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Boulder City, CO, United States

Arcone S.,U.S. Army | Campbell S.,University of Maine, United States | Pfeffer W.T.,Institute of Arctic and Alpine Research | Pfeffer W.T.,University of Colorado at Boulder
Journal of Environmental and Engineering Geophysics | Year: 2014

We discuss GPR reflection profiles that we recorded on glacial till and a colluvial diamict at several locations in New Hampshire, and from which we interpret water contents, depths and rates of signal loss. We used pulses centered from 150-200 MHz and 300-360 MHz. The boulder-rich sediments reside over granitic and metavolcanics, the horizons of which we recognize from the relative strengths and phase of their waveforms, underlying fractures, and well-developed diffraction asymptotes. The till produced an apparent dense distribution of diffractions with limited asymptotes and dispersion, and occasional minor stratification. We use these diffractions and moveout profiles to calculate relative dielectric permittivities between 17 and 27, values which suggest up to 30% volumetric water, and likely saturation within these over-consolidated sediments. The evidence for transitions from till to bedrock ranges from a simple horizon to complex horizon segments, all characterized by diffractions and amenable to single-layer migration. A gradational loss in diffraction strength with depth suggests gradational weathering or changes in grain size as the cause. Maximum profiled depths range from 4 m to at least 10 m, with estimated scattering attenuation rates of about 3.3 dB m-1. In contrast, one and possible two colluvial diamicts, which likely contained 3-m-size boulders, show short segments of stratification, rare diffraction asymptotes, allow more than 20-m penetration and provide scattering losses of about 0.5 dB m-1. We measured extremely low conductivity and calculated permittivities ranging from 9-12, which suggest high densities and volumetric water content of 4-12%. Low, single scattering loss and deep penetration in the till are consistent with evidence of ground waves traveling up to 40 m one way. The phase polarity of waveforms within till and colluvial events show they may originate from either high or low dielectric contrasts, likely related to water or large boulders, respectively.

Biederman J.A.,University of Arizona | Brooks P.D.,University of Arizona | Harpold A.A.,University of Arizona | Harpold A.A.,Institute of Arctic and Alpine Research | And 5 more authors.
Ecohydrology | Year: 2014

Seasonal snowpack in forested lands is the primary source of fresh water in western North America, where mountain pine beetle (MPB) infestation has resulted in rapid and extensive tree die-off. Forests significantly influence the amount and spatial distribution of peak seasonal snowpack, but the impacts of large-scale tree mortality on the processes controlling peak snowpack are not well understood. We evaluate the effects of widespread tree mortality on winter snow accumulation and peak seasonal snowpack across multiple spatial scales and several levels of MPB impact in the Central Rocky Mountains. Observations for winters 2010 and 2011 include continuous snow depths in 20 plots, distributed snow surveys at peak accumulation and climate observations above and below canopy including precipitation, temperature, humidity, wind and shortwave radiation. Stable water isotopes were observed for fresh snowfall and for snowpack. Plot-scale snowfall observations showed 20% lower interception (p<0.05) in grey-phase stands (needles lost) than in unimpacted stands. However, distributed snow surveys found no differences in peak seasonal snow water equivalent between unimpacted and grey-phase stands. Water isotopes of snowpack from MPB-killed stands indicated kinetic fractionation; enriched values demonstrated higher winter snowpack sublimation in MPB-killed forest. Following MPB infestation, reduced canopy sublimation of intercepted snow appeared to be compensated by increased snowpack sublimation, consistent with observations of higher snowpack insolation. Consequently, the effects of widespread tree mortality on peak seasonal snowpack, which is crucial for downstream water resources, will be influenced by compensation for lower interception by higher snowpack sublimation. © 2012 John Wiley & Sons, Ltd.

