QUANTIFYING THE COSMIC-RAY NEUTRON PROBE SUPPORT VOLUME IN HETEROGENEOUS SYSTEMS

Given the cosmic-ray neutron probe horizontal support at tens of hectares and tens of centimeters vertically; the probe has the potential to fill a critical measurement gap for validating and calibrating hyper-resolution land surface models. However, the estimate of probe support volume is complicated by two factors: 1) the probe sees all forms of hydrogen regardless of association and 2) the relationship between neutron counts and average hydrogen content is nonlinear resulting in non-uniqueness when averaging heterogeneous fields. In this work, I will first present a general framework accounting for all forms of hydrogen near the earth’s surface. Second, I will define the horizontal support and how it varies with changes in the mass and composition of the near surface atmosphere. Third, I will define the vertical support and how it changes with subsurface hydrogen content. Finally, I will present some hypothetical model scenarios of 1 and 2-dimensional soil moisture heterogeneity and its affect on average neutron count and thus area-average soil moisture. By quantifying the probe support volume and its dependence on system hydrogen content we can better constrain estimated area-average water flux and the parameters that control flux.