A Depth Control Stand for Improved Accuracy with the Neutron Probe

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). It is in this shallow zone that many plants have the largest root density and water uptake and where infiltration and evaporation typically cause 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 r2 values >0.98 and RMSE values of calibration <0.01 m3 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 water content or nature of the surface, thus enhancing accuracy of both the calibration and subsequent water content readings, both of which depend on standard count values. Also, 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, but without additional occupational exposure to radiation.