Next Generation Water Observing System (NGWOS) - Advancing Soil Moisture Monitoring

Soil Moisture Monitoring Research

Soil moisture—also called soil water content (SWC)—plays a key role in the water cycle. However, it is hard to measure SWC in a way that accurately represents a whole landscape. Traditional in-ground sensors measure only a very small area and are sensitive to local conditions. Many hydrologic studies need estimates that represent much larger areas that can be applied to entire watersheds.

To improve large‑area SWC measurement, the USGS - Next Generation Water Observing System (NGWOS) is testing both traditional in‑ground sensors and newer technologies, including cosmic‑ray neutron sensors (CRNS). CRNS can measure SWC over areas hundreds of meters wide and to a depth of about 20–30 cm, helping bridge the gap between small point measurements and watershed‑scale models.

How CRNS Measures Soil Moisture

• Cosmic rays strike Earth’s atmosphere and generate particles, including neutrons
• Neutrons slow down when they interact with water in the soil
• The above ground sensor counts “fast” (epithermal) and “slow” (thermal) neutrons
• Fewer fast neutrons = wetter soil

A short summary of how CRNS technology works is available in the CRNS overview video.

Field Testing

USGS is testing CRNS at five Innovation Test Bed (ITB) stations across the continental United States. These stations represent different land cover types, climates, geology, and hydrologic conditions. Three stations are located within NGWOS Integrated Water Science (IWS) basins and are co-located with existing USGS research stations.

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In 2023, USGS installed a dense network of 15 CRNS in the Roaring Fork Basin, Colorado. This network helps evaluate how well CRNS supports watershed‑scale water‑balance estimates (Figure 2). The Roaring Fork Basin already has a dense network of USGS streamgages, making a water-balance study possible. CRNS can also measure snow water equivalent (SWE). Four high-elevation CRNS stations in this network also include CRNS dedicated to continuous SWE measurements. 

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Each CRNS station measures epithermal and thermal neutron counts above land surface, along with air temperature, relative humidity, barometric pressure, and precipitation. Soil moisture and temperature are also measured with in-ground sensors at multiple depths to validate CRNS data. ITB stations also have multiple sub stations within the CRNS measurement footprint.