Submissions from 2014
Water Relationships in the U.S. Southwest: Characterizing Water Management Networks Using Natural Language Processing, John T. Murphy, Jonathan Ozik, Nicholson T. Collier, Mark Altaweel, Richard B. Lammers, Andrew Kliskey, Lilian Alessa, Drew Cason, and Paula Williams
Effect of Carex rostrata on seasonal and interannual variability in peatland methane emissions, Genevieve L. Noyce, Ruth Varner, Jill L. Bubier, and Steve Frolking
Simulating Water, Individuals, and Management using a coupled and distributed approach, Jonathan Ozik, Nicholson Collier, John T. Murphy, Mark Altaweel, Richard B. Lammers, Alexander A. Prusevich, Andrew Kliskey, and Lilian Alessa
Intercomparison of field measurements of nitrous acid (HONO) during the SHARP campaign, J R. Pinto, Jack E. Dibb, B H. Lee, B Rappengluck, E C. Wood, M Levy, R Y. Zhang, Barry Lefer, Xinrong Ren, J Stutz, C Tsai, L Ackermann, J Golovko, S C. Herndon, M Oakes, Q Y. Meng, J W. Munger, M Zahniser, and J Zheng
Mercury Speciation at a Coastal Site in the Northern Gulf of Mexico: Results from the Grand Bay Intensive Studies in Summer 2010 and Spring 2011, Xinrong Ren, Winston T. Luke, Paul Kelley, Mark Cohen, Fong Ngan, Richard Artz, Jake Walker, Steve Brooks, Christopher Moore, Phil Swartzendruber, Dieter Bauer, James Remeika, Anthony Hynes, Jack E. Dibb, John Rolison, Nishanth Krishnamurthy, William M. Landing, Arsineh Hecobian, Jeffery Shook, and L Gregory Huey
The reduction of HNO3 by volatile organic compounds emitted by motor vehicles, A P. Rutter, Q. G. J. Malloy, Y J. Leong, C V. Gutierrez, M Calzada, Eric Scheuer, Jack E. Dibb, and R J. Griffin
River ice responses to a warming Arctic—recent evidence from Russian rivers, Alexander I. Shiklomanov and Richard B. Lammers
Core handling and processing for the WAIS Divide ice-core project, Joseph Souney Jr, Mark S. Twickler, Geoffrey M. Hargreaves, Brian M. Bencivengo, Matthew J. Kippenhan, Jay A. Johnson, Eric D. Cravens, Peter D. Neff, Richard M. Nunn, Anais J. Orsi, Trevor J. Popp, John F. Rhoades, Bruce H. Vaughn, Donald E. Voigt, Gifford J. Wong, and Kendrick C. Taylor
Salt Marsh Values in a Changing World: Examining Sea Level Rise on Tidal Marshes with a Surface Elevation Table, Sarah Martina Tierney
The effect of nitrogen addition on soil organic matter dynamics: a model analysis of the Harvard Forest Chronic Nitrogen Amendment Study and soil carbon response to anthropogenic N deposition, Christina Tonitto, Christine L. Goodale, Marissa S. Weiss, Serita D. Frey, and Scott V. Ollinger
Temperature and peat type control CO2 and CH4production in Alaskan permafrost peats, C C. Treat, Wilfred M. Wollheim, Ruth Varner, Andrew S. Grandy, Julie Talbot, and Steve Frolking
Temperature and peat type control CO2 and CH4 production in Alaskan permafrost peats, Claire C. Treat, Wilfred M. Wollheim, Ruth K. Varner, Andrew S. Grandy, J. Talbot, and Stephen E. Frolking
Tropical-Forest Biomass Estimation at X-Band From the Spaceborne TanDEM-X Interferometer, Robert Treuhaft, Fabio Goncalves, Joao Roberto dos Santos, Michael M. Keller, Michael W. Palace, Soren N. Madsen, Franklin B. Sullivan, and Paulo M. L. A. Graca
