Technical Note: revisiting the general calibration of cosmic-rayneutron sensors to estimate soil water content

Abstract. Cosmic-ray neutron sensing (CRNS) is becoming increasingly popular for monitoring soil water content (SWC). To retrieve SWC from observed neutron intensities, local measurements of SWC are typically required to calibrate a location-specific parameter, N₀, in the corresponding transfer function. In this study, we develop a generalized conversion function that explicitly takes into account the different factors that govern local neutron intensity. That way, the parameter N₀ becomes location-independent, i.e., generally applicable. We demonstrate the feasibility of such a "general calibration function" by analysing 75 CRNS sites from four recently published datasets. Given the choice between the two calibration strategies – local or general – users will wonder which one is preferable. To answer this question, we estimated the resulting uncertainty of the SWC by means of error propagation. While the uncertainty of the local calibration depends on both the local reference SWC itself and its error, the uncertainty of the general calibration is mainly governed by the errors of vegetation biomass and soil bulk density. An interactive online tool is provided to support the decision decide which calibration strategy – local or general – is preferable in the user-specific application context (https://cosmic-sense.github.io/local-or-global).