TY - THES A1 - Rasche, Daniel T1 - Cosmic-ray neutron sensing for the estimation of soil moisture T1 - Cosmic-Ray Neutron Sensing zur Messung der Bodenfeuchte BT - from the atmosphere to the near-surface and to larger depths N2 - Water stored in the unsaturated soil as soil moisture is a key component of the hydrological cycle influencing numerous hydrological processes including hydrometeorological extremes. Soil moisture influences flood generation processes and during droughts when precipitation is absent, it provides plant with transpirable water, thereby sustaining plant growth and survival in agriculture and natural ecosystems. Soil moisture stored in deeper soil layers e.g. below 100 cm is of particular importance for providing plant transpirable water during dry periods. Not being directly connected to the atmosphere and located outside soil layers with the highest root densities, water in these layers is less susceptible to be rapidly evaporated and transpired. Instead, it provides longer-term soil water storage increasing the drought tolerance of plants and ecosystems. Given the importance of soil moisture in the context of hydro-meteorological extremes in a warming climate, its monitoring is part of official national adaption strategies to a changing climate. Yet, soil moisture is highly variable in time and space which challenges its monitoring on spatio-temporal scales relevant for flood and drought risk modelling and forecasting. Introduced over a decade ago, Cosmic-Ray Neutron Sensing (CRNS) is a noninvasive geophysical method that allows for the estimation of soil moisture at relevant spatio-temporal scales of several hectares at a high, subdaily temporal resolution. CRNS relies on the detection of secondary neutrons above the soil surface which are produced from high-energy cosmic-ray particles in the atmosphere and the ground. Neutrons in a specific epithermal energy range are sensitive to the amount of hydrogen present in the surroundings of the CRNS neutron detector. Due to same mass as the hydrogen nucleus, neutrons lose kinetic energy upon collision and are subsequently absorbed when reaching low, thermal energies. A higher amount of hydrogen therefore leads to fewer neutrons being detected per unit time. Assuming that the largest amount of hydrogen is stored in most terrestrial ecosystems as soil moisture, changes of soil moisture can be estimated through an inverse relationship with observed neutron intensities. Although important scientific advancements have been made to improve the methodological framework of CRNS, several open challenges remain, of which some are addressed in the scope of this thesis. These include the influence of atmospheric variables such as air pressure and absolute air humidity, as well as, the impact of variations in incoming primary cosmic-ray intensity on observed epithermal and thermal neutron signals and their correction. Recently introduced advanced neutron-to-soil moisture transfer functions are expected to improve CRNS-derived soil moisture estimates, but potential improvements need to be investigated at study sites with differing environmental conditions. Sites with strongly heterogeneous, patchy soil moisture distributions challenge existing transfer functions and further research is required to assess the impact of, and correction of derived soil moisture estimates under heterogeneous site conditions. Despite its capability of measuring representative averages of soil moisture at the field scale, CRNS lacks an integration depth below the first few decimetres of the soil. Given the importance of soil moisture also in deeper soil layers, increasing the observational window of CRNS through modelling approaches or in situ measurements is of high importance for hydrological monitoring applications. By addressing these challenges, this thesis aids to closing knowledge gaps and finding answers to some of the open questions in CRNS research. Influences of different environmental variables are quantified, correction approaches are being tested and developed. Neutron-to-soil moisture transfer functions are evaluated and approaches to reduce effects of heterogeneous soil moisture distributions are presented. Lastly, soil moisture estimates from larger soil depths are derived from CRNS through modified, simple modelling approaches and in situ estimates by using CRNS as a downhole technique. Thereby, this thesis does not only illustrate the potential of new, yet undiscovered applications of CRNS in future but also opens a new field of CRNS research. Consequently, this thesis advances the methodological framework of CRNS for above-ground and downhole applications. Although the necessity of further research in order to fully exploit the potential of CRNS needs to be emphasised, this thesis contributes to current hydrological research and not least to advancing hydrological monitoring approaches being of utmost importance in context of intensifying hydro-meteorological extremes in a changing climate. N2 - Wasser, das als Bodenfeuchte in der ungesättigten Bodenzone gespeichert ist, beeinflusst zahlreiche hydrologische Prozesse. Sie ist von großer Bedeutung für hydrometeorologische Extremereignisse, da