Hans-Peter Marshall

An Evaluation of the Hydrologic Relevance of Lateral Flow in Snow at Hillslope and Catchment Scales

David Eiriksson et al. — Thu, 28 Mar 2013 10:35:16 PDT

Lateral downslope ﬂow in snow during snowmelt and rain-on-snow (ROS) events is a well-known phenomenon, yet its relevance to water redistribution at hillslope and catchment scales is not well understood. We used dye tracers, geophysical methods, and hydrometric measurements to describe the snow properties that promote lateral ﬂow, assess the relative velocities of lateral ﬂow in snow and soil, and estimate volumes of downslope ﬂow. Results demonstrate that rain and melt water can travel tens of metres downslope along layers within the snowpack or at the snowpack base within tens of hours. Lateral ﬂow within the snowpack becomes less likely as the snowpack becomes saturated and stratigraphic boundaries are destroyed. Flow along the base can be prevalent in all snowpack conditions. The net result of lateral ﬂow in snow can be the deposition of water on the soil surface in advanced downslope positions relative to its point of origin, or direct discharge to a stream. Although both melt and ROS events can redistribute water to downslope positions, ROS events produced the most signiﬁcant volumes of downslope ﬂow. Direct stream contributions through the snowpack during one ROS event produced up to 12% of streamﬂow during the event. This can help explain rapid delivery of water to streams during ROS events, as well as anomalously high contributions of event water during snowmelt hydrographs. In catchments with a persistent snowpack, lateral redistribution of water within the snowpack should be considered a relevant moisture redistribution mechanism.

Monitoring Glacier Surface Seismicity in Time and Space Using Rayleigh Waves

T. D. Mikesell et al. — Thu, 07 Jun 2012 08:30:00 PDT

Sliding glaciers and brittle ice failure generate seismic body and surface wave energy characteristic to the source mechanism. Here we analyze continuous seismic recordings from an array of nine short-period passive seismometers located on Bench Glacier, Alaska (USA) (61.033°N, 145.687°W). We focus on the arrival-time and amplitude information of the dominant Rayleigh wave phase. Over a 46-hour period we detect thousands of events using a cross-correlation based event identification method. Travel-time inversion of a subset of events (7% of the total) defines an active crevasse, propagating more than 200 meters in three hours. From the Rayleigh wave amplitudes, we estimate the amount of volumetric opening along the crevasse as well as an average bulk attenuation ( Q ¯ = 42) for the ice in this part of the glacier. With the remaining icequake signals we establish a diurnal periodicity in seismicity, indicating that surface run-off and subglacial water pressure changes likely control the triggering of these surface events. Furthermore, we find that these events are too weak (i.e., too noisy) to locate individually. However, stacking individual events increases the signal-to-noise ratio of the waveforms, implying that these periodic sources are effectively stationary during the recording period.

Non-Destructive Quantification of Snowpack Properties

Achim Heilig et al. — Wed, 11 Jan 2012 16:02:15 PST

A temporal observation of the stratigraphy of seasonal snowpacks is only possible with non-invasive methods. Electromagnetic waves, specifically radar waves, proved to be the most appropriate technique to estimate internal snow parameters and media transitions non-destructively. Thereby, it is possible to estimate quantitatively snowpack stratigraphy and observe the snowpack evolution with time. Radar systems work as an active wave transmitter, which records reflection intensities with travel-time. Either the system modulates the signal on a defined frequency range as frequency modulated continuous wave systems (FMCW) or a short impulse is radiated at a center frequency and bandwidth. The stratigraphic resolution and the penetration depth of both systems depends on the system parameters. The frequency determines the penetration depth and sensitivity and the bandwidth determines the vertical resolution. In previous studies FMCW X- and Ku-band frequencies failed to penetrate a moist snowpack, but provided convincing results in resolving the snowpack stratigraphy. Pulsed 900 MHz antennas, as well as L- and C-band FMCW systems penetrated a wet snowpack up to one meter and measured adequate gradients in snow density. Current research in pulsed and modulated systems show that electromagnetic wave systems are convincing methods to quantitatively measure snow stratigraphy non-destructively.

