Good BWO weather continues. The same continues for successful streamer presentation and for responses to big stone fly nymphs as these bugs begin moving along the bottom towards banks.
Dr. Rob Van Kirk’s latest water report for the Henry’s Fork drainage follows.
Headlines:
- Heavy precipitation across the watershed last night and this morning increased water-year precipitation to 84% of average.
- At an outflow of 425 cfs, Island Park Reservoir gained 520 ac-ft yesterday, of which 180 ac-ft was from direct precipitation on the reservoir surface.
- Recent cold weather has delayed snowmelt, but peak SWE this year was 8.3 inches below average, equivalent to 620,000 ac-ft (4.5 times the capacity of Island Park Reservoir) of water that has not fallen on the watershed this year and will not contribute to stream flow or reservoir fill.
- Water-year natural flow to date is 75% of average and the lowest in the 1978-2022 record. The best-case outcome for the water year is 77% of average stream flow, 6th driest on record.
- The best water-management outcome possible is that spring and summer precipitation is high enough that reservoir draft is lower than expected, and the reservoir system ends the season better than expected.
Details:
A strong and unusually wet cold front brought heavy precipitation to the entire watershed last night and this morning. Precipitation prior to midnight was enough to move the water year total up a percentage point to 84% of average. Much more precipitation fell after midnight. Warmer temperatures over the past few days increased natural stream flow, but diversion increased just as much. Current natural flow is only about 1,500 cfs greater than diversion right now. After another reduction in outflow, Island Park Reservoir gained 520 ac-ft yesterday, 180 of which was from direct precipitation on the reservoir surface. Outflow will be reduced again tomorrow to around 275 cfs to fill the remaining 7,838 ac-ft as soon as possible, hopefully in 10 days or so.
Will additional snow gained over the past month increase water supply? The short answer is: only very slightly. Here’s why. Current snow water equivalent (SWE) is 80% of average for the date. That may sound good, but there are three reasons why it is not. First, 80% of average is 20% below average and still only in the bottom one-third of years since 1989. Second, on average, 15% of the snowpack has already melted at this point, so the current value is being compared to one that is already lower than the peak. Third, this year’s peak of 20.1 inches of SWE is 8.3 inches below the 1989-2021 average. That 8.3 inches did not fall on the watershed this winter and will not fall over the remainder of the spring, no matter how slowly our remaining snowpack melts.
About 70% of the Henry’s Fork watershed, or 1.49 million acres, accumulates snowpack. Over that area, the 8.3 inches of missing SWE (relative to average) is a little over 1 million ac-ft of volume in SWE. Around 60-65% of that, when melted, ends up as stream flow during the subsequent spring and summer. The remainder replenishes soils and aquifers to support future stream flow or is lost to evapotranspiration. So, that 8.3 inches of missing SWE is worth about 620,000 ac-ft of missing stream flow that will not flow down the river this spring and summer. For perspective, 620,000 ac-ft is 4.5 times the volume of Island Park Reservoir, 2.6 times the volume of the three reservoirs in the watershed combined, and 70% of total annual irrigation diversion in the watershed. Across the whole upper Snake River basin, the analogous deficit means that there is not enough water to fill the reservoir system beyond its current level of 59% full and meet ongoing irrigation demand.
Can a cool, wet spring and summer increase water supply? The short answer: again, very little. First, long-range forecasts call for a hot, dry summer. But, even if those are wrong, spring and summer rain cannot make up for missing snowpack, because most rain is taken up by plants and soils and does not contribute to stream flow the way winter snowpack does. So far this water year, natural streamflow is only 75% of average (25% below average) and the lowest in the 1978-2022 record. Considering poor base flow (the fall and winter “base” on which this year’s snowmelt is added) and a still very poor snowpack, even well above-average precipitation from here on out will yield natural flow over the remainder of the water year that is only around 78% of average. That would be much higher than last year’s May-September stream flow and an outcome that is very unlikely. But, even if this best-case scenario occurs, total water-year natural flow will end up at 77% of average and the sixth driest in the 1978-2022 record. It is just not possible to make up for 9 months of very low base flow (reflective of the long-term state of soils and aquifers) and a very low snowpack with spring and summer rain. Only several years of high snowpack will do that.
Can a cool, wet spring and summer increase end-of-summer reservoir levels? This one gets a definite “yes,” if precipitation is heavy enough. The previous two answers explain why overall water supply (natural streamflow) is not going to improve much over the next five months and also why the reservoir system will not fill much more than it already has. However, if rain is heavy enough and temperatures are cool enough, irrigation demand will be low because direct precipitation will make up a greater fraction of crop needs than average. In the Henry’s Fork watershed, precipitation makes up around 10-15% of irrigated crop needs in the watershed. A wet summer could move that up to 15-25%. That alone would reduce the need for diversion by 40,000-80,000 ac-ft. That’s 17-34% of the total volume of the Henry’s Fork watershed’s three reservoirs. In addition, continued rain will keep streamflow a little higher than it would otherwise be, in line with the 78% of average (vs the current 75% of average) referred to above. This would not necessarily limit reservoir draft but would keep natural flow water rights priorities a little higher, allowing irrigators to divert more water with their natural flow rights, on top of already limited storage rights they have. Thus, the best-case scenario would reduce the need for reservoir draft by 40,000-80,000 ac-ft. Compared with expected conditions, this would make the difference between Henry’s Lake, Island Park, and Grassy Lake combined ending up at 64% full instead of 39% full as expected. For reference, average carryover is 62% full.
If this happened across the whole upper Snake River system, then reservoir carryover will be higher than it would have been without the spring and summer rain. This scenario adds very little to the water supply, but it reduces the need for reservoir draft. This has happened in the past, albeit not in a year with this poor of a water supply and low system-wide reservoir carryover. Years with heavy spring/summer rain that greatly limited reservoir draft were 2010, 2011 and 2014. Even last year, rain limited reservoir draft due to decreased demand. However, the cost in 2014 and 2021 was in poor hay and grain yields and quality for many producers.Bottom line: the best outcome possible this year is lower-than expected reservoir draft that will put end-of-season reservoir storage near average here in the Henry’s Fork watershed. Carryover in the upper Snake River system as a whole will still below average but better than the ~10% carryover expected at this point. For agricultural producers, the “best case” water situation will likely come at the expense of crop yields and quality this year but provide a greater chance of reservoir fill and higher storage allocations next year. For anglers, this “best case” will keep outflow from Island Park Reservoir lower than expected this year, leading to better water quality this summer and higher winter flows next year. The cost will be very low stream flows between Island Park Dam and Chester Dam for much of the summer, due to decreased need for high outflows from Island Park Reservoir but ongoing low
