Snow hydrological measurements with ground-penetrating radar

Background

The water remaining when snow melts completely is called the snow water equivalent (SWE). It is an important factor in the context of forecasting floods and in view of hydropower utilisation. For hydrological investigation purposes, SWE data are often more revealing than snow depth measurements. However, measuring the SWE consumes considerable resources. Taking a single manual measurement generally takes just a few minutes, but in some circumstances can require several hours. Given that complex patterns underlie the distribution of snow in Alpine terrain, a large quantity of data has to be captured in order to determine the quantities of snow, which are subject to both spatial and temporal variation, that exist in a catchment zone. Efficient SWE measuring methods are therefore of great interest in snow hydrological research.

Project outline

The snow properties measured by ground-penetrating radar (GPR) allow the SWE to be estimated. The advantage of GPR lies in its ability to perform hundreds of measurements in the same time consumed by taking a single measurement using conventional methods. Evaluating GPR measurements is often no simple matter, however, especially if the snow is moist all the way through. Within the framework of this project, the use of GPR technology was optimised to enhance efficiency in the capturing of SWE measurements. For this purpose, a variety of system configurations were tested, and improved evaluation procedures were developed.

GPR system and measuring methods

A MALÅ GPR ProEx system with separable antennas, operating at frequencies of 400 and 1300 MHz, was used for this project. Such systems allow the testing of different antenna geometries and system configurations. A carrier system based on an expedition sledge was developed to provide portability and permit measurements to be taken in open terrain. In order to lower weight and costs, a set-up was designed to operate with just four antennas. To this end, two transmitter antennas and two receiver antennas were arranged in such a way that four differently spaced antenna pairs were available. This data collection method is practicable if the liquid water content is either low or known. If the snow moisture is variable or unknown, a fifth antenna pair is required. Its data are analysed to evaluate the frequency change in the radar signal. Together with the data captured by the other four antennas, this frequency change additionally allows the liquid water content of the snow to be determined. The snow characteristics measured by GPR were compared with manual measurements in numerous campaigns. The measuring accuracy obtained with the GPR system has now attained a level similar to that achieved by manual measuring methods.

System applications

The GPR system has already been used in several research projects and supplies valuable data which could not be captured manually without investing unreasonable resources.