Dual Field Calibration of Capacitance and Neutron Soil Water Sensors in a Shrinking-Swelling Clay Soil (original) (raw)

2004, Vadose Zone Journal

Multisensor capacitance sensors (MCS) are now popular alternamethods. Over the years, many NS calibration equations tives to neutron scattering (NS) soil water-monitoring devices. The have been proposed (e.g., Allen and Segura, 1990; Corobjectives of this study were to (i) quantify the effect of clay shrinkageswelling on the soil bulk density; (ii) determine field calibration equa-beels et al., 1999). One of the first guides on how to use tions for an MCS and an NS device; and (iii) compare the performance the neutron probe was written by Greacen et al. (1981); of the MCS with a NS meter under field conditions. The calibration recent guides were also published by Hignett and Evett was conducted in a duplex soil with sandy clay loam overlying clay (2002) and IAEA (2003). in South Australia. Six access tubes were installed in a 6 by 8 m grid. Since the release of the first prototype, NS has seen Three moisture treatments were replicated twice for every moisture several improvements such as weight and size reductions level. The bulk density of the top 20 cm increased with increasing and the introduction of more efficient detectors that also water content; this increase was more pronounced in the upper 10-cm used safer radioactive sources. However, despite these horizon, which could be attributed to soil compaction. However, a improvements, safety regulations requiring costly licensnegative correlation was obtained between bulk density and water ing and training of users and considerable regulation content in the 30-to 100-cm depth layers reflecting the shrinking and swelling properties of the fine-textured subsoil. Results also show have caused the NS method to remain expensive to mainhighly significant effects of sampling depth and moisture level on NS and tain and difficult or impossible to use in some situations, MCS readings. Compared with linear calibration, a three-parameter particularly for unattended monitoring (Evett, 2000). model improved NS calibration and/or minimized the root mean square Neutron scattering soil water measurements are known errors for 6 out of the 10 sampling depths. Except for the 10-cm sampling to be affected by soil access tube material type, its thickdepth, individual calibration for each 10-cm soil layer improved the ness and size, and air gaps between access tubes and accuracy of the MCS as compared with the use of single calibration the soil profile (e.g., Allen and Segura, 1990). The inside equation for the entire profile. Site-specific calibration improved the diameter of the access tube should be large enough for accuracy of both the NS and MCS soil water-monitoring devices. the probe to move freely without friction (IAEA, 2003). An air gap of 1.5 to 4 mm is often considered satisfactory provided the hole is straight (e.g., Prebble et al., 1981). C ompetition is increasing between agricultural, do-Neutron meter count ratios were found to be influenced mestic, and industrial water use. Optimal manageby material type, access tube type (Abeele, 1979; Allen ment of water resources is especially critical when water and Segura, 1990), and thickness (Allen and Segura, supplies are limited and/or potential environmental prob-1990). Large air gaps between the probe and the access lems exist. One key requirement to optimal soil and tube may result in errors due to eccentric positioning water management is accurate knowledge of the soil water (Schrale, 1976). The effect of an air gap on neutron content. Most early in situ measurements of the water counts is similar to having a larger tube diameter (Abecontent were made with neutron moisture meters, often ele, 1979; Allen and Segura, 1990). An inverse linear referred to also as neutron scattering (NS). First introcorrelation exists between NS count ratio and access duced in the 1950s (e.g., Gardner and Kirkham, 1952), tube diameter of PVC and steel types (Abeele, 1979). NS proved to be very popular as a research and teaching Allen and Segura (1990) found that on average, count tool within the scientific community, but also for appliratios for PVC access tubes with no air gaps were 6% cation to a wide range of practical agricultural, environhigher than those for tubes with 10-mm air gaps (i.e., mental, and engineering problems. Its widespread use for auger holes having a diameter approximately 20 mm resulted partly from the ease and speed of measurement larger than the tube) and 14% higher than for tubes with 20-mm air gaps (auger holes having a diameter A. Fares, Natural Resources and Environmental Management Dep., approximately 40 mm larger than the tube).