Dual field calibration of capacitance and neutron soil water sensors in a shrinking-swelling clay soil

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Vadose Zone Journal


Multisensor capacitance sensors (MCS) are now popular alterna-tives to neutron scattering (NS) soil water-monitoring devices. The objectivesof thisstudy wereto (i)quantifythe effectof clayshrinkage-swelling on the soil bulk density; (ii) determine field calibration equa-tions for an MCS and an NS device; and (iii) compare the performance of the MCS with a NS meter under field conditions. The calibration was conducted in a duplex soil with sandy clay loam overlying clay in South Australia. Six access tubes were installed in a 6 by 8 m grid. Three moisture treatments were replicated twice for every moisture level. The bulk density of the top 20 cm increased with increasing water content; this increase was more pronounced in the upper 10-cm horizon, which could be attributed to soil compaction. However, a negative correlation was obtained between bulk density and water content in the 30-to 100-cm depth layers reflecting the shrinking and swelling properties of the fine-textured subsoil. Results also show highlysignificanteffectsofsamplingdepthandmoisturelevelonNSand MCS readings. Compared with linear calibration, a three-parameter model improved NS calibration and/or minimized the root mean square errors for 6 out of the 10 sampling depths. Except for the 10-cm sampling depth, individual calibration for each 10-cm soil layer improved the accuracy of the MCS as compared with the use of single calibration equation for the entire profile. Site-specific calibration improved the accuracy of both the NS and MCS soil water-monitoring devices. © Soil Science Society of America.

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