Rates of sustainable forest harvest depend on rotation length and weathering of soil minerals, Matthew A. Vadeboncoeur, Steven P. Hamburg, Ruth D. Yanai, and Joel D. Blum
Fungi, ecosystems and global change, Linda T. A. van Diepen, Erik A. Hobbie, and Jacqueline E. Mohan
Comparison of passive microwave and modeled estimates of total watershed SWE in the continental United States, Carrie M. Vuyovich, Jennifer M. Jacobs, and Steven F. Daly
Representation of multiple cropping systems in land use data sets, K Waha, A Bondeau, Steve Frolking, F T. Portmann, and S Siebert
Blending Natural Resources & Economic Development, Cameron P. Wake, Curt Grimm, and Alexandra Thorn
Past, Present, and Potential Future 100-year floods in the Lamprey River Watershed, Cameron P. Wake, Fay Rubin, Steve Miller, Robert M. Roseen, Ann Scholz, Thomas P. Ballestero, Michael Simpson, Julia Peterson, Lisa Townson, John Echeverria, Katherine Garvey, and Peg Elmer
Quantifying the effects of harvesting on carbon fluxes and stocks in northern temperate forests, W. Wang, Jingfeng Xiao, Scott V. Ollinger, A. R. Desai, J. Chen, and A. Noormets
Energy input is primary controller of methane bubbling in subarctic lakes, Martin Wik, Brett F. Thornton, David Bastviken, Sally MacIntyre, Ruth K. Varner, and Patrick M. Crill
Nitrate uptake dynamics of surface transient storage in stream channels and fluvial wetlands, Wilfred M. Wollheim, T. K. Harms, Bruce J. Peterson, K. Morkeski, Charles S. Hopkinson, Robert J. Stewart, M. N. Gooseff, and M. A. Briggs
Comparing MODIS daily snow albedo to spectral albedo field measurements in Central Greenland, Patrick Wright, Mike Bergin, Jack E. Dibb, Barry Lefer, Florent Domine, Tobey Carman, Carlo Carmagnola, Marie Dumont, Zoe R. Courville, Crystal B. Schaaf, and Zhuosen Wang
Satellite evidence for significant biophysical consequences of the “Grain for Green” Program on the Loess Plateau in China, Jingfeng Xiao
Uncertainty in model parameters and regional carbon fluxes: A model-data fusion approach, Jingfeng Xiao, Kenneth J. Davis, Nathan M. Urban, and Klaus Keller
A model base system for sustainable development of marine fishery, Jingfeng Xiao, H. Gong, J. Li, J. Yang, and W. Chen
Data-driven diagnostics of terrestrial carbon dynamics over North America, Jingfeng Xiao, Scott V. Ollinger, Steve Frolking, George C. Hurtt, David Y. Hollinger, Kenneth J. Davis, Yude Pan, Xiaoyang Zhang, Feng Deng, Jiquan Chen, Dennis D. Baldacci, Beverly E. Law, Ankur R. Desai, M Altaf Arain, Andrew D. Richardson, Ge Sun, Brian Amiro, Hank Margolis, Lianhong Gu, Russell L. Scott, Peter D. Blanken, and Andrew E. Suyker
Datadriven diagnostics of terrestrial carbon dynamics over North America, Jingfeng Xiao, Scott V. Ollinger, Steve Frolking, George C. Hurtt, David Y. Hollinger, Kenneth J. Davis, Yude Pan, Xiaoyang Zhang, Feng Deng, Jiquan Chen, Dennis D. Baldocchi, Bevery E. Law, M. Altaf Arain, Ankur R. Desai, Andrew D. Richardson, Ge Sun, Brian D. Amiro, Hank Margolis, Lianhong Gu, and Russell L. Scott
Impacts of recent droughts on North American terrestrial ecosystems, Jingfeng Xiao, Scott V. Ollinger, F Li, C Li, Steve Frolking, George C. Hurtt, Maria Rosa Guerrieri, Lucie Lepine, and Heidi Asbjornsen
