2009

Occurrence, Fate, And Ecosystem Implications Of Endocrine Disrupting Compounds In Select Rivers Of Minnesota

Anonymous — Tue, 21 Aug 2018 23:23:03 +0000

Occurrence, Fate, And Ecosystem Implications Of Endocrine Disrupting Compounds In Select Rivers Of Minnesota Anonymous (not verified) Tue, 08/21/2018 - 17:23 Jeff H Writer

Writer, Jeff H ¹ ; Barber, Larry ² ; Stephanie, Keefe ³ ; Brown, Greg ⁴ ;James, Gray ⁵ ; Schoenfuss, Heiko ⁶ ; Jahns, Nathan ⁷ ; Ferrey, Mark ⁸

¹ University of Colorado/USGS, Boulder, CO
² USGS, Boulder, CO
³ USGS, Boulder, CO
⁴ USGS, Boulder, CO
⁵ USGS, Denver, CO
⁶ St. Cloud State University, MN
⁷ St. Cloud State University, MN
⁸ Minnesota Pollution Control Agency

Select endocrine disrupting compounds were measured in four rivers in southwestern Minnesota. Additionally, caged and wild fish populations were assessed for indication of endocrine disruption using plasma vittelogenin, morphological data, and histopathology. To determine fate of these compounds, a tracer study was performed on one of the rivers (Redwood River) using Lagrangian sampling coupled with hydrologic modeling incorporating transient storage. Mass exchange (transient storage, sorption) and degradation were approximated as pseudo first order processes, and in-stream removal rates were then computed by comparing conservative tracer concentrations to organic compound concentrations. The distance required for 17β-estradiol and nonyl phenol to undergo a 50% reduction in concentration was >2 km and >10 km, respectively, indicating that endocrine disrupting compounds are transported several kilometers downstream from sources and therefore have the potential of adversely impacting the lotic ecosystem. Water quality results indicated the widespread distribution of low level concentrations (<100 ng/L) of endocrine disrupting compounds. Elevated plasma vitellogenin in caged and wild fish populations indicated that endocrine disruption was occurring.

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Mission Architectures For Measuring Hydrology Using Satellites

Anonymous — Tue, 21 Aug 2018 23:22:26 +0000

Mission Architectures For Measuring Hydrology Using Satellites Anonymous (not verified) Tue, 08/21/2018 - 17:22 David N Wiese

Wiese, David N ¹ ; Nerem, Steve ²

¹ Colorado Center for Astrodynamics Research
² Colorado Center for Astrodynamics Research

Since its launch in 2002, the Gravity Recovery and Climate Experiment (GRACE) mission has been providing measurements of the time-varying Earth gravity field. Scientists have used this data to measure terrestrial water storage in river basins, ice mass loss from Greenland, Antarctica, and glaciers, post glacial rebound signals, as well as other important geophysical processes. The GRACE mission architecture includes two satellites in near-circular, near-polar orbits separated in the along-track direction by approximately 220 km (e.g. collinear). A microwave ranging instrument measures changes in the distance between the spacecraft, while accelerometers on each spacecraft are used to measure changes in distance due to non-gravitational forces. This research focuses on quantifying the performance of alternative mission architectures for a follow-on mission to GRACE in hopes of improving temporal and/or spatial resolution. Previous research compared the traditional two-satellite collinear pair to a two-satellite and four-satellite cartwheel formation. Results showed that the cartwheel formation, which provides radial and along-track measurements due to the natural dynamics of the formation, offers improved sensitivity to the mass distribution of the Earth, and thus, higher spatial resolution in the derived gravity field. However, when short-period errors due to the atmospheric/ocean dealiasing (AOD) models are considered, the cartwheel formation, while reducing the longitudinal striping, does not offer higher spatial resolution in the derived gravity field. One way to reduce temporal aliasing errors, and thus increase the spatial resolution of the derived gravity field, is to have better temporal resolution, which can be accomplished through multiple satellite pairs. Current and future research is focused on performing an exhaustive study of possible mission architectures, including constellations, and optimizing a four-satellite architecture in hopes of reducing temporal aliasing errors. Preliminary work has been performed on constellations consisting of both 16 and 32 collinear satellite pairs in polar orbits, as well as a four-satellite architecture consisting of one polar orbiting collinear pair in a 5-day repeat period coupled with a lower inclined 63o collinear pair in a 23-day repeat period. An alternate processing strategy has been implemented in which daily estimates of the gravity field (to low degree and order) are made in hopes of reducing temporal aliasing errors. Results show the four-satellite architecture, while estimating daily 20x20 gravity fields, offers an order of magnitude improvement in recovering a hydrology signal over a traditional two-satellite collinear pair. It even outperforms the constellations considered thus far, albeit having a 23-day temporal resolution versus a 6-hour temporal resolution. All simulations are performed using NASA Goddard Spaceflight Center's GEODYN software package.

