Field Calibration Accuracy and Utility of Four Down‐Hole Water Content Sensors

Abstract

Soil water balance studies of profile water content, changes in stored water, crop water use, and spatial variability of water content and use require accurate soil water determinations that are representative across at least field‐sized areas. Several capacitance and other electromagnetic (EM) sensors are commercially available for use in access tubes to determine profile water content. Scientists and practitioners need to know if they are suitable replacements for the neutron moisture meter (NMM) in terms of accuracy and utility. In a field calibration of the NMM and three EM sensors in a Panoche clay loam soil in the San Joaquin Valley of California, three access tubes were installed in a site dried by plant water uptake and three were installed in an adjacent plot wetted to saturation and allowed to drain. Sensors were read and volumetric water content samples taken at several depths at each access tube; calibrations of water content vs. sensor reading were calculated for each depth and for appropriate combinations of depths by regression analysis. Calibrations for the EM sensors changed rapidly with depth, often requiring separate calibrations for every 10‐ or 20‐cm depth range, and were relatively inaccurate (RMSE of 0.015–0.063 m3 m−3). The NMM is the preferred choice for accurate profile water content and change in storage determination. In general, the EM sensors cannot be recommended for profile water content or change in storage determinations due to their relatively less accurate (larger RMSE values) calibrations, strong dependence of calibration slopes and exponents on depth, probable dependence of the calibrations on soil bulk electrical conductivity (BEC), and the likelihood of BEC changes in the field during the irrigation season.

Correcting Inaccurately Recorded Data due to Faulty Calibration of a Capacitance Water Content Probe

“…Due to their sensitivity to variations in soil structure and soil bulk electrical conductivity (EC), several investigators recommend not to use them in scientific research which require accurate measurements of soil water [2,3,[13][14][15]. However, the probes ease of use and reasonable prices urge scientists to use them all around.…”

“…If the capacitance probes' users performed the proper calibration and they want to benefit from the past data that were recorded before calibration, they can easily use one of the correction equations in this study to convert the past data to the correct form (see (13) to (17), or the general equation (7)). Using the correction procedure mentioned in this study corrects the faulty values and enables the owner of the capacitance probe to use his faulty-logged data with confidence.…”

“…One of the famous errors is the error that occurs when the manufacturer's software is used, by mistake, without changing any of the default values (Table 4, Scenario 1); that is,̃,̃,̃,̃, and̃equaled 65535, 0, 1, 1, and 0, respectively. Hence, after obtaining the correct calibration parameters ( , , ) and the correct air/water sensor calibrations ( , ), we can get the correct values of the water content, , by applying a reduced form of (7), (13), over the faulty logged water content values,̃:…”

Density-dependent calibration of multisensor capacitance probes in coarse soil

“…EnviroSCAN multisensor capacitance probes (MCPs) have been used extensively to monitor soil water content since 1991 (Buss 1993;Starr and Paltineanu 1998;Evett et al 2006;Starr and Rowland 2007;Mazahrih et al 2008). The capacitance sensors consist of one or more pairs of cylindrical metal electrodes mounted on a support rod that is inserted into a previously installed PVC access pipe.…”

“…If care is taken in the installation of the access tubes, these potential problems can be limited; however, a soil-specific calibration is still required. The sensor accuracy can also be affected by soil structure and bulk electrical conductivity changes in field (Mazahrih et al 2008;Evett et al 2009Evett et al , 2012. Careful access tube installation cannot overcome these latter effects.…”