Identification of greenhouse gas hot-spots and predicting district-wise GHG Intensities in rice based crop rotations in India using DNDC model, Jagadeesh Babu Yeluripati, Steve Frolking, Changsheng Li, D R. Nayak, and T K. Adhya
Dynamics of methane ebullition from a peat monolith revealed from a dynamic flux chamber system, Zhongjie Yu, Lee D. Slater, Karina V. R. Schafer, Andrew S. Reeve, and Ruth K. Varner
Adaptability of Irrigation to a Changing Monsoon in India: How far can we go?, E Zaveri, Danielle S. Grogan, K Fisher-Vanden, Steve Frolking, and D H. Wrenn
Adaptability of Irrigation to a Changing Monsoon in India: How far can we go?, E Zaveri, Danielle S. Grogan, K Fisher-Vanden, Steve Frolking, D H. Wrenn, and R Nichilas
Ice Cores from the St. Elias Mountains, Yukon, Canada: Their Significance for Climate, Atmospheric Composition and Volcanism in the North Pacific Region, Christian Zdanowicz, David Fisher, Jocelyne Bourgeois, Mike Demuth, James Zheng, Paul A. Mayewski, K Kreutz, Erich Osterberg, Kaplan Yalcin, Cameron P. Wake, Eric J. Steig, Duane Froese, and Kumiko Goto-Azuma
Net ecosystem productivity of temperate grasslands in northern China: an upscaling study, Li Zhang, Huadong Guo, Gensuo Jia, Bruce Wylie, Tagir Gilmanov, Danny Howard, Lei Ji, Jingfeng Xiao, Jing Li, Wenping Yuan, Tianbao Zhao, Shiping Chen, Guangsheng Zhou, and Tomomichi Kato
China's sizeable and uncertain carbon sink: A perspective from GOSAT, Li Zhang, Jingfeng Xiao, Li Li, Liping Lei, and Jing Li
Joint data assimilation of satellite reflectance and net ecosystem exchange data constrains ecosystem carbon fluxes at a high-elevation subalpine forest, J M. Zobitz, David J.P. Moore, Tristan Quaife, Rob Braswell, Andrew Bergeson, Jeremy A. Anthony, and Russell K. Monson
Submissions from 2013
Joint analysis of stressors and ecosystem services to enhance restoration effectiveness, J. David Allan, Peter B. McIntyre, Sigrid D. P. Smith, Benjamin S. Halpern, Gregory L. Boyer, Andy Buchsbaum, G. A. Burton Jr., Linda M. Campbell, W. Lindsay Chadderton, Jan J. H. Ciborowski, Patrick J. Doran, Tim Eder, Dana M. Infante, Lucinda B. Johnson, Christine A. Joseph, Adrienne L. Marino, Alexander A. Prusevich, Jennifer G. Read, Joan B. Rose, Edward S. Rutherford, Scott P. Sowa, and Alan D. Steinman
On the export of reactive nitrogen from Asia: NOx partitioning and effects on ozone, T H. Bertram, Paul J. Wooldridge, Jack E. Dibb, Robert C. Cohen, and A E. Perring
Putting the capital ‘A’ in CoCoRAHS: an experimental programme to measure albedo using the Community Collaborative Rain, Hail & Snow (CoCoRaHS) Network, Elizabeth A. Burakowski, Cameron P. Wake, Jack E. Dibb, and Mary D. Stampone
Putting the capital ‘A’ in CoCoRAHS: an experimental programme to measure albedo using the Community Collaborative Rain, Hail & Snow (CoCoRaHS) Network, Elizabeth A. Burakowski, Cameron P. Wake, Jack E. Dibb, and Mary D. Stampone
Snow spectral albedo at Summit, Greenland: measurements and numerical simulations based on physical and chemical properties of the snowpack, Carlo Carmagnola, Florent Domine, Marie Dumont, Patrick Wright, B Strellis, M Bergin, Jack E. Dibb, G Picard, Q Libois, L Arnaud, and S Morin
Evaluation of drought indices via remotely sensed data with hydrological variables, Minha Choi, Jennifer M. Jacobs, Martha C. Anderson, and David D. Bosch
