Land-Surface Hydrology at an Intermediate Spatial Scale Using Cosmic-Ray Neutrons

Observations of soil moisture, snow pack and canopy-intercepted water are crucial to quantifying land-surface energy and water balances, but field studies of land-surface processes have been hindered by an absence of appropriately scaled measurement techniques. There is a large gap between invasive point measurements at the 10^-2 to 10⁰ m scale and microwave retrievals at the 10³ to 10⁵ m, while up- and downscaling these measurements to the field scale remains problematic. We present a new technique for monitoring land-surface water that utilizes natural and spatially distributed neutrons generated by cosmic rays to take advantage of the unique neutron scattering and absorption properties of hydrogen. This technique is non-invasive, passive, and operates at the 10² m scale, thus filling a crucial and longstanding gap in measurement scales. The equipment has moderate power demands, generates small data streams, and is rugged enough for field surveys and long-term monitoring at remote sites. We will discuss the theoretical basis of the technique, its operational characteristics, and recent field applications to soil moisture and snow water equivalent monitoring.