sie sowohl die Prozesse zur Entstehung von Hochwassereignissen beeinflusst als auch pflanzenverfügbares Wasser in Dürreperioden bereitstellt, in denen Regen ausbleibt. Vor allem Bodenfeuchte in tieferen Schichten des Bodens wird zum Beispiel durch die geringere Dichte an Pflanzenwurzeln langsamer aufgenommen und reduziert. Die Bodenfeuchte in diesen tieferen Schichten kann daher vor allem in Trockenperioden zum Überleben der Pflanzen in landwirtschaftlichen Gebieten und natürlichen Ökosystemen beitragen. Im Kontext hydro-meteorologischer Extremereignisse kommt der Bodenfeuchte so eine besondere Bedeutung zu und ist daher Teil nationaler Monitoring- und Anpassungsstrategien an sich verändernde Klimabedingungen. Cosmic-Ray Neutron Sensing (CRNS) ist ein geophysikalisches Messverfahren, das natürlich vorkommende Neutronen aus kosmischer Strahlung zur Bodenfeuchtebestimmung nutzt. Die Intensität der über dem Boden gemessenen Neutronen ist dabei abhängig von der Menge anWasserstoff in der Umgebung des Neutronendetektors. Da in den meisten Bereichen an Land die Bodenfeuchte den größten Teil des Wasserstoffs ausmacht, lassen Veränderungen in der gemessenen Neutronenintensität auf veränderte Bodenfeuchtebedingungen schließen. Ein Vorteil dieser nichtinvasiven Methode ist ihr großer Messbereich von mehreren Hektar. Die, selbst über kurze Distanzen und Zeiträume auftretenden, Unterschiede werden somit repräsentativ gemittelt und gemessene Bodenfeuchtewerte können so besser für Vorhersagemodelle von Hochwasser- und Dürreereignissen genutzt werden. Trotz des Potentials von CRNS für das Monitoring von Bodenfeuchte bleiben zahlreiche offene Forschungsfragen, von denen einige im Rahmen dieser Arbeit betrachtet werden. Hierzu zählt die Bestimmung und Korrektur von Einflussgrößen, die das Neutronensignal zusätzlich zur Bodenfeuchte beeinflussen. Ebenso gehört die Ableitung von Bodenfeuchte aus dem Neutronensignal selbst sowie der Umgang mit stark unterschiedlichen Bodenfeuchtebedingungen im Messbereich dazu. Obwohl CRNS einen großen horizontalen Messbereich besitzt, ist die Messtiefe auf die oberen ca. 30 cm des Bodens begrenzt. Hierzu werden Ansätze untersucht, die Bodenfeuchte mathematisch in größere Tiefen zu extrapolieren und sie direkt dort zu messen, indem Neutronendetektoren in Bohrlöchern installiert werden. Mit der Betrachtung der Forschungsfragen kann diese Arbeit einen wichtigen Beitrag zur Weiterentwicklung von CRNS und der Anwendbarkeit der Methode z.B. im Rahmen nationaler Monitoring-Programme leisten, denen im Kontext zunehmend intensiverer hydro-meteorologischer Extremereignisse eine besondere Bedeutung zukommt. KW - cosmic-ray neutron sensing KW - soil moisture KW - Cosmic-Ray Neutron Sensing KW - Bodenfeuchte KW - soil hydrology KW - geophysics KW - Bodenhydrologie KW - Geophysik Y1 - 2024 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-636465 ER - TY - JOUR A1 - Schrön, Martin A1 - Oswald, Sascha A1 - Zacharias, Steffen A1 - Kasner, Mandy A1 - Dietrich, Peter A1 - Attinger, Sabine T1 - Neutrons on rails BT - Transregional monitoring of soil moisture and snow water equivalent JF - Geophysical research letters : GRL / American Geophysical Union N2 - Large-scale measurements of the spatial distribution of water content in soils and snow are challenging for state-of-the-art hydrogeophysical methods. Cosmic-ray neutron sensing (CRNS) is a noninvasive technology that has the potential to bridge the scale gap between conventional in situ sensors and remote sensing products in both, horizontal and vertical domains. In this study, we explore the feasibility and potential of estimating water content in soils and snow with neutron detectors in moving trains. Theoretical considerations quantify the stochastic measurement uncertainty as a function of water content, altitude, resolution, and detector efficiency. Numerical experiments demonstrate that the sensitivity of measured water content is almost unperturbed by train materials. Finally, three distinct real-world experiments provide a proof of concept on short and long-range tracks. With our results a transregional observational soil moisture product becomes a realistic vision within the next years. KW - soil moisture KW - transregional KW - multiscale KW - snow water equivalent KW - cosmic-ray neutron sensing KW - railway Y1 - 2021 U6 - https://doi.org/10.1029/2021GL093924 SN - 0094-8276 SN - 1944-8007 VL - 48 IS - 24 PB - Wiley CY - Hoboken, NJ ER - TY - JOUR A1 - Weimar, Jannis A1 - Köhli, Markus A1 - Budach, Christian A1 - Schmidt, Ulrich T1 - Large-scale boron-lined neutron detection systems as a 3He alternative for Cosmic Ray Neutron Sensing JF - Frontiers in water N2 - Cosmic-Ray neutron sensors are widely used to determine soil moisture on the hectare scale. Precise measurements, especially in the case of mobile application, demand for neutron detectors with high counting rates and high signal-to-noise ratios. For a long time Cosmic Ray Neutron Sensing (CRNS) instruments have relied on He-3 as an efficient neutron converter. Its ongoing scarcity demands for technological solutions using alternative converters, which are Li-6 and B-10. Recent developments lead to a modular