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

Abstract

Air-and water-filled voids around neutron moisture meter (NMM) access tubes have been cited as sources of volumetric water content (q 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 (r 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 r b and q 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 q v to in situ q v was significantly different (p < 0.05) from measured q v half the time (RMSE, 0.012-0.05 m 3 m −3 ). Small air-filled voids created drier estimates of q 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 q v , with biases of −0.068 and 0.080 m 3 m −3 , respectively. Although this work does not correct NMMpredicted q v to matrix q v , it does bound NMM error under field conditions in a cracking clay soil.Abbreviations: NMM, neutron moisture meter. Ackerson. 2018. Effect of air-and water-filled voids on neutron moisture meter measurements of clay soil. Vadose Zone J. 17:180137.

Testing a Water Redistribution Model in a Cracked Vertisol at Two Scales

“…The study found that errors were much smaller than had been simulated previously (Li et al, 2003). Based on the findings of Bagnall et al (2018), the “worst‐case” overestimation for NMM VWC measurements will occur at our driest measurement of VWC (0.277 m 3 m −3 ) in the presence of a roughly 2‐cm‐wide crack that surrounds the access tube and is totally full of water. This “worst‐case” overestimation for our study would be a NMM‐measured VWC of 0.390 m 3 m −3 when the true matrix VWC was 0.277 m 3 m −3 , a 37% overestimation.…”

“…Our experimental design will encounter the problem of making NMM measurements with soil cracks filled with water in a dry soil matrix. To address this problem, Bagnall et al (2018) experimentally created air‐ and water‐filled Al annuli that represent the range in crack volume expected to be encountered at the site. The study found that errors were much smaller than had been simulated previously (Li et al, 2003).…”

“…We expect errors to be smaller because cracks will not surround access tubes, nor will they be filled entirely with water. As the soil matrix wets, the NMM error associated with water-filled cracks decreases because the cracks become smaller and the contrast between crack VWC and soil matrix VWC is reduced (Bagnall et al, 2018). For example, if the soil VWC was 0.35 m 3 m −3 , the NMM would measure it as 0.43 m 3 m −3 in the presence of a ?2-cm crack (a 23% overestimation in VWC).…”

Event‐based deep drainage and percolation dynamics in Vertosols and Chromosols

“…Even in soil that has uniform properties at the ~1 m scale, this will add to the apparent randomness between sites. There may also be considerable under‐ or over‐estimation of soil moisture if any shrinkage cracks form around the perimeter of the probe (Bagnall et al, ), introducing further inter‐site and inter‐event variance. Integrating sites reduces the random errors associated with heterogeneity of soil and brings out the importance of rainfall volumes; and it may be speculated that adding more sites would further smooth out errors.…”

Current Insights into Nonuniform Flow across Scales, Processes, and Applications

“…Furthermore, soil water content is a primary explanatory variable for the formation and connectivity of shrinkage crack networks, making it critical to obtain accurate soil water content readings in these soils. Similarly, Bagnall et al (2018) assessed the influence that (i) air‐ and water‐filled cracks and (ii) uncertainty in bulk density sampling have on neutron moisture probe readings. Their results provide critical observational data on how cracks influence moisture readings from neutron probes depending on their size and state.…”