Idaho Water Supply Outlook Report April 1, 2019
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Natural Resources Conservation Service Idaho Water Supply Outlook Report April 1, 2019 Owyhee Reservoir in Eastern Oregon near the Idaho Border, March 30, 2019 In 1932, the U.S. Bureau of Reclamation completed the Owyhee Dam and the Owyhee Reservoir started filling for the first time. At the time of completion, the Owyhee Dam was the tallest of its kind in the world (417 feet) and served as a prototype for the Colorado River’s Hoover Dam. In the otherwise arid climate of Eastern Oregon and Southwestern Idaho, the reservoir provides critical irrigation water to approximately 118,000 acres of farmland across both states and four separate irrigation districts. The Owyhee River basin is huge, covering ~7,690 square miles in Oregon, Idaho, and Nevada. To put this in perspective, the entire Boise River basin is ~2,200 square miles! April 1 snowpack in the expansive Owyhee River basin is 130% of normal, and the reservoir is 61% full (88% of average), inflows began to increase in March and will continue in April as we transition into Spring. Please see the full report for details on current snowpack and expected water supplies. 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. 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 April 1, 2019 SUMMARY The stage is set. If it were not for February’s abundant precipitation, we would be telling a different story. Snowpacks across Idaho will provide an adequate irrigation supply and then some. The gradual melt of the snowpack in the lowest elevations during March helped to relieve some of the pressure of a rapid melt in these elevations. However, the mid-elevations across the state from the Weiser to the Henrys Fork basin still have a tremendous snowpack that is ripe and ready to melt. This mid-elevation snowpack covers a large area and still has the potential to melt rapidly with the arrival of rain and warmer temperatures. Spring weather the next few weeks or months will determine how this year’s snowpack melts and fills rivers and reservoirs. Based on the PRISM temperatures maps, March temperatures across the state and most of the West were near to slightly below normal. As we move into April, the days are getting longer which allows for more heating, but night time temperatures are critical, as freezing night time temperatures slows the melt process. Future precipitation intensity and duration is also critical in melting the snow and providing additional runoff. Consecutive days of rain saturates the soils and provides much greater runoff than precipitation falling every other day or every few days. Short-term weather outlooks illustrate that more moisture is on the way and may continue into mid-April. Reservoir operators are closely watching inflows and streamflow forecasts to assist in their delicate balancing act of making releases while ensuring reservoirs refill after peak inflows occur. More good news is that nearly all the reservoirs and natural lakes will fill this year. This should provide good carryover storage for next year for those starting to think about next year’s water supply. The exceptions are Bear Lake, Salmon Falls and Oakley reservoirs which are not likely to fill even using the 10% chance of exceedance forecasts. Owyhee Reservoir could fill depending upon spring rains falling or not. SNOWPACK Snowpacks vary across the state and within watersheds by elevation, so if you like details, be sure to look at the individual sites in your basin. The lowest snowpacks with respect to normal remain in northern Idaho and range from 80 to 100% of median. Interestingly, Bear Mountain SNOTEL at 5,400 feet just north of Clark Fork, Idaho in the Cabinet Mountains, usually one of the higher snow accumulation zones, only has 37.3 inches of snow water (69% of median). Normal April 1 amounts are 54.3 inches. Moving south the snowpack increases to 95% of median in the Clearwater basin and to 109% in the Salmon basin. This is where it gets interesting – the mid-elevation snowpack in the 6,000 foot elevation band across central Idaho has well above normal snow. Sites in the Weiser are at 150% of median, 300% in the Little Wood basin, and 130% of median in the Henrys Fork where a valley snowpack remains – Island Park SNOTEL site (6,290 feet) is at 125% of median with 47 inches of snow on the ground. Luckily, some of the lower elevation snow has gradually melted filling soil moisture voids from last year’s dry summer. Significant rain or above average temperatures will increase melt rates to levels that the soils cannot absorb and result in rapid increases in tributary streams. Overall, most basins south of the Salmon Basin range from 110 to 145% of median. The lowest are near 110% of median in the Upper Snake, Mud Lake, Little Lost and Salmon and highest are 150% in the Owyhee and Weiser basins and 160% for the Little Wood. The early April storms are bringing rain in the valleys and snow in the higher elevations. This pattern may continue into mid-April. After more of the mid-elevation snow melts, the higher elevation snowmelt will keep rivers flowing near average or better into the later summer months. PRECIPITATION In contrast to February, March precipitation was below average across nearly the whole state. Lowest percentages were a third of average in northern Idaho and increase to 40% in the Clearwater basin. Moving south, precipitation amounts increased more, but only to 50 to 80% of average across central and southern Idaho. Greater precipitation amounts that were closer to near normal fell in Oakley and Bear River basins because the primary March storm track was across the Great Basin. South of the Owyhee, Bruneau, Salmon Falls and Goose headwaters in Nevada and Utah, March precipitation totals reached 150 to 200% of average. This dry north and wet south pattern is more typical of past El Nino years. When the past two months precipitation for February and March are combined, this pattern really stands out and is interesting to note that basins south of the Salmon River received 100 to 150% of normal February to March totals while basins to the north received only 60 to 100% of normal amounts.