Soil moisture 101: What it means, how it’s measured

The National Integrated Drought Information System and the National Weather Service hosted the first in a two-part series of webinars on soil moisture data and applications. The first was held Feb. 22.

The webinar was also sponsored by the National Coordinated Soil Moisture Monitoring Network, a NIDIS-led multi agency initiative to deliver a coordinated high quality nationwide soil moisture information system to support public good. One of the goals of the series is to raise awareness of soil moisture as a key hydrologic indicator.

Mike Cosh, U.S. Department of Agriculture Agricultural Research Service, and Chris Fiebrich, Oklahoma Mesonet, were among the speakers during the webinar. Cosh discussed how soil moisture affects everyday life. He defined soil moisture as the ratio of the volume of water to the volume of soil. Fiebrich detailed the Oklahoma Mesonet system and its importance.

For Cosh, soil moisture can be determined several ways. He starts with a small physical sample. He collects a sample of soil and places it in a small container. Once in the laboratory he weighs it and it’s placed in an oven, where it runs for 24 hours. Upon completion, all the water is removed and he’s left with the dry soil.

“Then we know the water and that ratio is really what we’re talking about here,” he said. “Volumetric soil moisture seems very simple, but it’s pretty complex.”

And the drying process takes time. But finding the soil moisture from other means—like remote sensing or modeling, or regular incision monitoring—tends to be quicker.

Typically when measuring soil moisture Cosh wants an in situ network constructed. The network is installed into a sidewall of a hole that’s dug and it measures the dielectric content of the surrounding soil.

“Water has very high dielectric constant, soil generally has very low dielectric constant,” Cosh said. “Therefore, the higher the dielectric, the more moisture we can typically have a good observation range of moisture.”

The moisture can range from zero to 50%. This process is automated, and normally low electrical cost since a solar panel can run it. The network gives a small body of measurement, and generally this type of installation provides a good reference point to the area around it.

Soil moisture is a valuable parameter with a variety of uses—including agricultural management where water is being used.

“Soil moisture estimates, too, help us irrigate effectively,” he said. “We don’t want to over irrigate, we don’t want to under irrigate, we don’t want to have a drought. We don’t want to have water run off all of our valuable nutrients into the rivers.”

Knowing where the water is going at the surface helps mitigate runoff as well as having the measurements can allow correct assessment for the risk of wildfires since the plants can tell you how much water is there.

“Then you know what kind of fuel load you have. Knowing the fuel load is telling your risk for wildfires and forest fires,” Cosh said. “These things are all affected by soil moisture in the largest scheme of things.”

But it all starts with a simple measurement. One of the older methods of doing this is to actually have a physical station monitoring soil moisture say down the road at your local college at the airport.

“We need to understand soil moisture state so we can start to get an idea of what the evaporation rate is, what the energy cycle’s doing,” he said.

Soil moisture also controls what the solar radiation does as it hits the surface. Does the water turn into water vapor? This takes that energy out and turns it into evaporation instead of heating the soil.

“But here we start with a simple station a couple of sensors in the ground, solar panels running everything, providing the basis for all of the research that can happen and all of the decisions that can be made off of understanding a few parameters at the surface,” Cosh said.

Mesonet measurements

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Fiebrich discussed soil moisture data and products from an in situ perspective, along with how mesonets measure soil moisture. In Oklahoma, the Mesonet is jointly operated by Oklahoma State University and the University of Oklahoma and has sites spaced out every 19 miles.

“It’s a scale that allows us to detect medium-sized weather events, for instance, fronts and outflow boundaries and things like that,” he said. “About half of our mesonet sites are on privately owned land or landowners allowed us to put the site on their property and the other half are on publicly available land.”

A typical Oklahoma mesonet site often times has a 10-meter tower to allow for wind speed and direction observation at the top of the tower. Along the tower there are other instruments to measure air temperature, relative humidity, pressure, solar radiation, rainfall, as well as gathering soil temperature and soil moisture measurements.

Nationwide mesonets allow for great coverage of state and regional areas across the U.S, but not all record soil moisture data.

Fiebrich said mesonets across the U.S. try to emphasize data quality, reliability and completeness and do a lot of work with calibrations, quality assurance and want to be able to deliver near-real time data to users.

In Oklahoma, they have several products to measure the soil moisture. First, is the fractional water index that’s derived from the matrix potential measurements. The fractional water index goes from zero—essentially the wilting point for a plant to one. One is the field capacity, meaning it has unlimited access to water. These products update throughout the day and can indicated the index at 2-, 4-, 10- and 24-inches based on the sensors at those specific depths.

“We also create seven day changes in fractional water index which help us look at whether the state’s moisting or drying,” he said. “If drought is intensifying, and the like and then graphs of the time series of the data also (intensifies).”

When converting data into a product that a grower or producer would find helpful, the Mesonet generates plant available water data. Here, they integrate observations over four-, 16- and 32-inch depths, which correspond with some various plant depths—whether they’re seedlings, shallow rooted plants or deep-rooted plants. They generated these estimates with soil core samples that Oklahoma State analyzed with water retention curves. From this they were able to estimate how much water is in the profile.

“And then we also compare that to what the soil profile indicates would be the maximum water that integrated depth could hold and we can create percent plant available water apps also,” he said.

Another product is time series data. The Oklahoma Mesonet takes soil moisture observations every 30 minutes and can plot those readings over the source of the day, weeks and months. With this data they can see the variability in the readings during selected time periods.

Fiebrich and his colleagues also try to validate their estimates of soil moisture data. For example, they looked back on the data in the state during a time period when there were drought conditions.

“It was somewhat of a controlled situation where all the soils were dry and we had a big rain event move in and we then looked at how much the plant available water increased each site compared to how much rain fell,” he said.

They found some of the time when the rain fell, it all didn’t necessarily go into increasing the plant available water. Some ran off, some drained and infiltrated through the soil—which made sense in some instances.

“All in all these were some good, good findings to give us confidence in our estimates of plant available water,” Fiebrich said.

With all the good measurements they’re able to capture, there are some limitations of the system. There’s variability in locations, thus different soil moisture values. Also, their measurements are under vegetated sod.

“So if your field is bare, that could change the infiltration in the soil moisture there,” he said. “And then lastly, the soil structure could be different at our mesonet site and in your area.”

Kylene Scott can be reached at 620-227-1804 or [email protected].