HR: 0800h
AN: U21B-0810    [Abstracts]
TI: Cosmic-Ray Neutron Probe: Non-Invasive Measurement of Soil Water Content
AU: * Zreda, M
EM: marek@hwr.arizona.edu
AF: Hydrology and Water Resources Department, University of Arizona, Tucson, AZ 85721 United States
AU: Desilets, D
EM: ddesilet@hwr.arizona.edu
AF: Hydrology and Water Resources Department, University of Arizona, Tucson, AZ 85721 United States
AU: Ferre, T P
EM: ty@hwr.arizona.edu
AF: Hydrology and Water Resources Department, University of Arizona, Tucson, AZ 85721 United States
AB: Water content measurement methods have been designed primarily for either highly accurate, small scale applications (time domain reflectometry, neutron probe) or for rapid estimation over very large areas (satellite imagery), but no established technique exists for the scale intermediate to point measurement and satellite images. We are developing a novel technique for soil water determination that operates on the horizontal scale of dekameters. This scale of operation makes the method an appealing tool for both direct measurement at relevant scales and for calibration of satellite remote sensors. The method is based on the same principle that underlies conventional neutron probes: thermalization of neutrons by hydrogen atoms. The standard neutron probe works by emitting fast neutrons from a source in the instrument, and measuring the flux of neutrons that are scattered back. Our new probe uses a similar detector, but has cosmic-ray neutrons as a source. Results from a series of laboratory and field experiments show that our cosmic-ray probe is capable of measuring changes of water content resulting from irrigation or infiltration after a storm event. Our sensitivity experiments indicate that that 1% change in volumetric water content corresponds to approximately 1% change in neutron flux. Neutron intensities are sensitive to water in the upper 10-50 cm of soil and over a footprint of 10-100 m in diameter. Both the depth and the footprint decrease with increasing soil water content. Coupling neutron detectors with various shielding materials (we used cadmium and polyethylene) changes the energy sensitivity of the instrument, and thereby also the sensitivity to water content. The maximum sensitivity of the instrument to water (or hydrogen) content is achieved by placing the instrument about 20 cm below the surface or about 100-200 m above the surface (in a balloon or an aircraft). Our new method has several useful features: it is non-invasive and non-contact, allowing for broad coverage of undisturbed soil conditions; it does not contain a radioactive source, so it can be transported easily and left in the field unattended; it can be automated easily; it can be deployed below or above the ground surface; and it measures soil water at an intermediate scale of tens of meters. The cosmic-ray neutron probe can be used jointly with other techniques, thus bridging the gap between the small-scale point measurements and satellite imaging. In this fashion, it can be used to calibrate remote sensing instruments. This research was supported by grants from NSF (EAR-0126241) and ARO (43857-EV).
DE: 1835 Hydrogeophysics
DE: 1855 Remote sensing (1640)
DE: 1866 Soil moisture
DE: 1875 Vadose zone
DE: 1895 Instruments and techniques: monitoring
SC: Union [U]
MN: Fall Meeting 2005