Instruments and Methods A parallel-probe saturation profiler: a new technique for fast profiling of meltwater saturation in a seasonal snowpack J.C. KAPIL, Chandrika PRASHER, Moiz CHASMAI, P.K. SATYAWALI Snow and Avalanche Study Establishment, Manali, Himachal Pradesh 175103, India E-mail: jc_kapil@rediffmail.com ABSTRACT. We describe a parallel-probe saturation profiler (PPSP) for accurate and fast profiling of liquid-water saturations in a snowpack. This device utilizes the absorption of electrical energy, by water molecules under the action of an external electric field, due to instantaneous rotations from initially random orientations to the orientation of the applied electric field. Our observations show that the height of first peak signal (HFPS), i.e. the difference between the maxima and minima in the PPSP signal- response time series, is proportional to the liquid-water content and the water saturation of snow. The HFPS corresponding to different liquid-water contents were obtained from various naturally occurring snow types and were observed to be proportional to the water saturation of the snow, irrespective of snow types. For simultaneous measurements at corresponding depths in a snowpack, a position encoder supports the PPSP. This device was calibrated for various types of snowsamples and was then tested on the snow covers under different climatic zones of the Himalaya. The operation of the PPSP is easy and fast. The distribution of liquid water within a large snow cover can be estimated speedily using the PPSP, with a vertical resolution of 7 mm. INTRODUCTION Melting, refreezing and the passage of liquid water into a snowpack can drastically change its physical and mechan- ical characteristics. The heterogeneities within a snowpack (ice-lensing, fingering, ponding and ice-crust formations) are often caused by the presence of liquid water and its percolation through the sub-freezing snow cover. Since liquid-water saturation combines liquid-water content and porosity, it is often used as an independent variable while modelling the meltwater percolation in snow (Colbeck, 1972, 1973, 1974, 1975; Denoth and others, 1979; Ambach and others, 1981; Bengtsson 1982; Colbeck and Anderson, 1982; McGurk and Kattelmann 1986; Bøggild, 2006). We present a method to directly measure the water saturation in snow. This method is fast in operation and we can speedily record the saturation profiles from snow over a large area. Hot calorimetric (Akitaya, 1978, 1985) and freezing calorimetric (Jones and others, 1983) methods have long been used to measure the liquid-water contents in the snow samples. Kawashima and others (1998) made a portable calorimeter to measure this quantity, and Denoth (1994) designed a moisture meter, based on change in the dielectric constant of snow due to variations in the the liquid-water content. Time-domain reflectometry (TDR), based on the interaction of a microwave pulse with liquid water, is yet another method used to measure liquid-water content (Stein and Kane, 1983; Camp and Labrecque, 1992; Schneebeli and others, 1995; Spaans and Baker, 1995; Lundberg, 1996; Stein and others, 1997; Schneebeli and others, 1998). A flat- cable TDR system was used by Sta ¨hli and others (2004) to estimate the liquid-water content and water equivalent of snow from a spatially distributed snow cover at a fixed installation. Finnish scientists made a ‘snow fork’ which can also be used to record the density and the water-content profiles of a snowpack in a region approximately 20 mm in diameter around the probe (Sihvola and Tiuri, 1986). The liquid-water content measured by these devices is at some fixed points rather than over a continuous depth (Pfeffer and Humphrey, 1998). For the purpose of recording the vertical distribution of water saturation over a large area, existing methods of measuring the water contents are unfeasibly time- consuming. Additionally, a large number of sensors are required to obtain a saturation profile by digging a pit (which can also disturb the snow cover). We have developed a parallel-probe saturation profiler (PPSP) for accurate and fast recording of vertical saturation profiles within a snowpack. The basic physical principle behind the detection of liquid- water content utilizes the instantaneous absorption of electrostatic energy by water molecules (as electric dipoles), which is a consequence of their rotation from their initial random directions to the direction of an applied electric field. For simultaneous measurements of depth positions, an incremental encoder has been fabricated which is placed within the main housing of the instrument. A data logger can also be connected to the PPSP to store the data recorded under field and laboratory conditions. MECHANISM FOR ROTATION OF WATER DIPOLES UNDER EXTERNAL ELECTRIC FIELD A water molecule (H 2 O) is polar in nature, due to the asymmetry in its charge distribution. When water mol- ecules, or more specifically ‘water dipoles’, are intercepted by an external electric field, they tend to align in the direction of the applied field, as distinct from their initial random alignments. Figure 1a shows the random orienta- tions of water dipoles in the absence of an external electric field. Figure 1b shows the orientations of water dipoles in the direction of the external electric field. The positive- Journal of Glaciology, Vol. 55, No. 193, 2009 814