Northwest Sudden Drying: The Rapid Transition to Dry Soils in May

People think of the Pacific Northwest as being a wet place, with wet, sodden soils.

Perhaps that is true during the winter and early spring. 

But something startling happens in May in much of western Washington: a rapid drying of the soils.   

A transition so rapid that some folks begin irrigating their gardens by late May.

Can this really be true?   Let's check it out.

Here is the soil moisture (8 inches down) at Langley, on southern Whidbey island, for the past month.  A major drop in soil moisture percentage from roughly 26% to around 15%.

Graph courtesy of the WSU Agweather website

Perhaps a better perspective can be gain by looking at the soil moisture over an entire year at this location.  For example, 2017.

Moisture values are high (20-26%) during the winter, but in May and early June the bottom drops out, with values dropping below 8%.  Slow drying follows in July and August, with a huge increase in October.

How about 2015, a year that was particularly warm and dry?  Again, a big drying in May

Looking at other years and other western WA stations shows that the May superdrying is a normal occurrence, something that accords with my experience as a gardener.

But why such rapid drying in May?  To understand, one must know about an important meteorological term:  evapotranspiration.  

Evapotranspiration or (ET for short) is the sum of evaporation and plant transpiration from the Earth's land and ocean surface to the atmosphere.  Over land, it is mainly water evaporating from the earth's surface and water loss by plants.  

So ET is the loss, while precipitation is the gain.  If ET exceeds the precipitation coming in, surface soil moisture declines.

Evapotranspiration is enhanced by warming of the surface (e.g., by solar radiation or warm air passing over it), drying of the air above the ground, and stronger winds (which help mix moisture away from the surface).

Think about what is happening in May.  

Precipitation is declining rapidly.  Here are average monthly precipitation totals at Seattle-Tacoma Airport.  May (roughly 1.75 inches) is a third of the November and December totals and less than half of the March rainfall.   So much less coming in.

Solar radiation, which warms the surface--promoting evaporation, increases rapidly in May, as shown by the solar radiation for the year at Langley, WA:

Surface air temperature increases in May, relative humidity decreases, and soil temperature (which is very important for surface evaporation ) increases substantially during that month (see below)

In short, a major decline of incoming moisture (from precipitation) and a big increase in evaporation from strengthening solar radiation at the surface and warming temperatures, results in a rapid decline of solar moisture in May.  

And we should not forget the large increase of evapotranspiration from plants, which are leafed out and growing rapidly in may, sucking out moisture from the soil.

Today, I was preparing the soil in my garden and could not believe how dry it was.  The May drying has gotten a particularly good start, with drier than normal and warmer than normal conditions (see graphs below)

Blue lines is normal precipitation, purple is this year. 

Red is this year, purple is normal high, cyan is normal low.

So be prepared to water your garden.  And this year, there is another threat to our plants.  A very cute, furry threat that has been increasing in numbers rapidly:

A sign of global warming?  I will leave that for a future blog.