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Seasonal Changes In Benthic Diatom Community Composition In An Antarctic Stream Related To Stream Flow

Anonymous — Tue, 21 Aug 2018 23:21:49 +0000

Seasonal Changes In Benthic Diatom Community Composition In An Antarctic Stream Related To Stream Flow Anonymous (not verified) Tue, 08/21/2018 - 17:21 Lee F Stanish

Stanish, Lee F ¹ ; Kimball, Allison J ² ; McKnight, Diane M ³

¹ University of Colorado at Boulder
² University of Colorado at Boulder
³ University of Colorado at Boulder

Stream microbial mats are dynamic communities of phototrophic and heterotrophic organisms that develop over intra-seasonal and inter-seasonal time scales. Diatom community composition is influenced by successional processes and scouring events that together act to shape stream benthic habitats. In ephemeral streams of the McMurdo Dry Valleys (Antarctica), previous work has demonstrated that in streams within the Fryxell Lake Basin, the diatom composition in microbial mat communities is determined largely by the annual and historical flow regime. This study found that the relative abundances of locally-occurring diatom species increases under a colder, lower stream flow regime. Conversely, warmer, higher-flow conditions favor widespread diatom species, and concomitant decreases in diatom species diversity occur at either extreme. This study aims to determine changes in diatom community composition at intra-seasonal time scales in microbial mats in Canada Stream, Fryxell Basin. Preliminary results support the finding that brief cold periods favor local diatom species while warmer periods favor widespread species. Furthermore, the results suggest that there is a ‘lag’ period between the occurrence of a cold snap and the changes in diatom diversity and community composition, suggesting that extreme cold events are important disturbance mechanisms shaping diatom community composition in ephemeral stream microbial mats.

Esposito, R.M.M., S.L. Horn, D.M. McKnight, M.J. Cox, M.C. Grant, S.A. Spaulding, P.T. Doran, K.D. Cozzetto. 2006. Antarctic climate cooling and response of diatoms in glacial meltwater streams. Geophysical Research Letters 33: L07406.

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Interactions Between Two Turbulent Passive Scalar Plumes Using A Novel Two-Dye Planar Laser-Induced Fluorescence System

Anonymous — Tue, 21 Aug 2018 23:21:01 +0000

Interactions Between Two Turbulent Passive Scalar Plumes Using A Novel Two-Dye Planar Laser-Induced Fluorescence System Anonymous (not verified) Tue, 08/21/2018 - 17:21 Michael Soltys

Soltys, Michael ¹ ; Crimaldi, John ²

¹ University of Colorado
² University of Colorado

Turbulent fluid flow plays an important role in many physical and biological mixing processes. This is exemplified by broadcast spawning, the reproductive strategy used by corals and many other marine invertebrates. Current biological models predict that the efficiency of broadcast spawning is vastly insufficient to sustain coral populations (Denny, 1988), but the existence of these animals suggests otherwise. These models rely on time-averaged processes over long timescales. Recent research on the structure of turbulent plumes has revealed that over short timescales structured filaments of high concentration form in turbulent flow before being dispersed to low concentrations at longer timescales (Crimaldi et. Koseff, 2006). This breakthrough in the understanding of turbulent flows has led to models that help explain the success of broadcast spawning and other mixing processes. In a two-dimensional model by Crimaldi et. al. (2009), it has been shown that vortex stirring helps scalars to coalesce therefore increasing their reaction rate, however until this time no laboratory procedure has been developed to confirm these findings.

