Just how dry was the spring of 2015?

Back on April 1, with snowpack and winter streamflow data in hand, I predicted that the spring of 2015 would be the driest in the last 35 years and quite possibly the driest since the extended drought of the 1930s. You can read my April 1 predictions at http://henrysfork.org/spring-2015-shaping-be-driest-record. So, how good were my predictions? 

I had predicted that natural watershed inflow between Henry's Lake and Island Park Reservoir would average 344 cfs over the April-June spring runoff period, if precipitation over this period were the lowest on record. Including statistical uncertainty, I projected average watershed inflow to lie between 231 cfs and 513 cfs, with 95% confidence. The actual value turned out to be 484 cfs, well within the statistical uncertainty, but also quite a bit higher than my estimate of 344 cfs. The actual value turned out to be much higher because precipitation in May was much higher than average, and I had used the historic minimum as an input to my calculations.

I also predicted that natural streamflow at Island Park Dam would be around 450 cfs from late May through mid-June. "Natural streamflow" is the flow that would be present in the river in absence of regulation by Henry's Lake and Island Park dams. Natural flow averaged 553 cfs from May 20 through June 15, 23% higher than my prediction, again due to above-average rainfall in May. However, natural streamflow at Island Park Dam averaged 446 cfs from June 15 through June 30, reflecting hot, dry weather during this time period.

I had also predicted that streamflow at Island Park Dam would be much higher than the natural flow of 450 cfs if irrigation demand were high during the spring. This turned out to be true as well.  

Read the full details here, in a document prepared by Christina Morrisett, HFF's environmental modeling intern from Stanford University. Christina did all of the data analysis, created the graphics, and helped write the text. When she applied for this internship, neither she nor I knew that one of her primary tasks this summer would be analyzing what turned out to be the 3rd driest spring since the 1930s. Without her skill and diligence, we would not have this information out to you in such a timely manner.