Comparing the accuracy of soil moisture estimation derived from environmental and spectroscopic gamma radiation measurements

Continuous monitoring of root-zone soil moisture status is important to ensure the effective management of water resources for agricultural production, and to improve our understanding of land-atmosphere interactions in a changing climate. Utilizing gamma radiation to monitor soil moisture at the field scale is an emerging non-invasive technique that can also bridge the gap between point and remote sensing measurements. The measurement principle relies on the increased attenuation of gamma radiation emitted from soil with increasing soil moisture content. In a previous study, we successfully obtained soil moisture estimates from low-cost environmental gamma radiation (EGR) detectors. However, since EGR detectors provide the bulk response to gamma radiation over a wide energy range (0 to 3000 keV), EGR signals are influenced by several confounding factors, e.g., skyshine radiation, atmospheric and soil radon variability. To what extent these confounding factors deteriorate the accuracy of soil moisture estimates obtained with EGR is still not fully understood. Therefore, the aim of this study is to compare EGR measurements with K-40 gamma radiation (at 1460 keV) measurements that are much less influenced by these confounding factors. For this, two different kinds of gamma radiation detectors were installed close to each other at an agricultural field in Selhausen, Germany: an EGR detector based on a G-M counter tube (MIRA, ENVINET GmbH) and a spectroscopic scintillation-based detector (SARA, ENVINET GmbH). The field was also equipped with in-situ soil moisture sensors to measure reference soil moisture and a climate station to measure meteorological parameters. The EGR measurements were corrected for atmospheric radon-washout during precipitation events and the contributions of meteorologically influenced secondary cosmic radiation were also eliminated. In case of the spectroscopic measurements, no further corrections were applied as the analysis was only focused on the K-40 energy window. Both sets of gamma radiation measurements were related to reference soil moisture using a functional relationship derived from theory. We found that daily soil moisture can be predicted more accurately from K-40 gamma radiation (RMSE 4 vol.%) than from EGR (RMSE 6 vol.%). Regardless of the higher prediction uncertainty obtained due to the influence of the confounding factors at low energy, the long-term availability of ERG data, e.g., in Europe via EURDEP, makes it interesting for continental scale analysis of soil moisture.