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Geoscientific Instrumentation, Methods and Data Systems An interactive open-access journal of the European Geosciences Union
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Volume 7, issue 1
Geosci. Instrum. Method. Data Syst., 7, 83-99, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.
Geosci. Instrum. Method. Data Syst., 7, 83-99, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 09 Mar 2018

Research article | 09 Mar 2018

Intercomparison of cosmic-ray neutron sensors and water balance monitoring in an urban environment

Martin Schrön1, Steffen Zacharias1, Gary Womack2, Markus Köhli1,3,4, Darin Desilets2, Sascha E. Oswald5, Jan Bumberger1, Hannes Mollenhauer1, Simon Kögler1, Paul Remmler1, Mandy Kasner1,6, Astrid Denk1,7, and Peter Dietrich1 Martin Schrön et al.
  • 1Dep. Monitoring and Exploration Technologies, Helmholtz Centre for Environmental Research GmbH – UFZ, Leipzig, Germany
  • 2Hydroinnova LLC, Albuquerque, USA
  • 3Physikalisches Institut, Heidelberg University, Heidelberg, Germany
  • 4Physikalisches Institut, University of Bonn, Bonn, Germany
  • 5Institute of Earth and Environmental Science, University of Potsdam, Potsdam, Germany
  • 6Institute of Geosciences and Geography, University of Halle-Wittenberg, Halle, Germany
  • 7Dep. of Geosciences, University of Tübingen, Tübingen, Germany

Abstract. Sensor-to-sensor variability is a source of error common to all geoscientific instruments that needs to be assessed before comparative and applied research can be performed with multiple sensors. Consistency among sensor systems is especially critical when subtle features of the surrounding terrain are to be identified. Cosmic-ray neutron sensors (CRNSs) are a recent technology used to monitor hectometre-scale environmental water storages, for which a rigorous comparison study of numerous co-located sensors has not yet been performed. In this work, nine stationary CRNS probes of type CRS1000 were installed in relative proximity on a grass patch surrounded by trees, buildings, and sealed areas. While the dynamics of the neutron count rates were found to be similar, offsets of a few percent from the absolute average neutron count rates were found. Technical adjustments of the individual detection parameters brought all instruments into good agreement. Furthermore, we found a critical integration time of 6h above which all sensors showed consistent dynamics in the data and their RMSE fell below 1% of gravimetric water content. The residual differences between the nine signals indicated local effects of the complex urban terrain on the scale of several metres. Mobile CRNS measurements and spatial simulations with the URANOS neutron transport code in the surrounding area (25ha) have revealed substantial sub-footprint heterogeneity to which CRNS detectors are sensitive despite their large averaging volume. The sealed and constantly dry structures in the footprint furthermore damped the dynamics of the CRNS-derived soil moisture. We developed strategies to correct for the sealed-area effect based on theoretical insights about the spatial sensitivity of the sensor. This procedure not only led to reliable soil moisture estimation during dry-out periods, it further revealed a strong signal of intercepted water that emerged over the sealed surfaces during rain events. The presented arrangement offered a unique opportunity to demonstrate the CRNS performance in complex terrain, and the results indicated great potential for further applications in urban climate research.

Publications Copernicus
Short summary
Cosmic-ray neutron sensing (CRNS) is a unique technology to monitor water storages in complex environments, non-invasively, continuously, autonomuously, and representatively in large areas. However, neutron detector signals are not comparable per se: there is statistical noise, technical differences, and locational effects. We found out what it takes to make CRNS consistent in time and space to ensure reliable data quality. We further propose a method to correct for sealed areas in the footrint.
Cosmic-ray neutron sensing (CRNS) is a unique technology to monitor water storages in complex...