Hinckley E.-L.S.,Institute of Arctic and Alpine Research | Hinckley E.-L.S.,University of Colorado at Boulder | Bonan G.B.,U.S. National Center for Atmospheric Research | Bowen G.J.,University of Utah | And 17 more authors.
Ecosphere | Year: 2016

Human impacts on biogeochemical cycles are evident around the world, from changes to forest structure and function due to atmospheric deposition, to eutrophication of surface waters from agricultural effluent, and increasing concentrations of carbon dioxide (CO2) in the atmosphere. The National Ecological Observatory Network (NEON) will contribute to understanding human effects on biogeochemical cycles from local to continental scales. The broad NEON biogeochemistry measurement design focuses on measuring atmospheric deposition of reactive mineral compounds and CO2 fluxes, ecosystem carbon (C) and nutrient stocks, and surface water chemistry across 20 eco-climatic domains within the United States for 30 yr. Herein, we present the rationale and plan for the ground-based measurements of C and nutrients in soils and plants based on overarching or "high-level" requirements agreed upon by the National Science Foundation and NEON. The resulting design incorporates early recommendations by expert review teams, as well as recent input from the larger natural sciences community that went into the formation and interpretation of the requirements, respectively. NEON's efforts will focus on a suite of data streams that will enable end-users to study and predict changes to biogeochemical cycling and transfers within and across air, land, and water systems at regional to continental scales. At each NEON site, there will be an initial, one-time effort to survey soil properties to 1 m (including soil texture, bulk density, pH, baseline chemistry) and vegetation community structure and diversity. A sampling program will follow, focused on capturing long-term trends in soil C, nitrogen (N), and sulfur stocks, isotopic composition (of C and N), soil N transformation rates, phosphorus pools, and plant tissue chemistry and isotopic composition (of C and N). To this end, NEON will conduct extensive measurements of soils and plants within stratified random plots distributed across each site. The resulting data will be a new resource for members of the scientific community interested in addressing questions about long-term changes in continentalscale biogeochemical cycles, and is predicted to inspire further process-based research. © 2016 Hinckley et al.

Hinckley E.-L.S.,Institute of Arctic and Alpine Research | Ebel B.A.,U.S. Geological Survey | Barnes R.T.,Bard College | Anderson R.S.,Institute of Arctic and Alpine Research | And 5 more authors.
Hydrological Processes | Year: 2014

In the Colorado Front Range, forested catchments near the rain-snow transition are likely to experience changes in snowmelt delivery and subsurface water transport with climate warming and associated shifts in precipitation patterns. Snowpack dynamics are strongly affected by aspect: Lodgepole pine forested north-facing slopes develop a seasonal snowpack, whereas Ponderosa pine-dotted south-facing slopes experience intermittent snow accumulation throughout winter and spring. We tested the degree to which these contrasting water input patterns cause different near-surface hydrologic response on north-facing and south-facing hillslopes during the snowmelt period. During spring snowmelt, we applied lithium bromide (LiBr) tracer to instrumented plots along a north-south catchment transect. Bromide broke through immediately at 10- and 30-cm depths on the north-facing slope and was transported out of soil waters within 40days. On the south-facing slope, Br- was transported to significant depths only during spring storms and remained above the detection limit throughout the study. Modelling of unsaturated zone hydrologic response using Hydrus-1D corroborated these aspect-driven differences in subsurface transport. Our multiple lines of evidence suggest that north-facing slopes are dominated by connected flow through the soil matrix, whereas south-facing slope soils experience brief periods of rapid vertical transport following snowmelt events and are drier overall than north-facing slopes. These differences in hydrologic response were largely a function of energy-driven differences in water supply, emphasizing the importance of aspect and climate forcing when considering contributions of water and solutes to streamflow in catchments near the snow line. © 2012 John Wiley & Sons, Ltd.

Schaefer H.,NIWA - National Institute of Water and Atmospheric Research | Mikaloff Fletcher S.E.,NIWA - National Institute of Water and Atmospheric Research | Veidt C.,University of Heidelberg | Lassey K.R.,NIWA - National Institute of Water and Atmospheric Research | And 11 more authors.
Science | Year: 2016

Between 1999 and 2006, a plateau interrupted the otherwise continuous increase of atmospheric methane concentration [CH4] since pre-industrial times. Causes could be sink variability or a temporary reduction in industrial or climate sensitive sources. We reconstruct the global history of [CH4] and its stable carbon isotopes from ice cores, archived air and a global network of monitoring stations. A box-model analysis suggests that diminishing thermogenic emissions, probably from the fossil-fuel industry, and/or variations in the hydroxyl CH4-sink caused the [CH4]-plateau. Thermogenic emissions didn’t resume to cause the renewed [CH4]-rise after 2006, which contradicts emission inventories. Post-2006 source increases are predominantly biogenic, outside the Arctic, and arguably more consistent with agriculture than wetlands. If so, mitigating CH4-emissions must be balanced with the need for food production.

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