Cosmic-ray hydrometeorology

Soil water content can be inferred from subaerial measurements of cosmic-ray neutron intensity. The hydrogen in soil water dominates the moderating power in the land surface environment. Through its ability to moderate and absorb neutrons, hydrogen exerts a strong control on neutron fluxes in the fast to thermal energy range. Cosmic-ray measurements are passive, noninvasive, noncontact, and represent a sample area of tens of hectares and a depth of tens of centimeters. The method has moderate power demands and data processing and transmission requirements, which makes it particularly well suited for long-term monitoring and field campaigns. A promising direction for future research is coupling neutron observations to land surface models and possibly even inverting neutron data to obtain soil properties and evapotranspiration. Furthermore, advances in neutron detection technology, for example, in the area of directionally sensitive neutron detectors (Mascarenhas et al., 2006), have the potential to open new applications and spatial scales for hydrologic measurements.