On soil bulk density and its influence to soil moisture estimation with cosmic-ray neutrons

Abstract. Cosmic-ray neutron sensing (CRNS) is a non-invasive technique that is used to quantify soil moisture in a representative footprint of 10–20 ha and 15–80 cm depth. In a stationary mode, CRNS is widely employed to monitor the moisture dynamics on agricultural land, which may undergo active changes of the soil compartments, e.g. due to plowing. On mobile platforms, CRNS measurements aim at mapping the spatial soil moisture distribution across various types of soil and land use. To date, the potential effect of variable soil bulk density on the neutron measurements has not been investigated in detail. In fact, most sensors are calibrated only once on site-specific soil properties. Therefore we hypothesize that unaccounted spatiotemporal changes of soil bulk density may have impact on the quality of CRNS soil moisture products. In this study, we quantify the effect of the soil density on the neutron response by neutron transport simulations and a dedicated lab experiment. The results indicate a significant dependency of neutrons on soil bulk density, which also depends on the soil moisture state. For the hypothetical cases with constant ratio between soil density and water content, the neutron intensity remains unaffected. Correction functions are proposed to improve the performance of two widely used approaches to convert neutrons to soil moisture (Desilets et al. 2010, and Köhli et al. 2021). The latter approach together with the proposed correction can be recommended for practical use, as it accurately represented the simulated neutron response to soil moisture and soil bulk density with a constant – and potentially universal – calibration parameter.