This poster will present a novel two-dye planar laser induced fluorescence (PLIF) system which can be used to study the interactions between two passive scalars separated by a third passive scalar in a turbulent flow. This system uses an argon-ion and a krypton-ion laser each paired with a fluorescent dye, much like traditional PLIF systems discussed in Crimaldi (2008). A pair of high-speed cameras collect images of the plumes which are then post-processed to determine concentration fields of the individual plumes. The system will be used to better understand the processes that result in the success of broadcast spawning and other mixing phenomena.

Denny, M., 1988. Biology and the Mechanics of the Wave-Swept Environment. Princeton Univ. Press, Princeton, NJ.
Crimaldi, JP. “Planar laser induced fluorescence in aqueous flows.” EXPERIMENTS IN FLUIDS 44.6 (2008): 851-863.
Crimaldi, JP, JR Cadwell, and JB Weiss. “Reaction enhancement of isolated scalars by vortex stirring.” PHYSICS OF FLUIDS 20.7 (2008). 22 Feb 2009
Crimaldi JP, Koseff JR. 2006. Structure of turbulent plumes from a momentumless source in a smooth bed. Environ Fluid Mech 6:573 – 592.

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Evapotranspiration Depletion Of Saltcedar Managed Areas In The New Mexico Arid Environment Along The Rio Grande

Anonymous — Tue, 21 Aug 2018 23:20:16 +0000

Evapotranspiration Depletion Of Saltcedar Managed Areas In The New Mexico Arid Environment Along The Rio Grande Anonymous (not verified) Tue, 08/21/2018 - 17:20 Jose A Solis

Solis, Jose A ¹

¹ Univerisity of Colorado at Boulder

Many federal, state, and local water management agencies have been extremely concerned with the infestation of this exotic saltcedar (Tamarix spp.) plant due to its reputation for high water consumption and the difficulty and cost involved in its management. Several methods of control have been implemented in an effort to reduce saltcedar evapotranspiration (ET), improve ecological health of riparian regions along the river, and re-establish indigenous plants such as cottonwood, willows and saltgrass among others. Management practices for controlling saltcedar include mechanical, biological, chemical and ecological competitive means.

This study investigated evapotranspiration (ET) depletion of saltcedar managed areas where three common methods i) mowing, ii) herbicide treatment and iii) plowing or clearing have been practiced. The eddy covariance method was used to measure ET at four sites which included Caballo mowed saltcedar site, Monticello herbicide saltcedar treated site, Bosque del Apache National Wildlife Refuge (NWR) plowed or cleared saltcedar site, and Bosque del Apache NWR monotypic dense saltcedar site.

A comparison of mowed, herbicide treated and plowed ET to a dense monotypic saltcedar stand as a baseline resulted in mean ET potential reduction of 25%, 52%, and 30%, respectively. After deducting mean precipitation from the measured ET, assuming all precipitation evaporated, a reduction in ET of 38% was achieved when mowed saltcedar site was compared to the dense site; ET reduction of 66% and 37% was achieved when herbicide treated and plowed sites were compared to dense site.

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Chemical Characteristics Of Dissolved Humic Substances From Arsenic-Rich Shallow Groundwater In Bangladesh

Anonymous — Tue, 21 Aug 2018 23:18:53 +0000

Chemical Characteristics Of Dissolved Humic Substances From Arsenic-Rich Shallow Groundwater In Bangladesh Anonymous (not verified) Tue, 08/21/2018 - 17:18 Bailey E Simone

Simone, Bailey E ¹ ; Legg, Teresa ² ; McKnight, Diane ³ ; Nemergut, Diana ⁴ ;Zheng , Yan ⁵ ; Mladenov, Natalie ⁶

¹ University of Colorado at Boulder
² University of Colorado at Boulder
³ University of Colorado at Boulder
⁴ University of Colorado at Boulder
⁵ Queens College
⁶ University of Colorado at Boulder

Dissolved organic material present in groundwater has been shown to play a significant role in mobilizing arsenic through several pathways in Bangladesh aquifers and other regions with As-rich sediments (Bauer, 2006). Firstly, labile organic substrates may stimulate the growth of heterotrophic bacteria, resulting in anoxic conditions, which drives the use of alternate electron acceptors beyond oxygen, such as nitrate and ferric iron present in oxides in sediment (Mladenov, 2008; Horneman, 2004). Another potentially important pathway is the electron shuttling by dissolved humic substances where the quinone moieties in the humic substances are reduced by microbial processes (Van Geen, 2004). These reduced humic molecules transfer electrons to ferric iron in iron oxides along the flow path. This redox cascade promotes the release of arsenic and potentially other toxic metals. Although all humic substances studied have shown some electron shuttling capability, there is a large variation among fulvic and humic acids of different origins. We have isolated preparative quantities of fulvic and transphilic acid from two well depths at a site in Bangladesh with high arsenic concentrations in the groundwater, as well as surface water. The fluorescence characteristics of these samples, which were modeled using parallel factor analysis (PARAFAC) indicate that a terrestrial source of DOM is released concurrently with high arsenic concentrations.

