When watershed, agricultural, or other environmental research calls for soil moisture measurements, we will often study the wide array of sensors available to inexpensively and continuously measure the water content in the soil. We’ll compare price, accuracy, the ease of use, and maybe even sometimes the additional features that the devices on the market have to offer.
After a field season, you may develop soil moisture graphs like the one shown below.
The lines on the graph represent your eyes under the ground; one more measurement to allow you to correlate what you are seeing above ground to what your plants’ roots are experiencing below ground. There isn’t a single hour that goes by where you can’t look at your graph to see the amount of water in the soil. Did you realize that before June of 1980, no one was able to create these types of graphs?
The advent of the soil moisture calibration equation
In June of 1980, G. Clarke Topp and his team members, J.L Davis and P. Annan, published a watershed paper which included what would come to be known as the Topp equation. The equation is so commonly used that some people don’t even know they’re using it. For instance, when the above graph was made, the Topp equation was used. Do you use the Topp equation? If you use the mineral soil factory calibration with our 5TE, 5TM, or some of Decagon’s other soil moisture sensors, you’re using the Topp Equation.
The Topp Equation makes it possible for us to measure an electrical property of the soil (the dielectric permittivity) and correlate that electrical property with the water content in the soil. The beauty of the equation is that it is pretty accurate in most typical soils, saving us the job of doing custom calibrations.
The equation specifically is:
Θ (m3/m3) = 4.3 X 10-6 * ε3 - 5.5 X 10-4 * ε2 + 2.92 X 10-2 * ε -5.3 X 10-2
Topp and his team showed that the volumetric water content of soil can be determined from the apparent dielectric constant of the soils, independent of soil type, soil temperature, and soluble salt content.
If you would like to learn more, join us for a virtual seminar where Clarke Topp will be discussing his work on the Topp equation and the future of TDR.
Topp, G.C., J.L. David, and A.P. Annan 1980. Electromagnetic, Determination of Soil Water Content: Measurement in Coaxial Transmission Lines. Water Resources Research 16:3. p. 574-582.