Title: The Cosmic-Ray Neutron Rover - Mobile 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 non-invasively 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 the presence of roads
introduces a bias in the CRNS estimation of field soil moisture compared to
non-road scenarios. However, this effect becomes insignificant at distances
beyond a few meters from the road. Measurements from 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 successfully removed
with an analytical correction function that accounts for these parameters.
Additionally, an empirical approach is proposed that can be used on-the-fly
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, any 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.