Van Geen, A., 2004, Decoupling of As and Fe release to Bangladesh groundwater under reducing conitions. Part II: Evidence from sediment incubations: Geochimica et Cosmochimica, v.68, p 3475-3486
Mladenov, N., 2008, Dissolved organic matter accumulation, reactivity, and redox state in ground water of a recharge wetland: Wetlands, v.28, p.747-759
Bauer, M., 2006, Mobilization of arsenic by dissolved organic matter from iron oxides soils and sediments: Science of the Total Environment, v.354, p.179-190

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Water Supply Risk On The Colorado River: Can Management Mitigate?

Anonymous — Tue, 21 Aug 2018 23:18:08 +0000

Water Supply Risk On The Colorado River: Can Management Mitigate? Anonymous (not verified) Tue, 08/21/2018 - 17:18 Balaji Rajagopalan , Kenneth Nowack

Rajagopalan, Balaji ¹ ; Nowak, Kenneth ² ; Prairie, James ³ ; Hoerling, Martin⁴ ; Harding, Benjamin ⁵ ; Barsugli, Joseph ⁶ ; Ray, Andrea ⁷ ; Udall, Bradley ⁸

² Presenting Author

¹ Department of Civil Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309 USA
² Department of Civil Environmental and Architectural Engineering, University of Colorado, Boulder, CO 80309 USA
³ Bureau of Reclamation, University of Colorado, Boulder, CO 80309 USA
⁴ NOAA Earth System Research Laboratory, Boulder, CO 80305 USA
⁵ AMEC Earth & Environmental, Inc., Boulder, CO 80302 USA
⁶ NOAA Earth System Research Laboratory, Boulder, CO 80305 USA
⁷ NOAA Earth System Research Laboratory, Boulder, CO 80305 USA
⁸ Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309 USA

With climate change looming, continued population growth, and the likelihood of multi-year droughts, the future reliability of Colorado River water supply is in question. We assess the risk to Colorado River water supply for the next 50 years (2008-2057). Under current practices in the absence of climate change we find a 5% risk of reservoir depletion through 2026 increasing to 9% by 2057, demonstrating resilience to demand growth and natural climate variability. A 20% reduction in Colorado River average flow due to climate change by 2057, increases risk through 2026 to less than 12%, but greatly increases risk to 52% in 2057. However, we find management alternatives can greatly reduce risk – under aggressive management the risk reduces to 32%. A lower rate of climate change induced flow reduction, demand adaptation and aggressive management can further reduce the risk to around 10% - suggesting substantial flexibility in existing management could mitigate the increased risk.

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Estimating Runoff In Glacier-Covered Watersheds In The Nepal Himalaya Using Area-Altitude Distributed Models

Anonymous — Tue, 21 Aug 2018 23:17:07 +0000

Estimating Runoff In Glacier-Covered Watersheds In The Nepal Himalaya Using Area-Altitude Distributed Models Anonymous (not verified) Tue, 08/21/2018 - 17:17 Adina Racoviteanu

Racoviteanu, Adina E ¹ ; Armstrong, Richard ² ; Alford, Don ³

¹ Dept. of Geography and Institute of Arctic and Alpine Research, University of Colorado
² National Snow and Ice Data Center, University of Colorado
³ 1831 Poly Drive, Billings, Montana

Glaciers are a component of the hydrologic regime of many large mountain ranges and regions, including the Himalayas. However, the hydrologic regime of Himalayan catchment basins, and the role of glaciers in the hydrologic regime of this mountain range is not well understood. As a result of a general unavailability of hydrologic, climatic and topographic data for Himalayan catchment basins, the application of hydrologic concepts and models developed for mountain catchments in Europe or North America is difficult, and sometimes, misleading. Current concerns over the impact of the retreat of Himalayan glaciers on water supplies poses an urgent need to resolve questions related to specific aspects of the water budget cycle, and to determine the impact of glacier change on regional-scale water supplies. The present study focuses on assessing and quantifying the role of glaciers in the hydrologic regime of the Nepal Himalaya. The overarching goal is to estimate the potential impact of a continued glacier retreat in this region.

