New Medium Scale Soil Moisture Measurement Technique
Between dielectric soil moisture sensors with a volume of influence measured in liters and remote sensing systems which measure soil moisture on the scale of kilometers, there is a gap—a gap Dr. Larry Murdoch of Clemson University has been working to fill. In this post, read about the DELTA (Displacement Extensometer for Lysimetric Terrain Analysis), an instrument that measures water content measurements over an area with a 25 m radius.
A New Idea:
Dr. Murdoch was a graduate student in structural geology and geomechanics in the mid-1980s, working on the mechanics of hydraulic fractures in soil. He developed techniques for environmental “fracking” to clean up contaminated soil, long before the recent applications by the oil industry that have caused fracking to become a household word. Fracking causes movements in soil, and Dr. Murdoch developed methods for measuring those movements in order to monitor fracture displacement. This led to work on sensitive borehole extensometers that could measure small strains in rock during well testing.
In the course of his hydrology work, Dr. Murdoch became interested in how much water content was in the vadose zone (the unsaturated soil above the water table). He wondered if he could use the strain measuring technique to quantify it. He decided to bore a hole into the vadose zone and insert a simplified extensometer device that could measure the strain as the soil expands and contracts. This would allow him to gauge the weight change of the overburden. Then, because other mass changes are relatively minor compared to the water in the soil, that weight change would enable him to determine water content.
Since soil compresses more than bedrock, Dr. Murdoch developed a method where he inserted two anchors and cylinders that are pressed up against the soil borehole. In the middle of these cylinders is a fiberglass rod held tight by the bottom anchor which is able to move inside the top anchor. The anchors move up and down from the stress on the soil, and this movement is transferred to the rod where it can be measured with a high-resolution displacement transducer.
Dr. Murdoch’s device is so sensitive that when it is buried 6 m, it will register clear strain signals as his student walks over it. The weight of a person causes around 50 nanometers of displacement at the Clemson Field site, but the instrument itself can resolve displacement approaching 1 nanometer. And the diameter of measurement on the surface is about 4 times the depth. So if you install the system at 7m, you’d be measuring about a 25 m diameter circle on top.
Like almost all other water content techniques, the challenge is removing all other confounding factors that affect the measurement. It has been said that all sensors are temperature sensors first. Not surprisingly, one thing that causes errors in the system is temperature, though Dr. Murdoch’s team has dealt with that by getting the system deep in the soil and putting the electronics near it so the temperature change is small. Barometric pressure also produces cyclical loading of soil mass and requires correction over a range of periods. And, since the calculation of water content requires an estimate of the soil elasticity, changes in soil moisture also may affect the measurement. Considerable work has been done and significant progress has been made in dealing with these and other issues with the extensometer approach.
The amazing thing is that Dr. Murdoch’s system can resolve less than a millimeter of rain water falling on the soil surface, and it can match trends over time. In addition, you can easily calibrate the system by getting your 190-pound student to walk over the top of it and then checking that the compressibility of the soil matches that weight.
Another advantage of the system is its ability to be buried. In order to plow, for example, all you have to do is pull the sensor up, take off the top plastic casing, and cap it, and the grower can drive a plow over the top. Finding the installation can be challenging, so it must be located by precision GPS or survey equipment prior to burial. But, if done correctly, the site can be monitored for long periods of time.
Though not yet a final technology, the Delta extensometer did correlate well with point measurements of water content and shows a lot of promise. The instrument was developed with funding from the National Science Foundation. Colby Thrash, a grad student at Clemson, has done much of the recent work. Dr. Murdoch’s team will publish a paper describing the technique soon in Water Resources Research.
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