Aspect Influences on Soil Water Retention and Storage

I. J. Geroy et al. — Thu, 05 Jan 2012 15:41:36 PST

Many catchment hydrologic and ecologic processes are impacted by the storage capacity of soil water, which is dictated by the profile thickness and water retention properties of soil. Soil water retention properties are primarily controlled by soil texture, which in turn varies spatially in response to microclimate-induced differences in insolation, wetness and temperature. All of these variables can be strongly differentiated by slope aspect. In this study, we compare quantitative measures of soil water retention capacity for two opposing slopes in a semi-arid catchment in southwest Idaho, USA. Undisturbed soil cores from north and south aspects were subjected to a progressive drainage experiment to estimate the soil water retention curve for each sample location. The relatively large sample size (35) supported statistical analysis of slope scale differences in soil water retention between opposing aspects. Soils on the north aspect retain as much as 25% more water at any given soil water pressure than samples from the south aspect slope. Soil porosity, soil organic matter and silt content were all greater on the north aspect, and each contributed to greater soil water retention. These results, along with the observation that soils on north aspect slopes tend to be deeper, indicate that north aspect slopes can store more water from the wet winter months into the dry summer in this region, an observation with potential implications on ecological function and landscape evolution.

Rutschblock-Scale Snowpack Stability Derived from Multiple Quality-Controlled SnowMicroPen Measurements

Christine Pielmeier et al. — Thu, 31 Mar 2011 15:39:07 PDT

Stability prediction from SnowMicroPen (SMP) profiles would support avalanche forecasting operations, since objective stability information could be gathered more quickly than with standard tests, thereby allowing sampling at higher resolution and over larger spatial scales. Previous studies have related the snow properties derived from the SMP to observed snow properties at Rutschblock (RB) and compression test failure planes. The goals of this study are to show to what extent snowpack stability for artificial triggering, based on RB, can be derived from SMP measurements and how multiple measurements at the RB scale improve the results. Measurements at 36 different sites were used for the development of a classification scheme. Each site included a RB test, a manual profile, and 6 to 10 adjacent SMP measurements, for a total of 262 SMP profiles. A recently improved SMP theory was applied to estimate the micro-structural and mechanical properties of manually defined weak layers and slab layers. SMP signal quality control and different noise treatment methods were taken into consideration in the analysis. The best and most robust predictor of RB stability was the weak layer micro-scale strength. In combination with the SMP-estimated mean density of the slab layer, the total accuracy of predicting RB stability classes was 85% over the entire dataset, and 88% when signals with obvious signal dampening (11% of the dataset) were removed. The total accuracy increased when multiple SMP measurements at the RB scale were used to calculate the mean weak layer strength, when compared to using just one SMP measurement at a site. The analysis was robust to trends and offsets in the absolute SMP force, which was a frequent signal error. However, it was sensitive to dampened or disturbed SMP force micro variance. The sensitivity analysis also showed that the best predictor of instability, the weak layer micro-scale strength, was robust to the choice of SMP signal noise removal method.

Quantifying Changes in Weak Layer Microstructure Associated with Artificial Load Changes

Eric Lutz et al. — Thu, 31 Mar 2011 15:39:05 PDT

Researchers and practitioners have long utilized a variety of penetrometers to investigate the snowpack. Identifying definitive relationships between penetrometer-derived microstructural information and stability has been challenging. The purpose of this study is two-fold: 1. We propose a simple field test to establish relationships between load and penetrometer-derived microstructural estimates, 2. We utilize the SnowMicroPen (SMP) to quantify changes in weak layer residual strength and microstructural dimension associated with an artificial loading event. Our dataset is from Moonlight Basin, Montana and includes three modified loaded-column tests, each paired with 5 SMP profiles. Depth hoar comprised the targeted weak layer. Results indicate that loading caused the residual strength and rupture frequency to decrease significantly. Much like a compression test at a micro-scale, the force required for the SMP to rupture individual structures as well as the micro-scale strength decreased significantly when the slab stress was increased by artificially adding blocks of snow. A decrease in observed rupture frequency within the weak layer (or an increase in the distance between ruptured structures) also occurred after the loading event, probably because some structures within the weak layer had already failed or were so close to failing that the penetrometer could not detect their rupture. Due in part to the large difference in loads, microstructural differences between the natural and loaded columns were significant enough that only one profile would have been necessary to determine a significant difference in residual strength. Artificial removal of slab stress resulted in greater rupture forces and larger microstructures, likely due to elastic rebound.

