Strength Envelopes from Single and Multi Geosynthetic Interface Tests     

Timothy D. Stark  Fawad S. Niazi  Timothy C. Keuscher 《Geotechnical and Geological Engineering》2015,33(5):1351-1367

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Reconstructing a millennial‐scale record of flooding in a single valley setting: the 2011 flood‐affected Lockyer Valley,south‐east Queensland,Australia          下载免费PDF全文

Jacky Croke  Christopher Thompson  Robert Denham  Heather Haines  Ashneel Sharma  Timothy Pietsch 《第四纪科学杂志》2016,31(8):936-952

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Long-term ecosystem and biogeochemical research in Loch Vale watershed,Rocky Mountain National Park,Colorado     

Jill S. Baron  David W. Clow  Isabella A. Oleksy  Timothy Weinmann  Caitlin Charlton  Amanda Jayo 《水文研究》2021,35(3):e14107

Loch Vale watershed was instrumented in 1983 with initial support from the National Acid Precipitation Assessment Program to ask whether ecosystems of Rocky Mountain National Park (RMNP) were affected by acidic atmospheric deposition. Research and monitoring activities were expanded in 1991 by the U.S. Geological Survey Water, Energy, and Biogeochemical Budgets program to understand the processes, and their interactions, controlling water, energy, and biogeochemical fluxes. With help from many collaborators we have characterized trends and patterns in atmospheric deposition, climate, and hydrology, including glaciers and other ice features. Instead of acidic deposition, we documented high atmospheric inputs of reactive nitrogen (Nr), and have studied the ecological consequences in soils, surface water, and vegetation. Using paleolimnology, we documented the onset of human-caused change to lake primary producers ca. 1950 in response to increased Nr deposition and warming. Our results provided the basis for the Colorado Nitrogen Deposition Reduction Plan, a state policy that aims to reduce Nr emissions to protect resources in RMNP by 2032. Carbon cycle research revealed mountain wetlands now release more carbon than they store, and respiration and methane flux occurs even during winter through deep snow packs. Trend analyses found export of Nr to be closely tied to atmospheric inputs, but can lag in response to drought. Current research explores consequences of the combination of warming, changes in precipitation dynamics, and atmospheric deposition of Nr and dust on stream and lake CO₂ dynamics, lake biology and trophic state, and soil carbon composition. Dramatic increases in park visitors have prompted studies on the effects of recreational use on water quality. New tools such as remote sensing and high frequency instream water quality sensors are being applied to lake and stream studies. Monitoring, combined with experiments, models, and spatial comparisons is an essential foundation for science-based resource management.  相似文献   

Surface water input from snowmelt and rain throughfall in western juniper: potential impacts of climate change and shifts in semi‐arid vegetation          下载免费PDF全文

Ryan J. Niemeyer  Timothy E. Link  Mark S. Seyfried  Gerald N. Flerchinger 《水文研究》2016,30(17):3046-3060

In the western USA, shifts from snow to rain precipitation regimes and increases in western juniper cover in shrub‐dominated landscapes can alter surface water input via changes in snowmelt and throughfall. To better understand how shifts in both precipitation and semi‐arid vegetation cover alter above‐ground hydrological processes, we assessed how rain interception differs between snow and rain surface water input; how western juniper alters snowpack dynamics; and how these above‐ground processes differ across western juniper, mountain big sagebrush and low sagebrush plant communities. We collected continuous surface water input with four large lysimeters, interspace and below‐canopy snow depth data and conducted periodic snow surveys for two consecutive water years (2013 and 2014). The ratio of interspace to below‐canopy surface water input was greater for snow relative to rain events, averaging 79.4% and 54.8%, respectively. The greater surface water input ratio for snow is in part due to increased deposition of redistributed snow under the canopy. We simulated above‐ground energy and water fluxes in western juniper, low sagebrush and mountain big sagebrush for two 8‐year periods under current and projected mid‐21st century warmer temperatures with the Simultaneous Heat and Water (SHAW) model. Juniper compared with low and mountain sagebrush reduced surface water input by an average of 138 mm or 24% of the total site water budget. Conversely, warming temperatures reduced surface water input by only an average of 14 mm across the three vegetation types. The future (warmer) simulations resulted in earlier snow disappearance and surface water input by 51 and 45 days, respectively, across juniper, low sagebrush and mountain sagebrush. Information from this study can help land managers in the sagebrush steppe understand how both shifts in climate and semi‐arid vegetation will alter fundamental hydrological processes. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Surface water–groundwater interaction at restored streams and associated reference reaches     

