ARS | Publication request: Sensitivity of the Enviroscan soil moisture capacitance probe to the presence of artificial macropores

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 2010
Publication Date: January 1, 2011
Citation: Rowland, R.A., Pachepsky, Y.A., Guber, A.K. 2011. Sensitivity of the Enviroscan soil moisture capacitance probe to the presence of artificial macropores. Soil Science. 176:1-9.

Interpretive Summary: Frequent measurement of soil water content has recently made been possible with capacitance soil moisture probes. Large macropores, cracks, or other large heterogeneities can exist in soil near a capacitance probe before the installation, can be formed during the installation, or can appear near the probe after installation because of plant root and soil animal activity and soil swelling-shrinking. Such large pore spaces can be filled with water when the bulk soil is dry or can release water much faster than the rest of soil, under these circumstances, the sensor will react to both empty pores and the bulk soil, resulting in an inaccurate reading of the water content of the bulk of soil which is of actual interest. We proposed and implemented a simple assembly of capillaries to mimic macropore effects on the Enviroscan soil moisture sensor reading. Results of the work showed that substantial macroporosity near the access tube may have only a moderate effect on the measurements if the water fills the empty space between the access tube and soil. The effect of macropores decreased drastically as macropores were moved away from the probe. Results of this work are important for personnel in commercial and research facilities involved in soil moisture monitoring in that the bulk soil near probe can be used for site-specific calibration of probes and evaluation of their performance. The experimental design proposed in this work can be used with other types of capacitance probes, and can provide essential data for modeling the effect of soil moisture variability of capacitance probe response.

Technical Abstract: Capacitance probes (CP) have been used to measure soil water contents in various applications. Effects of large macropores, cracks and other large heterogeneities within the CP sensitivity volume are sources of concerns related to such applications. The objective of this work was to evaluate the sensitivity of the Enviroscan probes to the presence of simulated macropores positioned at different distances from the access tube. The artificial macropores with the OD of 0.63 cm and the ID of 0.41 cm were manufactured from polystyrene tubes. They were placed in contact with the access tube and distances of 1.5 cm and 2.5 cm in air, water, and wetted sand. Results of measurements were presented and analyzed that were obtained by sequential filling with water eight macropores at the same distance from the access tube. The dependencies of the raw frequencies on the number of filled macropores were linear with R2 > 0.98. The macropore sensitivity, defined as the decrease in frequency per one filled macropore, deceased exponentially with the distance from the access tube. At the same distance from the access tube, the macropore sensitivity depended linearly on the scaled frequency. The effect of the water-filled macropores on the bulk soil water content estimate decreased with the increase in soil water content. The estimated error of measurement was approximately 0.001 cm3cm-3 (0.0005 cm3cm-3) per 1% of soil volume near the access tube occupied by the macropore for the bulk water content of bulk soil of 0.10 cm3cm-3 (0.30 cm3cm-3). The estimated errors were six times smaller at the distance of 1.5 cm from the access tube. Overall, macropores near the access tube had only moderate effect on the soil water content measurements with CP when they occupied less than 30 % of soil volume near the access tube.