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

Abstract

Measurements of root‐zone soil moisture across spatial scales of tens to thousands of meters have been a challenge for many decades. The mobile application of Cosmic Ray Neutron Sensing (CRNS) is a promising approach to measure field soil moisture noninvasively by surveying large regions with a ground‐based vehicle. Recently, concerns have been raised about a potentially biasing influence of local structures and roads. We employed neutron transport simulations and dedicated experiments to quantify the influence of different road types on the CRNS measurement. We found that roads introduce a substantial bias in the CRNS estimation of field soil moisture compared to off‐road scenarios. However, this effect becomes insignificant at distances beyond a few meters from the road. Neutron measurements on the road could overestimate the field value by up to 40 % depending on road material, width, and the surrounding field water content. The bias could be largely removed with an analytical correction function that accounts for these parameters. Additionally, an empirical approach is proposed that can be used without prior knowledge of field soil moisture. Tests at different study sites demonstrated good agreement between road‐effect corrected measurements and field soil moisture observations. However, if knowledge about the road characteristics is missing, measurements on the road could substantially reduce the accuracy of this method. Our results constitute a practical advancement of the mobile CRNS methodology, which is important for providing unbiased estimates of field‐scale soil moisture to support applications in hydrology, remote sensing, and agriculture.

Multiscale soil moisture estimates using static and roving cosmic-ray soil moisture sensors

“…The dry roads will be over-represented in the measured neutron intensity as the sensitivity of neutron intensity to hydrogen is greater at the dry end of the scale (Andreasen et al, 2017a). A more recent study by Schrön et al (2017) using neutron transport simulations and dedicated field experiments supports the findings of Köhli et al (2015). Schrön et al (2017) found that the effects of roads are greatest when surrounding soil moisture is much higher than road moisture content.…”

“…A more recent study by Schrön et al (2017) using neutron transport simulations and dedicated field experiments supports the findings of Köhli et al (2015). Schrön et al (2017) found that the effects of roads are greatest when surrounding soil moisture is much higher than road moisture content. In the survey areas in which our broad-scale rover surveys were undertaken, more than 70 % of the roads were unsealed and many of the sealed roads were only one lane wide; while this does not remove the issue it does lessen the potential impact on reported results considerably.…”

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

“…In 2015, the neutron transport model URANOS (Ultra Rapid Neutron‐Only Simulation) (Köhli et al, 2015, 2018) was developed to reduce the number of model assumptions and make easy‐to‐use neutron simulations available for the hydrological and soil science communities. Since then, the revised model results have been confirmed in many experimental studies (Heidbüchel et al, 2016; Schattan et al, 2018; Schrön et al, 2017, 2018a, 2018b; Fersch et al, 2018). The footprint is a function of air pressure, soil moisture, and air humidity and is also slightly affected by the vegetation cover (Köhli et al, 2015).…”

“…The sensitivity of CRNS should also be improved to get a much higher signal/noise ratio to detect small‐scale drip irrigation. Non‐standard CRNS probes, such as the Cosmic‐ray Rover (Desilets et al, 2010; Schrön et al, 2018a) are comprised of a larger proportional counter tube and hence feature shorter integration times, allowing for higher count rates in the same time interval (Köhli et al, 2018). Multiple CRNS probes at the same field would also increase the total detected neutron intensity and decrease the statistical noise (Schrön et al, 2018b; Jakobi et al, 2018).…”

Synergies for Soil Moisture Retrieval Across Scales From Airborne Polarimetric SAR, Cosmic Ray Neutron Roving, and an In Situ Sensor Network

“…The neutron detectors used for environmental research are described in detail by Schrön, Zacharias, et al (): they are typically shielded by polyethylene to detect the epithermal neutron component, which is an energy band most sensitive to nearby hydrogen abundance (Köhli et al, ). In this work, we use a mobile neutron detector from Hydroinnova LLC (United States) based on helium‐3 gas (see Schrön, Rosolem, et al, , for details). The counting rate is higher than that of a standard stationary CRNS probe by a factor of ∼11.…”