Sunday, June 17, 2012

The Spring Drought: A Harbinger for the Future?

It's another scorcher out there today.  Everything is parched and today I'm running the soaker hoses to give my drought tolerant plants just a hint of water.

Spring is a critical season for water the Salt Lake Valley.  On average, May, April, and May are the three wettest months of the year, as shown by the blue bars in the graph below.

Climatological and 2011-12 water year precipitation at the Salt Lake
International Airport.  Source: NWS and Western Region Climate Center
This year, we reached just above average in April, but March and May were way below average.  Average precipitation for March through May is is 5.63 inches, but this year we got 3.61 inches, 64% of average.  June is a transitional month climatologically, with the first part of the month sometimes bringing a few storms.  The monthly average precipitation is .98 inches, but this year we've gotten only a trace.

The water cycle is affected not only by precipitation, but also evaporation and transpiration by plants, which we refer to collectively as evapotranspiration.  Evapotranspiration is influenced by a number of factors (sun, wind, etc.), but one of the most important is temperature.  All else being equal, higher temperatures mean greater evapotranspiration rates and faster drying of the soils.  At the Salt Lake City International Airport, March was 5.4ºF above average, April 3.4ºF above average, May 1.7ºF above average.  Although we've had a couple of cold spells, June is running just above average thus far too.  Thus, the soils this spring are drying more quickly.

Climate model precipitation projections for the middle 21st century vary over northern Utah, but most call for May and June to be a bit drier than they were in the later half of the 20th century.

Source: Thomas Reichler, University of Utah
That being said, the difference relative to climatology is fairly small compared to current year-to-year variations in precipitation, so I'm not ready to hang my hat on a prediction of less precipitation in the coming decades.  I am, however, confident that we will see more frequent springs with higher temperatures and more rapid drying of the soils in the coming decades, as we have this year.   This will likely yield increased demand for residential irrigation, assuming current landscaping practices continue.

For more discussion, see section 5 of our climate report to former Gov. Jon Hutsman Jr.'s Blue Ribbon Advisory Council on Climate Change.

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