For better prognoses in avalanche and flood warning as well as improvements in predicting the filling stages of Nordic and Alpine hydro-power reservoirs, it is necessary to improve the determination of snow pack properties such as liquid water content, density and snow water equivalent.
With in the EU-project SNOWPOWER (NNE5-2000-251) we have been developing and testing a new measurement set-up for automated large-scale determination of these properties. Unshielded three wire flat band cables are connected to both high and low frequency impedance analysers. The cables are installed prior to winter season or laid out at certain snow heights and are enclosed afterwards by snow fall. The electromagnetic characteristics of the cable depend mainly on the dielectric properties of the snow and thus allow the determination of its density and liquid water content simultaneously, non-destructively and long-term.
The sensor lines can have lengths of approx. 100 m, thus, in a cross laid pattern, the covered area can be as large as a radar pixel size and can be used for calibrating remote sensing images used for snow water equivalent determination.
Due to mechanical influences or partial melting it is common, that air gaps between sensor and snow are formed and the measurements are disturbed. A new measuring mode compensates for this effect by measuring the three wire sensor cable in two ways each with different penetration depths of the electromagnetic field. In this way an air gap has different influences on the measurement of the dielectric coefficients and can thus be calculated and a correction algorithm can be derived.
With the high frequency time domain reflectometry and a suitable reconstruction algorithm it is also possible to do snow moisture and density profiling along the sensor cables to get additional information about spatial resolution of natural snow inhomogeneities like ice lenses and percolation zones and observe their temporal behaviour.
The new flat-band cable sensors were tested at a high-elevation field site in
The natural settling of the snow cover showed up nicely in the horizontal cable measurements. The measured increase of the snow density was in accordance with manual measurements. No liquid water was detected until the beginning of the melting season, but then the determined liquid water content gave plausible results both compared to lysimeter data taken on the test field and with regard to the spatial variation of flow fingers that we normally experience in a natural snow pack. We also observed a faster response and a larger spatial variability at cables installed nearer to the surface compared to cables positioned deeper in the snow pack. The calculation of the snow water equivalent from these measured data gave encouraging results.
Stacheder, M.; Huebner, C.; SCHLAEGER, S. & Brandelik A. (2005): Combined TDR and low-frequency permittivity measurements for continuous snow wetness and snow density determination.- In: KUPFER, K. (Ed.): Electromagnetic Aquametry, 16: 367-382, Springer-Verlag Berlin Heidelberg.