Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2003
Publication Date: 10/31/2003
Citation: Evett, S.R., Tolk, J.A., Howell, T.A. 2003. A depth control stand for improved accuracy with the neutron probe. Vadose Zone Journal. 2:642-649. Interpretive Summary: The neutron probe has been the established reference method for non-destructive soil water content measurement for more than 50 yr. It is widely used in agriculture, engineering, hydrology, and the earth sciences both by scientists and by technical workers. Because it can be lowered down an access tube into the soil and underlying strata to practically any depth, it is useful for measuring water content not only in the soil but in underlying unconsolidated material and rock. However, it has been justly criticized for inaccuracy within 12 inches of the soil surface where most crop roots grow and where precipitation and irrigation cause the largest soil water changes. We show that the inaccuracy is due to lack of accurate depth placement of the probe, which is particularly sensitive to nearness to the surface at depths less than 10 inches. We also present the design of a depth control device that ensures accurate depth placement; and we show how this allows both accurate probe calibration and accurate soil water measurement on a routine basis. The device is easily made and improves worker health because it allows workers to use the probe in the standing position rather than kneeling and bending for every measurement.
Technical Abstract: The neutron thermalization method for soil water content measurement is well established as being accurate for deep soil profile measurements. However, the method has been criticized as inaccurate for shallow measurements (<30 cm depth). It is in this shallow zone that many crops have the largest root density and water uptake and where infiltration typically causes the largest changes in water content. We show how neutron probe depth influences soil 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 surface so that probes may be accurately calibrated and successfully used in the field for measurements at shallow depths. Using the stand, calibrations for the 10 cm depth may be obtained routinely with linear regression r**2 values >0.98 and root mean squared errors of calibration <0.01 m**3/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 nature or water content of the surface, thus enhancing accuracy of both the calibration and subsequent water content readings, both of which depend on standard count values. Finally, the stand serves to prevent repetitive strain injuries to backs and knees caused by bending and kneeling to place the gauge on top of access tubes.