A Depth Control Stand for Improved Accuracy with the Neutron Probe

rameterized for a given porous medium by slope and intercept values obtained through field calibration (HigThe neutron thermalization method for soil water content measurenett and Evett, 2002). For probe designs common in the ment is well established as being accurate for deep soil profile measurements. However, the method has been criticized as inaccurate for 1960s, about 95% of the measured slow neutrons were shallow measurements ( 30 cm). It is in this shallow zone that many from a nearly spherical volume of radius R (cm), with plants have the largest root density and water uptake and where the value of R dependent on the medium’s volumetric infiltration and evaporation typically cause the largest changes in water content ( v, m3 m 3), (IAEA, 1970). water content. We show how neutron probe depth influences soil R 15( v) 1/3 [1] water readings in the top 30 cm of soil, and we describe a depth control stand that serves to control probe depth relative to the soil Thus, the volume of sensitivity was predicted to have a surface so that probes may be accurately calibrated and successfully radius of roughly 20 cm for saturated media, up to 40 used in the field for measurements at shallow depths. Using the stand, cm for air-dry media. calibrations for the 10-cm depth may be obtained routinely with linear For profile water content measurements, a cylindrical regression r2 values 0.98 and RMSE values of calibration 0.01 m3 probe is lowered to different depths for measurements m 3. The stand is also useful for elevating the gauge high enough above the surface so that standard counts are not influenced by the in an access tube installed vertically in the porous mewater content or nature of the surface, thus enhancing accuracy of dium. Despite the relatively large volume measured, the both the calibration and subsequent water content readings, both of depth interval between readings has been as small as which depend on standard count values. Also, the stand serves to 0.15 m, or even 0.10 m (Carrijo and Cuenca, 1992; Vanprevent repetitive strain injuries to backs and knees caused by bending dervaere et al., 1994). Depth intervals as small as 0.15 and kneeling to place the gauge on top of access tubes, but without m may provide added value in soils with large and rapid additional occupational exposure to radiation. changes in water content with depth (McHenry, 1963; Stone, 1990). This is because sensitivity decreases exponentially with distance from the probe. A depth interval T neutron thermalization method of soil water of 0.10 m was reported to improve precision of profile content measurement was developed more than 50 yr water content (Carrijo and Cuenca, 1992), but the imago and has long been considered an accurate method, provement may have been due to the increased countonce calibrated, for deep measurements of porous meing time resulting from the additional readings rather dia water content in the vadose zone. However, the than any new information about the profile (Stone and method has been characterized as inaccurate at depths Weeks, 1994). Depth intervals of 0.2 m are sometimes 30 cm because of the loss of neutrons to the air at used where water content changes slowly with depth. these shallow depths and due to calibration equations When not in use, the probe is locked within a shield case with values of RMSE 0.01 m3 m 3. The theory, interfilled with high density polyethylene or similar hydrogeferences, calibration methods, and achievable accuranous material. Modern equipment called the neutron cies are well established (Hignett and Evett, 2002). moisture meter (NMM) consists of this shield case, with Briefly, the method uses a source of fast neutrons that an electronic counter, display, and keyboard attached, are slowed to ambient temperature (thermalized) by and a cable of useful length connecting the counter to repeated collisions with the nuclei of soil materials, the probe (Fig. 1). Marks or clamps on the cable are forming a cloud of slow neutrons around the source. used as reference points for placement of the probe The slow neutrons that pass through a detector tube at fixed depths. are then counted electronically. The detector tube and To avoid damage by machinery, access tubes are typifast neutron source are packaged together in a probe cally installed in crop rows with only 10 to 15 cm of that slides inside an access tube for soil water assessment the tube exposed above the soil surface. It is common below the soil surface. Because H is by far the most practice to place the shield case on top of the access effective element for slowing neutrons, and because tube before releasing the probe from the shield and rapid changes in soil H content are almost completely lowering it for readings in the soil. Doing so may lead due to changes in soil water content, the count of slow to errors for two reasons. First, when the shield case is neutrons is proportional to soil water content. For modplaced on top of the access tube, the shield material may ern probes, the proportionality is linear and can be painfluence near-surface counts to a degree that depends strongly on the height of the case above the soil and S.R. Evett, J.A. Tolk, and T.A. Howell, USDA-ARS, P.O. Drawer the depth of the probe (Stone et al., 1993). Second, in 10, Bushland, TX 79012. Received 24 Jan. 2003. Special Section— field use, the actual height of access tubes above the Advances in Measurement and Monitoring Methods. *Corresponding soil is likely to change with tillage, rainfall-induced comauthor ([email protected]). paction, erosion or deposition, or other factors, resulting Published in Vadose Zone Journal 2:642–649 (2003).  Soil Science Society of America Abbreviations: EMT, electromechanical tubing; NMM, neutron moisture meter. 677 S. Segoe Rd., Madison, WI 53711 USA