neutron detector consisting of several B-10-lined proportional counter tubes, which feature high counting rates via its large surface area. The modularity allows for individual shieldings of different segments within the detector featuring the capability of gaining spectral information about the detected neutrons. This opens the possibility for active signal correction, especially useful when applied to mobile measurements, where the influence of constantly changing near-field to the overall signal should be corrected. Furthermore, the signal-to-noise ratio could be increased by combining pulse height and pulse length spectra to discriminate between neutrons and other environmental radiation. This novel detector therefore combines high-selective counting electronics with large-scale instrumentation technology. KW - CRNS KW - neutron KW - detector KW - soil moisture KW - readout electronics KW - boron-10 KW - helium-3 alternative Y1 - 2020 U6 - https://doi.org/10.3389/frwa.2020.00016 SN - 2624-9375 VL - 2 PB - Frontiers Media CY - Lausanne ER - TY - JOUR A1 - Schrön, Martin A1 - Rosolem, Rafael A1 - Köhli, Markus A1 - Piussi, L. A1 - Schröter, I. A1 - Iwema, J. A1 - Kögler, S. A1 - Oswald, Sascha A1 - Wollschläger, U. A1 - Samaniego, Luis A1 - Dietrich, Peter A1 - Zacharias, Steffen T1 - Cosmic-ray Neutron Rover Surveys of Field Soil Moisture and the Influence of Roads JF - Water resources research N2 - 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. Plain Language Summary 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 roads. We employed physics simulations and dedicated experiments to quantify the influence of different road types on the CRNS measurement. We found that the presence of roads biased the CRNS estimation of field soil moisture compared to nonroad scenarios. Neutron measurements could overestimate the field value by up to 40 % depending on road material, width, surrounding field water content, and distance from the road. We proposed a correction function that successfully removed this bias and works even without prior knowledge of field soil moisture. Tests at different study sites demonstrated good agreement between corrected measurements and other field soil moisture observations. 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. KW - road effect KW - field-scale KW - soil moisture KW - cosmic ray neutrons KW - mobile survey KW - COSMOS rover Y1 - 2018 U6 - https://doi.org/10.1029/2017WR021719 SN - 0043-1397 SN - 1944-7973 VL - 54 IS - 9 SP - 6441 EP - 6459 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Meißl, Gertraud A1 - Formayer, Herbert A1 - Klebinder, Klaus A1 - Kerl, Florian A1 - Schöberl, Friedrich A1 - Geitner, Clemens A1 - Markart, Gerhard A1 - Leidinger, David A1 - Bronstert, Axel T1 - Climate change effects on hydrological system conditions influencing generation of storm runoff in small Alpine catchments JF - Hydrological processes : an international journal N2 - Floods and debris flows in small Alpine torrent catchments (<10km(2)) arise from a combination of critical antecedent system state conditions and mostly convective precipitation events with high precipitation intensities. Thus, climate change may influence the magnitude-frequency relationship of extreme events twofold: by a modification of the occurrence probabilities of critical hydrological system conditions and by a change of event precipitation characteristics. Three small Alpine catchments in different altitudes in Western Austria (Ruggbach, Brixenbach and Langentalbach catchment) were investigated by both field experiments and process-based simulation. Rainfall-runoff model (HQsim) runs driven by localized climate scenarios (CNRM-RM4.5/ARPEGE, MPI-REMO/ECHAM5 and ICTP-RegCM3/ECHAM5) were used in order to estimate future frequencies of stormflow triggering system state conditions. According to the differing altitudes of the study catchments, two effects of climate change on the hydrological systems can be observed. On one hand, the seasonal system state conditions of medium altitude catchments are most strongly affected by air temperature-controlled processes such as the development of the winter snow cover as well as evapotranspiration. On the other hand, the unglaciated high-altitude catchment is less sensitive to climate change-induced shifts regarding days with critical antecedent soil moisture and desiccated litter layer due to its elevation-related small proportion of sensitive areas. For the period 2071-2100, the number of days with critical antecedent soil moisture content will be significantly reduced to about 60% or even less in summer in all catchments. In contrast, the number of days with dried-out litter layers causing hydrophobic effects will increase by up to 8%-11% of the days in the two lower altitude catchments. The intensity analyses of heavy precipitation events indicate a clear increase in rain intensities of up to 10%. KW - climate change KW - hydrophobic effects KW - small Alpine catchments KW - soil moisture KW - storm runoff events KW - system conditions Y1 - 2016 U6 - https://doi.org/10.1002/hyp.11104 SN - 0885-6087 SN - 1099-1085 VL - 31 IS - 6 SP - 1314 EP - 1330 PB - Wiley CY - New York ER -