3-D reconstruction of ash vesicularity: Insights into the origin of ash-rich explosive eruptions, Simone Colucci, Danilo M. Palladino, Gopala K. Mulukutla, and Alexander A. Prusevich
3-D reconstruction of ash vesicularity: Insights into the origin of ash-rich explosive eruptions, Simone Colucci, Danilo M. Palladino, Gopal K. Mulukutla, and Alexander A. Proussevitch
Soil warming does not acclimatize to warmer temperatures when modeled over seasonal timescales, Alexandra R. Contosta, Serita D. Frey, Scott V. Ollinger, and Andrew B. Cooper
The use of forest stand age information in an atmospheric CO2 inversion applied to North America, Feng Deng, Jing M. Chen, Y. Pan, W. Peters, R. Birdsey, K. McCullough, and Jingfeng Xiao
Modeling impacts of fertilization alternatives on nitrous oxide and nitric oxide emissions from conventional vegetable fields in southeastern China, Jia Deng, Zaixing Zhou, Xunhua Zheng, and Changsheng Li
Natural peat degradation of equatorial peatlands in inland Borneo as a threshold-response to enhanced Late Holocene El Niño activity, R Dommain, Steve Frolking, P H. Glaser, Hans Joosten, S Kurnianto, and S G. Neuzil
Forest disturbance spectrum of the Amazon, Fernando D.B. Espirito-Santo, Michael Keller, Manuel Gloor, Yadvinder Malhi, Sassan Saatchi, Michael W. Palace, Steve Frolking, and Oliver L. Phillips
The preservation of atmospheric nitrate in snow at Summit, Greenland, Dorothy L. Fibiger, Meredith G. Hastings, Jack E. Dibb, and L Gregory Huey
A global fingerprint of macro-scale changes in urban structure from 1999 to 2009, Steve Frolking, Tom Milliman, Karen C. Seto, and Mark A. Friedl
Using a simple, process-based model to address (and raise) questions about relationships between climate, land-use, and decadal to millennial peatland carbon cycling, Steve Frolking, Julie Talbot, S Kurnianto, and C C. Treat
The size range of bubbles that produce ash during explosive volcanic eruptions, Kimberly Genareau, Gopal K. Mulukutla, Alexander A. Proussevitch, Adam J. Durant, William I. Rose, and Dork L. Sahagian
Soil respiration in a northeastern US temperate forest: a 22‐year synthesis, M. A. Giasson, A. M. Ellison, R. D. Bowden, Patrick M. Crill, Erik A. Davidson, J. E. Drake, Serita D. Frey, J. L. Hadley, M. Lavine, J. M. Melillo, J. W. Munger, K. J. Nadelhoffer, L. Nicoll, Scott V. Ollinger, K. E. Savage, P. A. Steudler, J. Tang, Ruth K. Varner, S. C. Wofsy, D. R. Foster, and A. C. Finzi
Large scale maps of cropping intensity in Asia from MODIS, Josh M. Gray, Mark A. Friedl, and Steve Frolking
Large scale maps of cropping intensity in Asia from MODIS, Josh M. Gray, Mark A. Friedl, Steve Frolking, Navin Ramankutty, and Andrew Nelson
Decreased water flowing from a forest amended with calcium silicate, Mark B. Green, Amey S. Bailey, Scott W. Bailey, John J. Battles, John L. Campbell, Charles T. Driscoll, Timothy J. Fahey, Lucie C. Lepine, Gene E. Likens, Scott V. Ollinger, and Paul G. Schaberg
Reply to Smith and Shortle: Lacking evidence of hydraulic efficiency changes, Mark B. Green, Amey S. Bailey, Scott W. Bailey, John J. Battles, John L. Campbell, Charles T. Driscoll, Timothy J. Fahey, Lucie C. Lepine, Gene E. Likens, Scott V. Ollinger, and Paul G. Schaberg