Timothy L. Daniluk  Laura K. Lautz  Ryan P. Gordon  Theodore A. Endreny 《水文研究》2013,27(25):3730-3746

Stream restoration goals include improving habitat and water quality through reconstruction of morphological features found at analogous, pristine stream reaches. Enhancing hyporheic exchange may facilitate achieving these goals. Although hyporheic exchange at restoration sites has been explored in a few previous studies, comparative studies of restored versus reference or control streams are largely absent. We hypothesized that restoration cross‐vanes enhance hyporheic exchange, resulting in biogeochemical alteration of stream water chemistry in the streambed. Two streams restored using cross‐vanes to control erosion and improve habitat were compared with their associated reference reaches, which provided the basis for the restoration design. Thirteen temperature profile rods with vertically stacked sensors were installed at each site for 2 weeks. Heat tracing was used to quantify vertical flux in the streambed from the diurnal temperature fluctuations in the subsurface. Stream water and bed pore waters from mini‐piezometers were analysed for ion and nutrient chemistry. In general, mean vertical flux rates through the streambed were small throughout reference sites (?0.3 to 0.3 m/day) and at most locations at restored sites. Immediately adjacent to cross‐vanes, vertical flux rates were larger (up to 3.5 m/day). Geochemistry of pore waters shows distinct differences in the sources for the reference and restored sites. Strong downwelling zones adjacent to cross‐vanes showed high dissolved oxygen (10.75 mg/l) and geochemistry in the streambed similar to surface water. Reference sites had lower dissolved oxygen in the streambed (0.66–5.14 mg/l), and geochemical patterns suggest a mixture of discharging groundwater and surface water in the hyporheic zone. Restored sites also clearly show sulfate and nitrate reduction occurring in the streambed, which is not observed at the reference sites. The stream restoration sites studied here enhance rapid hyporheic exchange, but upwelling of groundwater has a stronger influence on streambed geochemistry at reference sites. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Quantifying groundwater dependency of riparian surface hydrologic features using the exit gradient          下载免费PDF全文

Barton R. Faulkner  Scott G. Leibowitz  Timothy J. Canfield  Justin F. Groves 《水文研究》2016,30(13):2167-2177

Numerical groundwater flow models necessarily are limited to subsurface flow evaluation. It is of interest, however, to examine the possibility that, for unconfined aquifer systems, they could be used to proportionately measure the magnitude of seepage they estimate when these aquifers intersect the landscape surface. Our goal in this study was to determine the degree to which an unconfined groundwater model can estimate run‐off or seepage at the land surface during winter time wet season conditions, as well as in the dry season, when evapotranspiration is a major part of the water balance, using a lowland basin‐fill example study area in the Pacific Northwest. The exit gradient is a metric describing the potential for vertical seepage at the landscape surface. We investigated the spatial relationship of mapped surface features, such as wetlands, streams and ponds, to the model‐predicted mapped exit gradient. We found that areas mapped as wetlands had positive exit gradients. During the wet season, modelled exit gradients predicted seepage throughout extensive areas of the groundwater shed, extending far beyond mapped wetland areas (355% increase), associated with previously observed increases in nitrate‐nitrogen in streams in wet season. During the dry season, exit gradients spatially corresponded with wetland areas. The increase in in‐stream nitrogen corresponds with shorter residence times in carbon‐rich wetland zones because of the onset of saturation overland flow. We present results that suggest that the exit gradient could be a useful concept in examining the groundwater–surface water linkage that is often under represented physically in watershed flow models. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.  相似文献   

A micromechanics model for molecular diffusion in materials with complex pore structure          下载免费PDF全文

