The COsmic-ray Soil Moisture Observing System (COSMOS): a non-invasive, intermediate scale soil moisture measurement network

Soil moisture at a horizontal scale of around 700 m and depths of 15 to 70 cm can be inferred from measurements of cosmic-ray neutrons that are generated within soil, moderated mainly by the hydrogen atoms in water, and emitted back to the atmosphere. The intensity of the resulting field of neutrons above the ground is sensitive to water content changes, largely insensitive to soil chemistry and inversely correlated with hydrogen content of the soil. Measurement of this intensity with a portable neutron detector placed above the ground takes minutes to hours, permitting highresolution, long-term monitoring of undisturbed soil moisture. The large footprint makes the method suitable for weather and short-term climate forecast initialization and satellite validation, while the measurement depth makes the probe ideal for studying plant/soil/atmosphere interactions. The intensity of cosmic-ray neutrons is also sensitive to water above the ground in snow, vegetation, or intercepted water, this water being in principle distinguishable from soil moisture. Instruments using this method are being deployed in the COsmic-ray Soil Moisture Observing System (COSMOS), which comprises initially a network of 50 probes (to provide a proof of concept) and subsequently 500 probes distributed across the contiguous USA. Additional COSMOS probes are also now being deployed on an experimental basis in the UK, Australia, and China.