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Cosmic rays continually bombard Earth, generating fast neutrons in the ground and atmosphere as a by-product of nuclear disintegrations. The presence of this easily measurable “background” radiation suggests the possibility of a novel and harmless method for measuring water content in soil and the water equivalent depth of snow, two critical variables controlling surface processes.

A method utilizing cosmic rays has several advantages over soil and snow monitoring tools now available to hydrologists (e.g., microwave remote sensing, invasive electromagnetic methods, snow tubes, and snow pillows). It is passive, requires only moderately expensive equipment, of the order of U.S. $10,000, and can be insensitive to soil salinity depending on the neutron energies selected. Most important, a cosmic ray water sensor is noninvasive and can measure water content or snow water equivalent over a scale desirable for input to hydrologic models (100 meter radius) but unattainable by other instruments. While the ability to measure soil water and snowpack in the field with cosmic rays has been validated, this method has applications to other water-rich systems, including biological ones.