Neutron Soil Moisture Probe Operation in Saline Environments

Abstract

Neutron soil moisture probes enable soil moisture profiles to be measured in a rapid and nondestructive manner. This study examined experimentally and analytically the impact of salinity on neutron probe measurements. Experimentally, a decrease in the neutron count was associated with all common saline species (by as much as 50% when exposed to 10 wt.% aqueous solutions), with the decrease resulting from the absorption of lower energy neutrons by chloride nuclei. The experimental results also showed that the heterogeneity of the boundary region at the saline soil surface affected the neutron probe response. A seven-group finite element model of the neutron probe operation successfully predicted the response of the probe to saline conditions, with only a slight overprediction of the rate of decline in the neutron count as salinity increased. The finite element model simulation of the neutron probe operation in more realistic agricultural scenarios showed that the impact of salinity on the neutron count versus soil moisture curve is likely to be minor except in extreme cases. Extreme (electrical conductivity = 15 dSm) levels of salinity, however, are likely to cause significant deviations from the (nonsaline) calibration curve, particularly at higher soil moisture contents. The impact on the neutron count of nonuniform salinity distribution throughout the soil column was predicted to be highly dependent on the relative positioning of the areas of salinity and the neutron probe source and detector.