Anatomy of a Drought, Part 2

 

While I think it is natural for most of us to think about the climate in terms of the calendar year, hydrologists think about precipitation in terms of the "water year."  In most formulations, the water year runs from October 1 through September 30.  This is because precipitation that falls during the months of October, November, and December is considered to be "stored" until the next runoff period or growing season.  Taking approximately the past 75 water years in Iron County, Utah, we notice a possible pattern:  One dry year leads to another.  The table (above) contains the precipitation totals for the ten driest water years since about WWII.  The matching colors represent back to back years.  In other words, six of the top ten driest years came in pairs.  Last year, 2019/2020, sits in tenth place.  If the pattern holds, should we expect the current year to be dry, too?  Indeed.  In fact, we are on a record pace:  Since October 1, we have received so little precipitation that, even if we were to return to a normal rainfall pattern today, the water year would be the fourth driest on record (below).  (Of course, when that happens, the 2019/2020 water year will move out of the top ten.)  The other interesting thing to note in these data is that we are on track to have three dry years out of the past four (highlighted in orange).  I hope this is not another pattern.