April 1 is an important date in water management in the western U.S. because it is the typical date of maximum water content in the snowpack at most mountain locations. Based on yesterday's data, I predict that April-June streamflow in the Henry's Fork at Island Park this spring will be the lowest in the last 35 years and possibly the lowest since the extended drought of the 1930s. And that's no April fooling.
You can read the details here, in a handout I have prepared for tomorrow's meeting of the Island Park Drought Management Planning Committee. Highlights are:
- Inflows into Henrys Lake and Island Park Reservoir this past winter ranked around the 30th percentile of all water years since 1979. This means that the winter of 2014-2015 was drier than 25 of the last 35 years.
- April 1 snow-water-equivalent (SWE) at sites that are good predictors of streamflow in the upper Henry's Fork watershed range from 25% to 70% of the 30-year median; SWE at some sites is at its lowest value ever recorded.
- Optimistic predictions for spring inflow into Henry's Lake put it at the 25th percentile of the last 35 water years.
- My predicted inflow into Island Park Reservoir over the next three months is lower than any recorded since 1979.
- Both Henry's Lake and Island Park Reservoir will be physically full by the end of April (Henry's Lake is already full), but if my inflow projections are realized, even passing natural flow through both full reservoirs will result in streamflows of only around 450 cfs from late May through mid-June. Flows could be higher if we receive a large amount of rain between now and then or, ironically, if irrigation demand is so high this spring that water will need to be released from Island Park Reservoir beginning in June.
- Without rain this spring and/or summer, reservoir storage carryover, reservoir inflows, and hence winter flows below Island Park Reservoir next winter will be extremely low.
We may be faring better than California, but we are entering our fourth consecutive year of drought. Hope for rain.