Soil properties and elements other than hydrogen that can affect the field calibration of a neutron water meter

Abstract

A neutron water meter (NWM) operates on the principle that emitted high-energy neutrons are thermalised by elastic collisions with atomic nuclei present in soil, such as that of the hydrogen atom. Thermalised neutrons, however, are affected by other nuclear-matter interactions such as their capture by soil elements and properties, thereby decreasing the counts and leading to an underestimation of soil water content (SWC). The objective of this study was to determine which soil and concretion properties and elements (apart from hydrogen) would have a neutron capture effect during the field calibration of a NWM, and whether statistical relationships could be established. Soil and concretion samples from the field calibration of a NWM were analysed for a number of properties and elements. Linear regression analyses revealed that, apart from volumetric SWC, soil and concretion (chloride, potassium, iron and electrical conductivity of soil-paste extract [EC_e]) properties were correlated with NWM count ratio. A multiple regression equation containing SWC, soil potassium and EC_e, as well as concretion iron, explains 87% of the variation in NWM count ratio. It is recommended that measurements be made on a wider range of soils to establish a more rigorous calibration equation that contains SWC, as well as other soil and concretion parameters.

Keywords: barn value, calibration, elemental interference, regression, soil water content, thermalised neutrons