Effect of sensor installation on the accurate measurement of soil water content

Abstract

We evaluated the effects of different methods of installing a capacitance soil moisture sensor on the sensor's output with six soil types and three methods of sensor installation. For Method 1, the sensor was first placed in a container and then buried. For Method 2, the entire sensor (including prongs and circuitry) was inserted into a prepacked soil sample. For Method 3, the sensor prongs were installed directly into a prepacked soil sample, and then soil was placed around the circuitry. The sensor outputs of Method 1 were significantly smaller for the volumetric soil water content (θ) of most of the soil types compared with the sensor outputs of the other two methods. Large differences in sensor outputs were observed in relation to the installation method, which resulted in a large estimation error when using a soil moisture sensor. For example, the difference in θ values calculated by the calibration equation obtained by Methods 1 and 2 was 0.076 m3 m−3, a maximum for the Andisol sample. We observed a significant linear relation between the maximum differences in θ (which might result from the different sensor installation methods) and the coarse pore volume determined from soil water characteristic curves. Compaction around the sensor induced by sensor installation, which might be considerable for soil with many coarse pores, was considered the main reason for an increase in the soil moisture sensor's output value. For the sensor installed by Methods 2 or 3, a second-order polynomial equation could be used to translate sensor output to θ accurately except for soils with > 40% clay content or very low bulk density. Highlights: An appropriate calibration equation is important to monitor soil water content precisely. Effects of different sensor installation methods on calibration equations have not been determined. Method of installation considerably affected output values of a capacitance sensor. Soil compaction by sensor installation increases the sensor output: more so in coarse pore soil.