Instruments and Methods Recording Microscale Variations in Snowpack Layering Using Near-Infrared Photography

Ken D. Tape et al. — Mon, 28 Mar 2011 11:09:15 PDT

Deposition of snow from precipitation and wind events creates layering within seasonal snowpacks. The thickness and horizontal continuity of layers within seasonal snowpacks can be highly variable, due to snow blowing around topography and vegetation, and this has important implications for hydrology, remote sensing and avalanche forecasting. In this paper, we present practical field and postprocessing protocols for recording lateral variations in snow stratigraphy using near-infrared (NIR) photography. A Fuji S9100 digital camera, modified to be sensitive to NIR wavelengths, was mounted on a rail system that allowed for rapid imaging of a 10m long snow trench excavated on the north side of Toolik Lake, Alaska (688380 N, 1498360 W). Post-processing of the images included removal of lens distortion and vignetting. A tape measure running along the base of the trench provided known locations (control points) that permitted scaling and georeferencing. Snow layer heights estimated from the NIR images compared well with manual stratigraphic measurements made at 0.2m intervals along the trench (n = 357, R2 = 0.97). Considerably greater stratigraphic detail was captured by the NIR images than in the manually recorded profiles. NIR imaging of snow trenches using the described protocols is an efficient tool for quantifying continuous microscale variations in snow layers and associated properties.

Snow-Mediated Ptarmigan Browsing and Shrub Expansion in Arctic Alaska

Ken D. Tape et al. — Mon, 18 Oct 2010 09:07:32 PDT

Large, late-winter ptarmigan migrations heavily impact the shoot, plant, and patch architecture of shrubs that remain above the snow surface. Ptarmigan browsing on arctic shrubs was assessed in the vicinity of Toolik Lake, on the north side of the Brooks Range in Alaska. Data were collected in early May 2007, at maximum snow depth, after the bulk of the ptarmigan migration had passed through the area. In an area of tall shrubs, half of the buds on Salix alaxensis were browsed by ptarmigan. Three percent of the buds that were buried beneath the snow were browsed, 90% of the buds that were less than 30 cm above the maximum snow level were browsed, and 45% of the buds above that height were browsed. Ptarmigan browsing was found to be a major height limiter for tall shrubs, thereby controlling shrub architecture by brooming stems at the snow surface and inducing stump shoots. These results were qualitatively extrapolated by photographing shrub morphology over a region approximately 300 km wide across a series of north-flowing arctic rivers with headwaters in the Brooks Range. Ptarmigan “hedging” of shrub patches, and shrub growth under a warmer climate, are opposing forces mediated by snow distribution.

Autonomous FMCW Radar Survey of Antarctic Shear Zone

Gary Koh et al. — Thu, 07 Oct 2010 15:04:08 PDT

Radar survey of the Antarctic shear zone was conducted using an ultra-wideband (2-10 GHz) frequency modulated continuous wave (FMCW) radar. The radar was mounted on a sled and pulled by a robot that was specifically designed to operate in a harsh polar environment. Our FMCW radar had good penetration through Antarctic snow and we observed snow stratigraphy to a depth of 20 m. The radar images also revealed multiple crevasses in the shear zone. Our results demonstrate that autonomous survey using high frequency radar is feasible and safe approach for detecting hidden crevasses.

Accurate Inversion of High-Resolution Snow Penetrometer Signals for Microstructural and Micromechanical Properties

Hans-Peter Marshall et al. — Thu, 07 Oct 2010 15:04:07 PDT

Measurements of snow using a high-resolution micropenetrometer can be used to discriminate between different snow types; in lower-density snow the signal is sensitive to microstructure, and micromechanical properties can be estimated. Although a physics-based snow penetration theory was first developed almost a decade ago, since that time the majority of studies using snow micropenetrometers have focused on using direct hardness measurements in statistical relationships. We use Monte-Carlo simulations to rigorously test the existing physics-based snow micropenetration theories over a wide range of parameters. These tests revealed four major sources of error in the inversion, which are corrected in this analysis. It is shown that this improved inversion algorithm can recover micromechanical parameters in synthetic data with much greater accuracy over the entire range of micromechanical properties observed in natural snow. Detailed examples of the inversion results are shown for eight different snow types, collected in both Alaskan and alpine snowpacks. The resulting micromechanical properties are distinctly different, indicating that a snow characterization from snow micropenetrometer estimates of micromechanical properties is likely possible. Estimates of the microscale elastic modulus, microscale strength, and structural element length make sense physically when compared to the qualitative descriptions of the different snow types. Microscale strength estimates are used to estimate macroscale strength values, and results from 33 different snow samples, covering a wide range of densities and snow types, are consistent with previously reported values from macroscale tests.