May 12, 2022USDA-NRCS releases May Idaho water report
Below average temperatures and ample precipitation stalled the snowpack melting trend reported last month. Despite this spring weather windfall, peak snowpack at almost every SNOTEL site south of the Clearwater Basin remains well below average for WY 2022. Basin-wide percent of median peak snowpack is below normal in all basins.
Another way of understanding how this year’s snowpack ranks compared to previous winters is to look at percentiles. Current snow water equivalent (SWE) peak compared to the period of record at almost all measurement locations is near the 30th percentile. Again, the notable exception is the Clearwater Basin sites, which remain slightly above the 50th percentile.
Many water users this month have wondered, “Why do the basin percent of normal snowpack values keep increasing and how beneficial have these storms been for water supply?”. When snowpack is increasing, or at least holding steady during the typical melt season, comparisons to the 30-year normal can be misleading. Essentially, the basin percent of normal calculation compares today’s snowpack to the same day in the 30-year record, not the historical peak snowpack value. We recommend using figure 3 (in the report) to assess basin-wide peak snowpack compared to median peak snowpack. Water users can also use SNOTEL sites to directly compare this year’s SWE value to the median
In cold, wet springs like this year, the issues with not comparing water year peak
snowpack to peak 30-year median values are best illustrated by looking at the
snowpack accumulation charts as shown on the cover; we will use the Snake River above Heise as an example. You can access these charts by clicking on any basin or SNOTEL site in the interactive map.
Last month, it appeared snowpack peaked in this basin on March 24 with 14.7” SWE (71% of normal compared to the 30-year peak of 20.6” SWE). After a brief warm spell, the snowpack began to increase again, leading to what appears will be the true snowpack peak on April 25 at 17.0” SWE (83% of normal).
If this year’s April 25 SWE value is compared to the 30-year median value on April 25 (17.4”), then it looks like snowpack in this basin is healthier than it actually is. Snowpack would appear to be 98% of normal rather than 83% of normal when comparing peak basin snowpack values directly. In other words, because our snowpack isn’t melting as fast as normal for this basin, the difference (gap) between the WY 2022 and historical median snowpack is narrowing and thus appearing like the snowpack is in better shape than it actually is. Moral of the story? Use figure 3 for a true assessment of snowpack health or individual SNOTEL sites this year.
April showers brought total water year precipitation to near normal levels in all major basins. During April, all basins received near to well above normal precipitation (94% to 158%). It was highly variable in space and time whether it fell as snow or rain across the state. As previously mentioned, some basins weren’t quite as wet as the rest of the state this past month. Those include: Pend Oreille-Kootenai, Coeur d’Alene-St. Joe, Bruneau, Salmon Falls, Goose Creek Basin, and some sites within the Wood and Lost basins.
Since October 1, water year total precipitation is above 90% in many basins. Basins with the lowest total water year precipitation are the Weiser (88%), Payette (89%), Boise (91%), Big Wood (88%), Salmon (93%), Henrys Fork-Teton (88%), and Snake River above Heise (92%). All other basins are near or slightly above normal.
Water supply and impacts
Water users should still prepare for a short irrigation season and possible curtailments. Reservoir storage is below normal across Idaho. Total reservoir storage in the Upper Snake system above American Falls Dam is 59% full and well below normal at 80% of average storage on May 1. There is 1.12 million acre-feet less water currently in the system compared to this time last year.
Magic reservoir storage is at 32% of normal with delivery currently scheduled to begin on May 8. The Boise Reservoir system is currently 92% of normal (61% of capacity, 93,760 acre-feet less than WY 2021 levels on May 1), while the neighboring Payette system is 95% of normal (67% of capacity, 42,280 acre-feet less than WY 2021).
As cold weather slowed down snowmelt runoff and irrigation demand began, reservoir fill in the Boise system flat-lined the second half of April. If irrigation diversions exceed or balance natural flow, reservoir fill rates will decline with the potential for reservoirs to not fill as the snowpack melts. The Boise system total storage is predicted to reach ~650-750 KAF (thousand acre-feet) by the day of allocation. Reservoirs south of the Snake River are below normal with Salmon Falls (59%), Oakley (71%), Brownlee (81%), and Lake Owyhee at 65% of normal. Bear Lake is at 106% of normal. Reservoirs in the north are doing much better, ranging from 72% to 109% of normal.
The Surface Water Supply Index (SWSI) table below shows which basins are predicted to experience water supply shortages. This year, those include the Big Wood, Big Lost, Little Lost, Snake above Heise, Oakley, Salmon Falls, and Owyhee basins. The 1-month SPEI index show improved conditions across all of Idaho from the 3-month SPEI.
Our standard streamflow forecasts are the 50% exceedance forecasts. May
through July streamflow is predicted to be above normal in northern Idaho and below normal through most of the state. Streamflow forecasts increased slightly from last month. However, south of the Clearwater Basin, most streamflow forecast points are below the 35% percentile.
Whether the 50% or 70% exceedance forecasts come to fruition will depend on several factors. We anticipate if warm and dry conditions prevail through the remainder of spring, observed streamflow will more closely mirror the more conservative streamflow predictions (70%). Depending on how quickly the remaining snowpack melts will also influence the amount of runoff that makes it into our reservoirs and streams. At this point, a significant slowdown in the snowpack melt rate could negatively impact total water availability with more water lost to various ecological processes than if rapid melting occurs.