Field-Scale Neutron Probe Calibration and Variance Analysis for Clay Soil

Abstract

Although widely used for field measurements of soil moisture, problems continue with the use of the neutron probe in clay soils comprised of over 40% clay fractions, and in the determination of the intensity of field sampling necessary to characterize a site. We examine issues associated with field practice and calibration, data analysis and estimation of variance, and error in soil moisture measurements for silty clay soils in the Imperial Valley, California, using large calibration data sets from several different layouts of neutron access tubes. Our analysis indicates that for field calibrations: (1) bulk density corrections of the count ratio data do not necessarily improve the standard errors of the water content measurements; (2) soil sampling for calibration within the access tube hole at the time of tube installation results in smaller measurement errors; (3) calibrations improve as the average soil moisture of the profile at the time of the original sampling decreases; and (4) field-wide calibrations improve, and the sampling intensity required to characterize soil moisture content decreases with an increasing soil depth. We also address aspects of the neutron probe use associated with soil salinity and polyvinyl chloride (PVC) access tubing (chloride interference), inclusion of shallow sampling data in calibrations, and instrument, calibration, and spatially related standard variances affecting field measurements.