Non-invasive, Cosmic Ray Neutrons Approach for Area Wide Soil Moisture Measurement

Measurement of area wide soil moisture content is needed for a variety of applications such as large scale irrigation scheduling, yield forecasting and climate change studies. In past decades, measurement of area wide soil moisture has been a challenge since most devices are for small plots within the range of 0.05 to 1 m in diameter. As a result, a large number of measurements, which can be costly and time consuming, are required. The recent development of a cosmic ray neutrons approach represents a breakthrough in addressing this challenge (Zreda et al. 2008, Shuttleworth et al. 2010). Cosmic-ray neutrons monitor the background radiation in the air above the soil, the intensity of which depends primarily on soil moisture that was found to correlate with soil hydrogen content. The cosmic ray soil moisture probe integrates soil moisture content over an area of approximately 700 m in diameter to a depth of 70 cm, covering the rooting zones of most crops. As a result it can enhance point measurement devices to yield a reliable measure of area average soil moisture. The probe is insensitive to temperature, salinity, soil mineral chemistry and is non-invasive (Desilets et al. 2010), thus allowing measurements to be carried out under undisturbed soil conditions. The cosmic ray neutron probe responds to all forms of moisture, including liquid and frozen soil water, snow, and water in or on vegetation, allowing for the assessment of the total surface moisture. The probe will enable us to provide soil moisture readings at a large number of sites with different physical characteristics, from simple and easy (flat grasslands) to complex and difficult terrain.