Towards the establishment of a global COsmic-ray Soil Moisture Observing System

Soil moisture is an important component of the water balance despite accounting for a small volume relative to other hydrological cycle components. With the continuing evolution of land surface and global hydrological models, characterizing soil moisture dynamics at sub-kilometer scales is becoming ever important. To help with that, the cosmic-ray neutron sensing is an established technology that provides estimates of root-zone soil moisture at 200-300m radius. In simple terms, cosmic-ray neutron sensors can estimate root zone soil moisture through an indirect relationship between measured neutrons scattered from the soil and the amount of hydrogen atoms observed in the soil water.

Following its development in the late 2010s and the establishment of the first COsmic-ray Soil Moisture Observing System (COSMOS) network in the USA, a continuing adoption of this technology has been observed over the years, notably with the establishment of other national scale networks in Germany, Australia, and in the UK. As the cosmic-ray neutron sensing technology matures, so does our understanding on how to better isolate the soil moisture signal from other sources of hydrogen within the sensor footprint. However, despite recent improvements in our understanding, continental and global-scale datasets from cosmic-ray stations are still inexistent, partially due to a lack of proper data harmonization. This is simply because distinct networks operate under their own data processing protocols. This poses unwanted limitations to the use of these data by the wider scientific community.

Here, we introduce the initial steps towards the harmonization of cosmic-ray neutron sensors worldwide. The harmonization is performed using the state-of-the-art and recent developed Cosmic-Ray Sensor PYthon data processing tool, applied to publicly available data from more than 200 stations. We highlight examples of applications using this global harmonized dataset in hydrology, agriculture, and environmental sciences; and present an open discussion about challenges and opportunities in potentially establishing a Global COSMOS network.