FIELD-SCALE NEUTRON PROBE CALIBRATION AND VARIANCE ANALYSIS FOR CLAY SOIL

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 as sociated with field practice and calibration, data analysis and estima tion of variance, and error in soil moisture measurements for silty cl ay soils in the Imperial Valley, California, using large calibration d ata sets from several different layouts of neutron access tubes. Our a nalysis indicates that for field calibrations: (1) bulk density correc tions of the count ratio data do not necessarily improve the standard errors of the water content measurements; (2) soil sampling for calibr ation within the access tube hole at the time of tube installation res ults in smaller measurement errors; (3) calibrations improve as the av erage soil moisture of the profile at the time of the original samplin g decreases; and (4) field-wide calibrations improve, and the sampling intensity required to characterize soil moisture content decreases wi th an increasing soil depth. We also address aspects of the neutron pr obe use associated with soil salinity and polyvinyl chloride (PVC) acc ess tubing (chloride interference), inclusion of shallow sampling data in calibrations, and instrument, calibration, and spatially related s tandard variances affecting field measurements.