American Society of Civil Engineers

Comparison of Laboratory and Field Calibration of a Soil-Moisture Capacitance Probe for Various Soils

by Kristoph-Dietrich Kinzli, (corresponding author), M.ASCE, (Assistant Professor, Whitaker School of Engineering, Florida Gulf Coast Univ., Fort Myers, FL 33965. E-mail:, Nkosinathi Manana, (Deceased, No affiliation information available.; formerly, Graduate Research Assistant, Dept. of Civil and Environmental Engineering, Colorado State Univ., Fort Collins, CO 80523. E-mail:, and Ramchand Oad, (Professor, Dept. of Civil and Environmental Engineering, Colorado State Univ., Fort Collins, CO 80523. E-mail:

Journal of Irrigation and Drainage Engineering, Vol. 138, No. 4, April 2012, pp. 310-321, (doi:

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Document type: Journal Paper
Abstract: Throughout the American west, irrigated agriculture has been targeted to increase water-use efficiency because of increased urban demands. Soil-moisture sensors offer a method to achieve efficiency improvements, but have found limited use primarily because of high cost and lack of soil-specific calibration equations. In this paper, the Decagon EC-20 soil-moisture sensor (a low-cost capacitance sensor) has been examined and a unique laboratory-calibration method has been developed. Field-and laboratory-calibration equations were developed for six soil types (sand, sandy loam, silt loam, loam, clay loam, and clay) in the Middle Rio Grande Valley for alfalfa and grass hay fields. The average absolute error in volumetric water content for field calibration was 0.430 m3/ m3, and 0.012 m3/ m3 for the laboratory calibration. The factory-calibration equation for the EC-20 was also evaluated and found to yield an average absolute error of 0.049 m3/ m3. In this study, it was found that the EC-20 is a reliable, cost-effective, and accurate sensor, and it is recommended that the laboratory-calibration method presented in this paper be used to obtain maximum accuracy. It is also recommended that the field calibration of the EC-20 soil-moisture sensor be foregone, as this type of calibration exhibits large error rates that are associated with colocation of samples, voids, organic residues, and root densities. Additionally, it was found that the field-calibration method was time-consuming, covered a small range of moisture content values, and was destructive to the area around installed sensors, which could lead to additional measurement errors.

ASCE Subject Headings:
Soil water
Water use
United States

Author Keywords:
Soil-moisture sensor calibration
Middle Rio Grande
Field and laboratory calibration
Dielectric soil-moisture probe
Factory-equation accuracy
Low-cost capacitance sensor
Precision irrigation
Increasing water-use efficiency