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August 11, 2009 August 11, 2009 August 11, 2009 August 11, 2009
Fractional Water Index ? What in the world is that?
Does anyone out there like history? Well for the rest of you, please bear with me for a few moments.
One of the Ticker subscribers posed an important question that we?d like to address, but it requires a
bit of Mesonet history to understand.
Back in the day, and shortly after the Mesonet was commissioned in 1994, a plan was put in place to
augment a subset of stations with sensors to measure soil moisture. Over the years we?ve added soil
moisture sensors to nearly all our sites.
Now, soil moisture is an important variable to many including those who work in agriculture and even
meteorologists. But, soil moisture is actually a complicated variable because it is dependent on soil
type. Thus, when ?soil moisture? such as the volumetric water content of the soil is plotted on a map,
often we will see ?bulls-eyes? of soil moisture that may give a false impression of soil wetness but
really represent soil texture. In the end, we found was that most folks simply wanted to know was
?how moist is the soil??.
So, the Mesonet team used the sensor technology installed within the Oklahoma Mesonet and
borrowed a bit of creativity from other scientists in the United States Department of Agriculture to
apply a simple index to the data called the Fractional Water Index (FWI). Basically, the closer the index
is to a value of 1, the wetter the soil. Similarly, the closer the index is to a value of 0, the drier the soil.
This simple index allows anyone to look at a map or graph of FWI, compare sites or depths, and make
relative assessments of how ?moist? the site is and how ?moist? it is compared to other sites or depths.
So, lets look at some examples (note: maps/data grabbed on August 10th) - the following maps are
for FWI at 5, 25, 60 cm (2, 10, 24 inches) and can be found on the Oklahoma Mesonet website under
the soil moisture heading:
http://res.mesonet.org/~jbasara/Ticker/August_11_2009_Ticker/mesonet.daily.current.FW05.grad.png
http://res.mesonet.org/~jbasara/Ticker/August_11_2009_Ticker/mesonet.daily.current.FW25.grad.png
http://res.mesonet.org/~jbasara/Ticker/August_11_2009_Ticker/mesonet.daily.current.FW60.grad.png
First, those areas that are green are relatively moist while those areas that are brown are relatively dry.
This time of year there is a battle between evaporation (water out of the soil) and precipitation (water
into the soil). Thus, we find many pockets of wet/dry as locations have or have not received rain. Now,
if you are observant, you can see that as you move down in depth right now across Oklahoma, the soil
is actually drier in many locations. How in the world is that possible? Well, let?s look at a graph from
the past 90 days at El Reno (ELRE):
http://res.mesonet.org/~jbasara/Ticker/August_11_2009_Ticker/ELRE_FWI_graph.jpg
Early in the period (i.e., May), the 5 cm depth was drier than both 25 and 60 cm. This makes sense
given that May is in the middle of the wet season in Oklahoma and the soil is typically quite moist
throughout the profile. However, the 5 cm depth is quite close to the surface and experiences the
impacts of evaporation to the atmosphere, and as such was a bit drier than the deeper depths. As
the summer progressed it got quite warm and dry. At the same time, the roots of the vegetation can
tap the soil for water well beyond the 60 cm depth. Thus, the whole soil profile dried first at 5 cm and
followed by 25 and 60 cm. Note that while some minor rainfall events occurred, only the 5 cm depth
was impacted. That all changed a few weeks ago when the rain returned and as more has fallen, the
more the soil profile has responded and become wetter ? again first at 5 cm and then at 25 cm.
However, even with a recent rainfall event of nearly 2 inches, the 60 cm depth has not responded ?yet.
This is one reason why folks around here will try to remind everyone that a single (or even multiple
rain events) will not break a dry spell or drought.
So, hopefully this provides a better understanding of FWI and how it can be a really useful tool. My
personal recommendation is to check out the data on the Mesonet website and keep tabs on soil
moisture conditions here in Oklahoma. You may not realize this, but the Oklahoma Mesonet has
one of the largest soil moisture monitoring networks in the world! That?s just another aspect of
the Oklahoma Mesonet?s great history and the unique partnership that exists between Oklahoma
State University and the University of Oklahoma.
As always, here are some links to follow.
http://www.mesonet.org/
http://okc.mesonet.org/
And for those who need Oklahoma weather tweets or are socially networked, check us out and become
a fan:
http://twitter.com/ocsticker
http://twitter.com/OKCNET
http://www.facebook.com/pages/Oklahoma-Mesonet/101813120731
http://www.facebook.com/pages/Oklahoma-City-Micronet/108107256598
Ticker Author: Jeff Basara
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