Data Analysis
SOIL MOISTURE RESEARCH Calculating the amount of water in
the soil is critical to many people. Farmers can use soil
moisture data to aid in scheduling of crop watering. Firemen
use soil moisture data to pinpoint possible areas of danger
for wildfires. Soil moisture data can also be included in
computer models to improve weather forecasts. Oklahoma has
put itself in the forefront of public use and research of
soil moisture with the Oklahoma Mesonet’s Soil Moisture
Network.
In 1996, soil moisture sensors were installed at 60 Mesonet
sites at depths of 5, 25, 60 and 75 cm. Based upon the initial
success in using data from this original deployment, soil
moisture sensors were installed at 43 additional Mesonet
sites during 1998 and 1999 (Figure 1). A key aspect of the
network of soil moisture sensors is that estimates of both
soil-water potential and water content are collected every
30 minutes.
The sensors measure soil moisture by heating and then taking
the temperature of a wire, pausing for a few seconds, and
then taking the temperature a second time. Since heat dissipates
more slowly in water (high heat capacity), wet soils will
have a smaller change in temperature measured by the sensor
than dry soils. This temperature difference allows hydrological
variables such as soil water content, soil matric potential,
and Fractional Water Index (FWI) to be calculated.
Soil water content is the physical amount of water per volume
of soil. Different soil types (i.e. sand, clay, etc.) have
different sizes of particles, which results in varying amounts
of space available for water to fill. Soil matric potential
is the force, like pressure, needed to move water vertically.
When there is less water in the soil, a larger force is required
to move the water. Soil matric potential is useful in agricultural
situations, when it is important to know how effective certain
plants are at removing water from the soil. However, soil
matric potential is an exponential function, which makes
it more difficult to implement in real time. Thus, the Oklahoma
Mesonet put soil matric potential values into four categories.
This allowed statewide plots to be easier to understand.
Since soil water content depends heavily upon soil texture,
and soil matric potential is exponentially related to soil
wetness, FWI was developed for an easy to use and understandable
index. FWI tells us how far between the dry and wet extremes
of the sensor a particular sensor reading resides. This unitless
value ranges from very dry soil having a value of zero, to
saturated soils having a value of one.
Figure 2 shows a typical image of soil moisture at a depth
of 5 cm – about the length of your thumb – during
the early part of the summer. The darker brown areas show
drier soils, while the green areas indicate more moist soils.
Maps like these, as well as maps of categorized matric potential,
for all four soil depths are updated daily on the Oklahoma
Mesonet Interactive Products website for public viewing (select
Soil Moisture Maps). Custom maps showing a particular day
in the past can also be generated on the website by selecting
Custom Maps under Soil Moisture Maps.
In addition to statewide maps of soil moisture, the website
allows the user to view a particular site in more detail.
Figure 3 shows a 30 day time series of soil moisture at the
El Reno Mesonet site. The top plot shows FWI and the bottom
plot shows soil water content. Each colored line represents
a particular depth within the soil. Here the public can see
how the soil moistures values have dropped over the past
30 days. Similar to the statewide plots, custom maps of different
time periods can be generated on the website by selecting
Custom Graphs under Graphs (Soil Moisture).
This vast network of soil moisture sensors has led to many
research opportunities, many of which would not have been
possible without the Oklahoma Mesonet. The Oklahoma Climatological
Survey has produced a detailed climatology of Oklahoma soil
moisture. This climatology allows state water and agriculture
officials to better understand what typical values of soil
moisture should be, and when they are reaching extreme values
for a particular time period. Other studies have focused
on the droughts of 1998 and 2000, which seriously impacted
the state’s economy. A better understanding of how
droughts occur and discovering signals to warn of impending
droughts will aid the advanced preparations for those who
rely on soil moisture.
Further drought studies indicate fall and winter rainfall
may help alleviate near-surface soil moisture problems during
severe droughts, but deeper depths may continue to be impacted
by the dry conditions. Figure 4 shows the FWI values for
the Mesonet station in Hollis during 1998. The drought can
clearly be seen during the middle part of the year (summer),
but during the fall and winter, when rain has fallen, only
the top two depths (red and yellow lines) recover from the
drought conditions. This research demonstrated how a drought
can impact a location longer than originally thought.
There is still much to learn about soil moisture, and the
Oklahoma Mesonet will continue to be an integral part of
this process. More products will be made available in order
to provide even more detailed soil moisture information to
the public. Additional data sets will be produced for researchers
to analyze. The discoveries which result will greatly benefit
not only Oklahomans, but communities across the globe as
well. Just as other portions of the Oklahoma Mesonet have
pioneered how weather data is used, the soil moisture network
will surely follow in its footsteps. |