The methodology developed for this study involved establishing a relationship between the area-altitude distributions of catchment basins and glaciers, and the water and energy exchange gradients. Two area-altitude distributed process models were developed: an orographic runoff model and a glacier melt model. The orographic runoff model was based on the relationship between mean specific runoff, and the mean altitude of each basin. The area-altitude distribution of streamflow was calculated for 1000 meter belts, as the product of the specific runoff depth and the area of the belt. The glacier melt model was based on glacier melt from 100 meter area-altitude belts for the glacierized portion of each catchment and involved: defining an “ablation gradient” (the rate of increasing specific ice melt with decreasing altitude in the ablation zone, taken as 0.6 to 1.4m/100m in the Himalayas); defining the mean maximum altitude of the 00 C isotherm during the ablation period (~5400 m); determining the volume of ablation as the product of specific ice melt values taken from the ablation gradient and the area-altitude values of corresponding belts in the glacier ablation zone. Topography was defined by digital elevation data sets acquired from the Shuttle Radar Topography Mission (SRTM) for both models, and glacier outlines were obtained from the International Center for Integrated Mountain Development (ICIMOD), Nepal.

Preliminary results indicate that the contribution of glacier annual melt water to annual streamflow in the Nepal Himalayas varies among catchment basins from 2-13% of the total annual flow volume, measured at low altitude hydrometric stations. This represents 2-3% of the total annual streamflow volume of the rivers of Nepal. These results suggest that neither the timing nor the volume of the streamflow of rivers of Nepal will be affected significantly by a continued retreat of the glaciers.

Alford, D. (1992). Hydrological aspects of the Himalayan region. ICIMOD Occasional Paper No. 18, Kathmandu, Nepal, 68 pp.
Fujita, K., Takeuchi, N., and Seko, K., 1998, Glaciological observations of Yala Glacier in the Langtang Valley, Nepal Himalayas, 1994 and 1996, Bull. Glacier Research, 16, pp. 75-81.
Rees, H., Holmes, M., Young, A., Kansakar, S., 2004, Recession-based hydrological models for estimating low flows in ungaged catchments in the Himalayas, Hydrology and Earth Systems Sciences, V. 8, N. 5, pp. 891-902

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Melt Water Influence On En-Glacial Temperature Distribution

Anonymous — Tue, 21 Aug 2018 23:16:31 +0000

Melt Water Influence On En-Glacial Temperature Distribution Anonymous (not verified) Tue, 08/21/2018 - 17:16 Thomas Phillips

Phillips, Thomas ¹ ; Steffen, Konrad ² ; Rajaram, Harihar ³

¹ CIRES, ESOC, University of Colorado
² CIRES, ESOC, University of Colorado
³ University of Colorado

The Greenland Ice Sheet experiences a large amount of melt each summer forming moulins and a well developed en-glacial water network. Is this en-glacial network a new network each year or does it remain at least partially open over winter, allowing the melt water of the following year to use the same network, thus enhancing efficiency of en-glacial water transport? We try to answer these questions by exploring the potential influence of the winter snow layers and water bodies in the ice at the end of the melt season on the thermal regime during the winter. During the summer months the short wave radiation warms the surface of the Greenland ice sheet generating surface melt. Melt water penetrating deep into the ice can potentially carry some of the energy to depth. In the Arctic regions snow plays a major role in protecting the underlying surface from losing too much energy to the cold winter atmosphere. In addition, if some of the water entering moulins and crevasses does not reach the bed, but is rather stored in water bodies within the ice at the end of the melt season, the ice sheet will experience a large local latent heat flux that may moderate the cooling of ice during the winter months. In this study we use temperature profiles of the upper 10 meters of ice as well as a two dimensional heat transfer model to simulate the seasonal thermal evolution of an ablation zone. This area of the Greenland Ice Sheet experienced annual snow thicknesses of approximately 80cm. We simulate the temperature variation with time and depth for different years considering the amount of available melt water and the insulating snow height in winter. The possibility of a positive feedback between ice sheet warming due to radiation and melt water in the summer, increased snow cover accumulation and reduced cooling in winter is discussed.

