CosmicSense - Snow Dynamics

In many environments, snow constitutes an important part of the hydrological cycle. A substantial portion of the planets population relies on freshwater resources seasonally stored as snow. Especially in the face of climate change, these resources are increasingly under pressure. The knowledge of the spatial and temporal distribution of water stored in snow (snow water equivalent, SWE) is therefore crucial for improved water resources management and flood forecasting in snow- fed river basins. A number of approaches with specific advantages and drawbacks exist to monitor SWE including in- situ measurements, remote sensing, and hydrological modelling. In particular, spatial representativeness is a major problem for SWE monitoring in mountain regions as snow depth and SWE can vary substantially over small distances. The key benefit of aboveground cosmic-ray neutron sensing (CRNS) for estimating SWE is that the signal contains information of a wider area and is thus insensitive to small-scale variations in snow accumulation. Previous work confirmed the general suitability of CRNS for monitoring snow water resources in mountain regions. Currently, research gaps regarding the spatial transferability of the above- mentioned results still exist. Therefore, measurements at other locations are planned to cover different site characteristics and to refine the estimation of SWE from CRNS data. Measuring in different elevation and climate zones allows to cover various SWE amounts and changing environmental conditions within one campaign. Alpine sites in Austria will be complemented by a cascade of lower-lying sites in Germany. Continuous stationary field measurements will be supported by campaign-based mobile measurements of the spatial SWE distribution in cooperation with the module Roving & Airborne (RA). The obtained field data will be further analyzed using physically- based simulations of the neutron response. This will enable an improved understanding of the use of CRNS for snow monitoring in different environments. Validation data will be generated using laser scanning based snow cover observations and terrestrial photography. In contrast to measurements at single points, laser scanning produces a 3D point cloud of the current surface elevation. Together with snow-free measurements during the summer period, a high-resolution representation of the snow depth can be calculated. Local measurements of the snow density allow to convert snow depth variations to SWE. Terrestrial photography provides high- resolution data on snow-covered areas. All field work and simulation experiments will be conducted in close cooperation with other modules of the research unit Cosmic Sense II. In particular, the modules Roving & Airborne (RA), Neutron Simulations (NS), Hydrological Modelling (HG), Vegetation (VG), Smart Coverage (SC), and Root Zone Water (RZ) will be involved.

Cosmic-Ray Neutron Sensing is a method for detecting changes in snow and soil moisture based on residuals of naturally occurring cosmogenic background radiation. The measurement technique enables a signal from a larger area of several hectares around the measurement site and is particularly useful in areas where the landscape is diverse and snow and soil moisture amounts vary on short distances. Furthermore, the detector is placed on a pole above the surface which minimises the impact of the installation for the measured volume around it. Thus, measurements are possible even in areas where disturbances of the soil are to be kept as low as possible, or where the amount of stones and rocks makes it difficult to dig into the soil and install a conventional sensor. In mountain and arctic or antarctic environments, both conditions hold true. Furthermore, the natural cosmogenic background radiation increases with height and with latitude. The project therefore focussed on the specific characteristics of using CRNS in mountain regions, focussing on measurements of the amount of snow on the ground. The investigations at different pre-Alpine and Alpine sites allowed for improving the understanding of the CRNS signal response to a variety of site characteristics, elevations and climatic conditions. This included a joint field campaign together with other modules of the research unit at Rotmoostal in Austria, measurements in Southern Germany, Austria and Switzerland. The campaigns included both measurements of the cosmic-ray neutron background flux and complementary data on soil moisture, snow coverage and the total water stored in the snowpack. Additional measurements were taken in the Arctic (Ny Ålesund in Svalbard/Norway) and on board of the Polarstern research vessel on its way from Northern Germany to Antarctica. The analysis of the data revealed that the relationship between the CRNS signal and the amount of water around the detector highly depends on the location. An analysis combining locally measurement cosmic-ray neutron background flux and remote sensing data showed how the sites can be classified based on the typical patterns in the signal and their relation to snow-free or snow-covered conditions. Furthermore, the influence of air pressure on the signal could be shown to be more variable than reported in previous studies. The results help using the CRNS technique for measuring snow in mountainous environments, reduce the uncertainty of the derived hydrological variables and prove that it could also be applied in Arctic environments.