather conditions, (2) uncertainty in the forecasting procedure, and (3) errors in the data. The forecast, therefore, must be interpreted not as a single value but rather as a range of values with specific probabilities of occurrence. The middle of the range is expressed by the 50% exceedance probability forecast, for which there is a 50% chance that the actual flow will be above, and a 50% chance that the actual flow will be below, this value. To describe the expected range around this 50% value, four other forecasts are provided, two smaller values (90% and 70% exceedance probability) and two larger values (30%, and 10% exceedance probability). For example, there is a 90% chance that the actual flow will be more than the 90% exceedance probability forecast. The others can be interpreted similarly. The wider the spread among these values, the more uncertainty is in the forecast. As the season progresses, forecasts become more accurate, primarily because a greater portion of the future weather conditions become known; this is reflected by a narrowing of the range around the 50% exceedance probability forecast. Users should take this uncertainty into consideration when making operational decisions by selecting forecasts corresponding to the level of risk they are willing to assume about the amount of water to be expected. If users anticipate receiving a lesser supply of water, or if they wish to increase their chances of having an adequate supply of water for their operations, they may want to base their decisions on the 90% or 70% exceedance probability forecasts, or something in between. On the other hand, if users are concerned about receiving too much water (for example, threat of flooding), they may want to base their decisions on the 30% or 10% exceedance probability forecasts, or something in between. Regardless of the forecast value users choose for operations, they should be prepared to deal with either more or less water. (Users should remember that even if the 90% exceedance probability forecast is used, there is still a 10% chance of receiving less than this amount.) By using the exceedance probability information, users can easily determine the chances of receiving more or less water. *Starting in 2020, streamflow forecasts with poor prediction skill (jackknife r2 < 0.34) will no longer be issued. This will primarily affect January and June forecasts, with little change anticipated for February, March, April, and May forecasts. For more information, please contact Danny Tappa ([email protected])* USDA is an equal opportunity provider and employer. To file a complaint of discrimination, write: USDA, Office of the Assistant Secretary for Civil Rights, Office of Adjudication, 1400 Independence Ave., SW, Washington, DC 20250-9410 or call (866) 632-9992 (Toll-free Customer Service), (800) 877-8339 (Local or Federal relay), (866) 377-8642 (Relay voice users). IDAHO WATER SUPPLY OUTLOOK REPORT March 1, 2020 SUMMARY Precipitation February of 2020, unlike 2019, was not one for the record books in Idaho. Monthly precipitation was below normal for most of Idaho and ranged from ~25 to 125% (Figure 1). The Clearwater River basin received the most precipitation during February (125%). Once again this water-year, the basins that received the least precipitation with respect to normal were the Wood & Lost in south-central Idaho, where new record low monthly precipitation was observed for the month of February. Now five months into the water-year and wet season, and the Wood & Lost basins have yet to see a single month with above normal precipitation (January was near normal). Resulting, water-year precipitation totals are abysmal and are now approaching half of normal for the Oct. 1 – Mar. 1 period (Figure 2). While there’s still time during the remainder of our wet-season to make up ground, it’s looking likely the overall water picture will be below normal in the Wood & Lost basins. Near-term outlooks from NOAA’s Climate Prediction Center suggest an increased likelihood for above normal precipitation in early March, which would certainly be welcomed across south-central Idaho. Monthly and water-year precipitation data for all basins in Idaho can be accessed in tabular form here. Snowpack After a favorable weather pattern for mountain snowfall during most of January, high-pressure and drier conditions became much more prevalent during February. That being said, moderate snowpack gains were still observed throughout Idaho during February except in and near the Wood & Lost River basins. Most of Idaho experienced normal to slightly below normal average temperatures, owing in part to sufficient radiative cooling at night resulting from dry (cloudless) weather. As we transition into spring, this is an important process that helps to preserve snowpack because it results in a net energy loss from snow to the atmosphere (the opposite of what’s needed for snowmelt). March 1 snowpack percentages range from 90 to 110% for most of Idaho (Figure 3), except for the Wood & Lost basins (50 to 70%) and northern Idaho (110 to 120%). More snow is needed in the Wood & Lost basins in order to secure an adequate water supply. See Figure 3 for a map of basin specific May 1 snowpack conditions, or access the same information in tabular form here. Reservoirs & Streamflow All major reservoir projects in the Middle and Upper Snake basins continue to hold above normal storage, which will help to provide a buffer against anticipated below normal streamflow in and near the Wood & Lost basins. A statewide summary of current reservoir storage can be accessed here. For the majority of Idaho, streamflow forecasts for the primary runoff periods are expected to be ~80 to 120% of normal. As discussed previously related to current snowpack and precipitation totals, the Wood & Lost basins are again the exception with streamflow forecasts ranging from 20 (Camas Creek) to 60% of normal. Full basin specific forecast details can be accessed here and in Figure 4. Note: The streamflow volumes referenced in this report are the 50% Chance of Exceeding Forecast, unless otherwise noted. IDAHO SURFACE WATER SUPPLY INDEX (SWSI) March 1, 2020 The Surface Water Supply Index (SWSI) is a predictive indicator of surface water availability within a watershed for the spring and summer water use season. The index is calculated by combining pre- runoff reservoir storage (carryover) with forecasts of spring and summer streamflow. SWSI values are scaled from +4.0 (abundant supply) to -4.0 (extremely dry), with a value of zero indicating a median water supply as compared to historical occurrences. The SWSI analysis period is from 1981 to present. SWSI values provide a more comprehensive outlook of water availability by combining streamflow forecasts and reservoir storage where appropriate. The SWSI index allows comparison of water availability between basins for drought or flood severity analysis. Threshold SWSI values have been determined for some basins to indicate the potential for agricultural irrigation water shortages. Agricultural Water Most Recent Year Supply Shortage SWSI With Similar SWSI May Occur When BASIN or REGION Value Value SWSI is Less Than Spokane 0.7 2009 NA Clearwater 1.3 2019 NA Salmon -0.4 2016 NA Weiser -0.4 2005 NA Payette -1.3 2004 NA Boise -1.0 2003 - 1.8 Big Wood above Hailey -2.9 2004 - 2.9 Big Wood -1.0 2008 0. 1 Little Wood -1.3 2004 - 1.6 Big Lost -1.3 2013 0.5 Little Lost --- --- 1.2 Teton 0.4 2019 - 4.0 Henrys Fork 0.7 2014 - 2.5 Snake (Heise) 1. 6 2019 - 1.8 Oakley 2.4 2019 - 0.1 Salmon Falls above Jackpot 0.4 2010 NA Salmon Falls 1.
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