Snow Water Equivalent Observations Using Refracted GPS Signals

Abstract

An extensive amount of water is stored in snow covers, which has a high impact on flood development during snow melting periods. Early assessment of the snow water equivalent (SWE) in mountain environments enhances early-warning and thus prevention of major impacts. SWE is one essential climate variable and the knowledge about the water storage in mountain catchment areas has a high significance for the global water and energy supply, especially for regions depending on the snow melt. The main objective of this thesis is a thorough investigation of the contribution of geodetic GNSS remote sensing techniques to observe and quantify mountainous SWE. This investigation is based on differential GPS processing using refracted GPS phase signals received by commercially available off-the-shelf GPS antennas buried underneath a typical Alpine snowpack. The main tasks are: a) Identification of the theoretical and empirical characteristics and limitations of GPS L1 single-frequency observations using submerged GPS antennas, as well as the development of a mathematical model for SWE estimation (Paper I); b) Application of the developed model to an Alpine seasonal snowpack and investigation of the potential for SWE quantification (Paper II); c) Identification of the main impact of GPS processing on the accuracy of the derived SWE estimates when using refracted GPS signals (Paper III). Further evaluation is based on the possibility of using low-cost GPS equipment for SWE quantification (Paper IV). Liquid water is expected theoretically to exert the largest influence on a GPS signal propagation through a snowpack. An experimental setup has been established to investigate the influence of liquid water on the GPS observations by testing water levels up to the signal penetration depth of 35mm above the antenna. Results correspond well with theory and the water level above the submerged antenna can be estimated using the derived model with sub-millimetre accuracy. The potential of using refracted GPS signals for SWE estimation has been evaluated based on an experimental setup at an Alpine snowpack. A measurement network has been installed, consisting of a GPS reference station above the snowpack and a GPS antenna mounted on the ground underneath the snowpack. The empirical SWE results are validated against the state-of-the-art reference sensors, i.e., the snow pillow, snow scale, and manual observations over three seasons (2015/16 – 2017/18). The comparison shows a high level of agreement with a relative bias below 5% (RMSE of 38mmw.e.) over all three seasons, including the melting periods. SWE could be accurately estimated with a high temporal resolution of one hour. The applied ambiguity resolution strategy and the selection of an elevation cut-off angle and a weighting function could be identified as the three most important GPS processing options influencing the quality of the resulting SWE estimates.