Effect of Air‐ and Water‐Filled Voids on Neutron Moisture Meter Measurements of Clay Soil

Abstract

© Soil Science Society of America. Air-and water-filled voids around neutron moisture meter (NMM) access tubes have been cited as sources of volumetric water content (θ v ) measurement error in cracking clay soils. The objectives of this study were to experimentally quantify this potential error stemming from (i) uncertainty in bulk density (ρ b ) sampling and (ii) the impact of air-and water-filled voids. Air-and water-filled voids were simulated using ~0.6-cm (small) and ~1.9-cm (large) annuli around access tubes. After NMM measurements were taken in a tightly installed access tube, either a small or large annulus was installed in the same borehole. Additional NMM measurements were taken with the annulus filled with air, and then water and ρ b and θ v were measured. The RMSE of the NMM calibration using all 11 installations was 0.02 m 3 m −3 . However, if two cores were used for calibration, the ratio of NMM-measured θ v to in situ θ v was significantly different (p < 0.05) from measured θ v half the time (RMSE, 0.012–0.05 m 3 m −3 ). Small air-filled voids created drier estimates of θ v (bias, −0.039 m 3 m −3 ; p < 0.001), wherease small water-filled voids were not significantly different from the calibration. Air-and water-filled voids from larger annuli were significantly lower and higher (p < 0.001) than core-measured θ v , with biases of −0.068 and 0.080 m 3 m −3 , respectively. Although this work does not correct NMM-predicted θ v to matrix θ v , it does bound NMM error under field conditions in a cracking clay soil.