Hooke, R. LeB., 1995, Principles of Glacier Mechanics, Prentice Hall, Upper Saddle River, New Jersey 07458, 248 pages.
Piccini, L., Romeo, A. and G. Badino, 2001, Moulins and marginal contact caves in the Gornergletscher, Switzerland, NIMBUS 23-24, Associazione La Venta, 1994.
Thomsen, H. H., 1992, Work Report, Groenlands Geologiske Undersogelse, Kopenhavn, Danmark.
Thomsen, H. H., 1991, Dead Glacier Map, Groenlands Geologiske Undersogelse, Kopenhavn, Danmark.
Zwally, H. J., Abdalati, W., Herring, T., Larson, K., Saba, J. and K. Steffen, 2002, Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow. Science 297, 218-222.

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Three-Dimensional Quantification Of Meltwater Flow Through A Snowpack Using A Snow Guillotine

Anonymous — Tue, 21 Aug 2018 23:15:47 +0000

Three-Dimensional Quantification Of Meltwater Flow Through A Snowpack Using A Snow Guillotine Anonymous (not verified) Tue, 08/21/2018 - 17:15 Jennifer L Petrzelka

Petrzelka, Jennifer L ¹ ; Erickson, Tyler A ² ; Williams, Mark W ³

¹ University of Colorado and INSTAAR
² Michigan Tech Research Insitute and Department of Civil and Environmental Engineering
³ University of Colorado and INSTAAR

In areas containing seasonal snowpacks, snowmelt contributes significantly to the hydrological cycle. Thus, quantifying the spatial distribution of flow through a snowpack is essential to accurate hydrograph interpretation and representation in snowmelt runoff modeling. Movement of liquid water through snowpacks is generally recognized to occur in distinct flow paths rather than as uniform flow through a homogeneous porous medium. Most methods of investigating meltwater flow through snow involve sparse or invasive sampling. This makes it difficult to study scale and time-dependent processes such as the evolution of preferential flowpaths.

McGurk and Marsh (1995) developed a method for quantifying the spatial distribution of flowpaths using a metal framework with an attached blade that sliced uniform sections of a snowpack. Using light transmission, photographs were taken of the thick-cut sections to identify flow paths. The metal framework design of McGurk and Marsh inspired the development of the snow guillotine. The structure of the snow guillotine allows the ability to take cross-sectional slices at 1-cm intervals of a snowpit where dye tracer has been applied. Photographs are taken of each cross-section over a one meter distance. Application of image processing and geostatistical analysis allows collection of high resolution (1cm3), three-dimensional data on meltwater flow through a snowpack.

Three snow guillotine experiments were conducted over the course of two melt seasons. Datasets obtained from the experiments were analyzed to address the following questions: What portion of the snowpack transmits meltwater? Under what conditions do vertical meltwater pathways occur? Are vertical meltwater flowpaths continuous across layer boundaries? Can dye tracer experiments be used to identify snowpack layer transitions that cannot be easily identified from snowpit sampling? Results indicated that the fraction of the snowpack transferring liquid water was highly variable between experiments, as well as within individual experiments with respect to depth. All three experiments illustrated preferential flowpaths, with the majority of vertical flow occurring in the upper 20-55cm of the snowpack, while no preferential flowpaths are apparent below 100cm. Layer interfaces were found to significantly affect the volume of dye, indicating dominance by lateral flow at these boundaries. These findings were supported by the decrease in probability with depth of finding vertical flow and an increase in the probability of finding lateral flow at layer interfaces. Preferential flowpaths decreased in distinctness with time after the snowpack reached isothermal conditions at 0oC. Implementation of the snow guillotine significantly improves field measurements of meltwater flow while providing three-dimensional, quantitative data of unprecedented spatial resolution.

McGurk, B. J. and P. Marsh, Flow-finger continuity in serial thick-sections in a melting Sierran snowpack, in Biogeochemistry of Seasonally Snow-Covered Catchments, edited by K.A. Tonnessen, M.W. Williams, and M. Tranter, pp. 81-88, International Association of Hydrological Sciences, Wallingford, Oxfordshire, UK, 1995.

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