Assessing the feasibility of a directional CRNS-sensor for estimating soil moisture

Abstract. Cosmic Ray Neutron Sensing (CRNS) is a non-invasive tool for measuring hydrogen pools like soil moisture, snow, or vegetation. The intrinsic integration over a radial hectare-scale footprint is a clear advantage for averaging out small-scale heterogeneity, but on the other hand the data may become hard to interpret in complex terrain with patchy land use. This study presents a directional shielding approach to block neutrons from certain directions and explores its potential to gain a sharper view on the surrounding soil moisture distribution. Using the Mont-Carlo code URANOS, we modelled the effect of additional polyethylene shields on the horizontal field of view and assessed its impact on the epithermal count rate, propagated uncertainties, and aggregation time. The results demonstrate that directional CRNS measurements are strongly dominated by isotropic neutron transport, which dilutes the signal of the targeted direction especially from the far field. For typical count rates of customary CRNS stations, directional shielding of halfspaces could not lead to acceptable precision at a daily time resolution. However, the mere statistical distinction of two rates should be feasible.