Jithender J. Timothy  Günther Meschke 《国际地质力学数值与分析法杂志》2016,40(5):686-712

Molecular diffusion in fully saturated porous materials is strongly influenced by the pore space, which, in general, is characterized by a complex topological structure. Hence, information on macroscopic diffusion properties requires up‐scaling of transport processes within nano‐pores and micro‐pores over several spatial scales. A new model in the framework of continuum micromechanics is proposed for predicting the effective molecular diffusivity in porous materials. Considering a representative volume element, characterizing a porous material without any information about the pore space microstructure complexity, the uniform flux is perturbed by recursively embedding shape information hierarchically in the form of the ESHELBY matrix‐inclusion morphology to obtain the effective diffusivity as a function of the recurrence level and the porosity. The model predicts a threshold value for the porosity, below which no molecular diffusion can occur because of the presence of isolated pore clusters that are not connected and unavailable for transport. The maximum porosity, below which no molecular transport is possible, is predicted as one‐third for spherical inclusions. The model allows for extensions to more complex morphologies of the inclusions. We also identify, that the effects of the micro‐structure on molecular transport are characterized by porosity dependent long‐range and short‐range interactions. The developed framework is extended to incorporate realistic pore size distributions across several spatial scales by means of a distribution function within the hierarchical homogenization scheme. Available experimental results assert the model predictions. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Growth of continental crust in intra-oceanic and continental-margin arc systems: Analogs for Archean systems     

Kusky  Timothy  Wang  Lu 《中国科学:地球科学(英文版)》2022,65(9):1615-1645

Science China Earth Sciences - Earth’s continental crust has grown and been recycled throughout geologic history along convergent plate margins. The main locus of continental crustal growth...  相似文献   

The alternative of CubeSat-based advanced infrared and microwave sounders for high impact weather forecasting          下载免费PDF全文

LI Zhenglong  LI Jun  SCHMIT Timothy J.  WANG Pei  LIM Agnes  LI Jinlong  NAGLE Fredrick W.  BAI Wenguang  OTKIN Jason A.  ATLAS Robert  HOFFMAN Ross N.  BOUKABARA Sid-Ahmed  ZHU Tong  BLACKWELL William J.  PAGANO Thomas S. 《大气和海洋科学快报》2019,(2)

近年来,小卫星技术的发展给传统气象卫星提供了经济有效的替补方案。本文针对可能出现的传统气象卫星的缺失情形,借助快速的区域观测系统模拟试验,探索了利用小卫星填补空缺的可能性。研究表明,单个小卫星,无论是近红外高光谱还是微波探测仪,都能够对局地强风暴天气的预报有所改进。然而,为了填补传统卫星缺失带来的影响,需要发射三颗甚至更多的小卫星来增加观测的覆盖面,以弥补小卫星精度略低、通道略少的缺陷。  相似文献   

Weathering of ordinary chondrites from Oman: Correlation of weathering parameters with 14C terrestrial ages and a refined weathering scale          下载免费PDF全文

Florian J. Zurfluh  Beda A. Hofmann  Edwin Gnos  Urs Eggenberger  A. J. Timothy Jull 《Meteoritics & planetary science》2016,51(9):1685-1700

We have investigated 128 ¹⁴C‐dated ordinary chondrites from Oman for macroscopically visible weathering parameters, for thin section‐based weathering degrees, and for chemical weathering parameters as analyzed with handheld X‐ray fluorescence. These 128 ¹⁴C‐dated meteorites show an abundance maximum of terrestrial age at 19.9 ka, with a mean of 21.0 ka and a pronounced lack of samples between 0 and 10 ka. The weathering degree is evaluated in thin section using a refined weathering scale based on the current W0 to W6 classification of Wlotzka (1993), with five newly included intermediate steps resulting in a total of nine (formerly six) steps. We find significant correlations between terrestrial ages and several macroscopic weathering parameters. The correlation of various chemical parameters including Sr and Ba with terrestrial age is not very pronounced. The microscopic weathering degree of metal and sulfides with newly added intermediate steps shows the best correlation with ¹⁴C terrestrial ages, demonstrating the significance of the newly defined weathering steps. We demonstrate that the observed ¹⁴C terrestrial age distribution can be modeled from the abundance of meteorites with different weathering degrees, allowing the evaluation of an age‐frequency distribution for the whole meteorite population.  相似文献