Measuring soil moisture with cosmic-ray neutrons

Through its influence on evaporation rates, humidity levels, and other factors, the moisture content of soil has a significant impact on weather. Accurate measurements of that content, though important for meteorological, hydrological, and ecological forecasting, are difficult to make. Extrapolating point measurements to larger areas is inaccurate, and satellite-based remote-sensing methods are hindered by ground cover, surface roughness, and other limitations. A team from the University of Arizona and the Southwest Watershed Research Center in Tucson has shown that just above the ground surface, so-called fast neutrons with energies on the order of an MeV are quantifiably correlated with soil moisture and thus provide a noninvasive means for measuring the average moisture levels over regions several hundred meters wide and tens of centimeters deep. The neutrons are generated by cosmic rays. Upon collision with atmospheric nuclei, cosmic rays create showers of high-energy secondary particles, and those that reach Earth’s surface can penetrate it, collide with nuclei there, and produce among their debris fast neutrons, some of which escape back into the atmosphere. Marek Zreda and colleagues discovered that hydrogen, mostly found in water, dominates soil’s ability to moderate fast neutrons and that a strong inverse correlation, independent of soil chemistry, exists between moisture content and the intensity of the fast neutrons that escape out of the ground. The team demonstrated that with an independent measurement of the moisture content for calibration, a neutron detector a few meters above the ground can give precise measurements of soil moisture on the time scale of minutes to a few hours. As the figure shows, the hourly soil moisture determined by a cosmic-ray neutron detector (top) agrees with that determined by time-domain reflectometry probes (middle) and with the monitored daily precipitation (bottom). (M. Zreda et al., Geophys. Res. Lett. 35 , L21402, 2008, http://dx.doi.org/10.1029/2008GL035655 .)