Status and Perspectives on the Cosmic‐Ray Neutron Method for Soil Moisture Estimation and Other Environmental Science Applications

Abstract

Core Ideas Numerous studies have been conducted to develop and examine the accuracy of the method. Cosmic‐ray neutron soil moisture estimates compare well with independent measurements. These estimates are useful for modeling, data assimilation, and calibration of satellite products. Many studies have used the neutron detector for other applications; results have been promising. Since the introduction of the cosmic‐ray neutron method for soil moisture estimation, numerous studies have been conducted to test and advance the accuracy of the method. Almost 200 stationary neutron detector systems have been installed worldwide, and roving systems have also started to gain ground. The intensity of low‐energy neutrons produced by cosmic rays, measured above the ground surface, is sensitive to soil moisture in the upper decimeters of the ground within a radius of hectometers. The method has been proven suitable for estimating soil moisture for a wide range of land covers and soil types and has been used for hydrological modeling, data assimilation, and calibration and validation of satellite products. The method is challenged by the effect on neutron intensity of other hydrogen pools such as vegetation, canopy interception, and snow. Identifying the signal of the different pools can be used to improve the cosmic‐ray neutron soil moisture method as well as extend the application to, e.g., biomass and canopy interception surveying. More fundamental research is required for advancement of the method to include more energy ranges and consider multiple height levels.

Cosmic‐ray Neutron Rover Surveys of Field Soil Moisture and the Influence of Roads

“…The CRNS technology makes use of the extraordinarily high sensitivity of cosmicray neutrons to hydrogen nuclei and measures the concentration of epithermal neutrons above the soil surface. Since its introduction, the CRNS technology has quickly established itself in the field of hydrological observations (Andreasen et al, 2017) and is now used for soil moisture monitoring by many research groups worldwide (e.g., Bogena et al, 2013;Franz et al, 2013a;Peterson et al, 2016;Schrön et al, 2017;Zhu et al, 2016).…”

On the Information Content of Cosmic‐Ray Neutron Data in the Inverse Estimation of Soil Hydraulic Properties

“…Similar networks have been established in Australia (Hawdon et al, 2014), the United Kingdom (Evans et al, 2016), and Germany (Baatz et al, 2014). To date, almost 200 stationary CRNPs have been installed worldwide (Andreasen et al, 2017). The required conversion of neutron intensity into soil moisture and the correction for important influencing factors (e.g., biomass, snow, and litter layer water content) have been described in detail in the literature (e.g., Andreasen et al, 2017; Desilets et al, 2010).…”

“…To date, almost 200 stationary CRNPs have been installed worldwide (Andreasen et al, 2017). The required conversion of neutron intensity into soil moisture and the correction for important influencing factors (e.g., biomass, snow, and litter layer water content) have been described in detail in the literature (e.g., Andreasen et al, 2017; Desilets et al, 2010). Few recent studies have focused on the use of CRNP data for the estimation of SHPs with promising results.…”

Can Drip Irrigation be Scheduled with Cosmic‐Ray Neutron Sensing?

“…In dry soils, the measurement depth can go down to 80 cm and the diameter to >500 m at sea level. The aboveground neutron density is affected by changes in the cosmic‐ray intensity and by additional hydrogen sources, such as biomass and lattice water (Andreasen et al, 2017; Schreiner‐McGraw et al, 2016; Baatz et al, 2015). Standard procedures are available to correct these effects.…”