Theory and application of measuring mesoscale soil moisture by cosmic-ray fast neutron probe

Abstract

Abstract. In recent years, measurement of surface soil moisture by the cosmic-ray fast neutron probe has gradually attracted more attention The intensity of the fast neutrons above the ground is sensitive to water content changes, largely insensitive to soil chemistry and inversely correlated with hydrogen content of the soil. Measurement of this intensity with a portable neutron detector placed above the ground permits the possibility of monitoring soil moisture. By this passive, non-invasive and intermediate scale measurement, soil moisture at a horizontal scale of around 660 m and depths of 12 to 76 cm can be inferred. And the large footprint makes this method suitable for weather and short-term climate forecast initialization and for validation of soil moisture inversed from satellite sensors. In this paper, the results of the cosmic-ray instruments well reflected the variation trend of soil moisture at the field scale. We found that irrigation and infiltration were two most important factors that affected the variation of soil moisture during a growing season of maize in Zhangye and the soil moisture of different period in the growing season of maize was very different. At last, Wireless Sensor Network data were used to validate this result and we found their correlation was very well with a RMSE of 0.0275 m 3 m -3 in the non-irrigation time.

Cosmic ray neutrons provide an innovative technique for estimating intermediate scale soil moisture

“…Some of these high energy particle flows in space reach the earth's atmosphere, where they are affected by the earth's magnetic field. 15 The high energy particles are captured into the earth's atmosphere and collide with atmospheric nuclei to initiate a cascade of secondary cosmic rays. 14 Fast neutrons are created, as these secondary cosmic rays pass through the atmosphere and then through the top few metres of the biosphere, hydrosphere and lithosphere.…”

“…10 Some of the fast neutrons are adsorbed by the soil during the collision, whilst others will be scattered above the surface of the soil. 15 The cosmic ray neutrons lose energy with each collision, therefore high energy neutrons become fast neutrons (in the atmosphere), which further lose energy and become thermal neutrons (in the soil). As a result of fast neutrons being strongly moderated by hydrogen present in the environment, their measured intensities relate to changes in soil moisture, as well as other hydrogen sources at the earth's surface.…”

“…As a result of fast neutrons being strongly moderated by hydrogen present in the environment, their measured intensities relate to changes in soil moisture, as well as other hydrogen sources at the earth's surface. 8,9,16 Moderation of neutrons According to Ochsner et al 14 and Jiao et al 15 , the moderation process of cosmic ray neutrons depends on three factors:…”

“…Hydrogen has by far the highest stopping power of any element (~25 times greater), as the hydrogen atom has a high probability of scattering a neutron as a result of its fairly large elastic scattering cross-section. 15 Hydrogen is the most efficient element with regard to the decrement of energy per collision and has a low atomic mass and makes up a substantial portion of all the atoms in many soils, as a consequence of the presence of water in the soil. 15 The presence of water within the soil pores plays an important and central role in moderating the concentration of cosmic ray neutrons above the soil surface.…”

“…15 Hydrogen is the most efficient element with regard to the decrement of energy per collision and has a low atomic mass and makes up a substantial portion of all the atoms in many soils, as a consequence of the presence of water in the soil. 15 The presence of water within the soil pores plays an important and central role in moderating the concentration of cosmic ray neutrons above the soil surface. 17 Cosmic ray probe measurements…”