Soil Profile Water Content Determination: Sensor Accuracy, Axial Response, Calibration, Temperature Dependence, and Precision

Abstract

Although the neutron moisture meter (NMM) has served the need for accurate soil water content determinations well, increasing regulatory burdens, including the requirement that the NMM not be left unattended, limit the usefulness of the method. Newer methods, which respond to soil electromagnetic (EM) properties, typically allow data logging and unattended operation, but with uncertain precision, accuracy, and volume of sensitivity. In laboratory columns of three soils, we compared the Sentek EnviroSCAN and Diviner 2000 capacitance devices, the Delta‐T PR1/6 Profiler capacitance probe, the Trime T3 tube‐probe, all EM methods, with the NMM and conventional time domain reflectometry (TDR, also an EM method). All but conventional TDR can be used in access tubes. Measurements were made before, during, and after wetting to saturation in triplicate repacked columns of three soils ranging in total clay content from 17 to 48%. Each column was weighed continuously, and thermocouple determinations of temperature were made every 30 min throughout. All of the devices were sensitive to temperature except for the NMM, with conventional TDR being the least sensitive of the EM devices (sensitivity <∣0.0006∣ m3 m−3 C−1). The Trime T3 and Delta‐T PR1/6 devices were so sensitive to temperature (0.015 and 0.009 m3 m−3 °C−1, respectively, in saturated soil using soil‐specific calibration) as to be inappropriate for routine field determinations of soil profile water content. Temperature sensitivity was up to 12 times larger in saturated soils compared with values in air‐dry soils, corresponding to the much larger bulk electrical conductivities of these soils when saturated. All devices exhibited estimation precision better than 0.01 m3 m−3 under isothermal conditions. However, under nonisothermal conditions, estimation precision for the EM sensors worsened as the number of measurements (and time involved in taking readings) increased, and as the soils became wetter, resulting in precision values >0.01 m3 m−3 for the Trime and Delta‐T devices. Accuracy of the devices was judged by the root mean squared difference (RMSD) between column mean water contents determined by mass balance and those determined by the devices using factory calibrations. Smaller values of the RMSD metric indicated more accurate factory calibration. The Delta‐T system was least accurate, with an RMSD of 1.299 m3 m−3 at saturation. At saturation, the Diviner, EnviroSCAN, NMM, and Trime devices all exhibited RMSD values >0.05 m3 m−3, while TDR exhibited RMSD <0.03 m3 m−3 Soil‐specific calibrations determined in this study resulted in RMSE of regression values (an indicator of calibration accuracy) ranging from 0.010 to 0.058 m3 m−3 All of the devices would require separate calibrations for different soil horizons. Of the EM devices, only the Delta‐T PR1/6 exhibited axial sensitivity appreciably larger than the axial height of the sensor, indicating small measurement volumes generally, and suggesting that these systems may be susceptible to small‐scale variations in soil water content (at scales smaller than the representative elemental volume for water content) and to soil disturbance close to the access tube caused during installation.