The hidden regional costs of improving irrigation efficiency: a case study from India, Danielle S. Grogan, Steve Frolking, Richard B. Lammers, Dominik Wisser, A Prousevitch, and Stanley Glidden
Quantifying and mapping China’s crop yield gains from sustainable and unsustainable irrigation water use 1981-2000, Danielle S. Grogan, Fan Zhang, Stanley Glidden, Dominik Wisser, Alexander Prusevich, Changsheng Li, Richard B. Lammers, and Steve Frolking
History of nutrient inputs to the northeastern United States, 1930–2000, Rebecca L. Hale, Joseph H. Hoover, Wilfred M. Wollheim, and Charles J. Vorosmarty
Climate change at the ecosystem scale: a 50-year record in New Hampshire, Steven P. Hamburg, Matthew A. Vadeboncoeur, Andrew D. Richardson, and Amey S. Bailey
Controls on dissolved organic carbon quantity and chemical character in temperate rivers of North America, Kevin W. Hanley, Wilfred M. Wollheim, Joseph Salisbury, Thomas Huntington, and George Aiken
Fungal functioning in a pine forest: evidence from a 15N‐labeled global change experiment, Erik A. Hobbie, Linda T. A. van Diepen, Erik A. Lilleskov, Andrew P. Ouimette, Adrien C. Finzi, and Kirsten S. Hofmockel
Microbes in nature are limited by carbon and energy: the starving-survival lifestyle in soil and consequences for estimating microbial rates, John E. Hobbie and Erik A. Hobbie
Considering Climate Change in Road and Building Design, Jennifer M. Jacobs, Paul H. Kirshen, and Jo S. Daniel
Climate Variability and Impacts on Irrigation Water Demand: Research and Application in Northeast Florida, Jennifer M. Jacobs, Sudheer R. Satti, Michael D. Dukes, and J. W. Jones
Vegetation response to extreme climate events on the Mongolian Plateau from 2000 to 2010, Ranjeet John, Jiquan Chen, Zu-Tao Ou-Yang, Jingfeng Xiao, Richard Becker, Arindam Samanta, Sangram Ganguly, Wenping Yuan, and Ochirbat Batkhishig
Modeling carbon accumulation dynamics in tropical peat swamp forests, Sofyan Kurnianto, Steve Frolking, Matthew Warren, Kristell Hergoualc’h, Julie Talbot, Boone Kauffman, Ruth Varner, and Daniel Murdiyarso
Contributions of the world's glaciers to the hydrological cycle in the 21st Century, Richard B. Lammers, A Bliss, R Hock, A Prousevitch, Danielle S. Grogan, Stanley Glidden, Steve Frolking, and V Radic
Coupled Hydrological and Thermal Modeling of Permafrost and Active Layer Dynamics: Implications to Permafrost Carbon Pool in Northern Eurasia, S Marchenko, Dominik Wisser, Vladimir Romanovsky, W Chapman, Steve Frolking, and J E. Walsh
Riverine ecosystem services and the thermoelectric sector: strategic issues facing the Northeastern United States, Ariel Miara, Charles J. Vorosmarty, Robert J. Stewart, Wilfred M. Wollheim, and Bernice Rosenzweig
Connecting PhenoCam Sites with the ORNL DAAC MODIS Global Subsetting and Visualization Tool, Thomas E. Milliman, Steve Frolking, A D. Richardson, Mark A. Friedl, S Santhana Vannan, Michael Toomey, and Stephen Klosterman
Nitrogen cycling, forest canopy reflectance, and emergent properties of ecosystems, Scott V. Ollinger, Peter B. Reich, Steve Frolking, Lucie Lepine, David Y. Hollinger, and Andrew D. Richardson
Climate sensitivity in the Anthropocene, M Previdi, B G. Liepert, D Peteet, J Hansen, D H. Beerling, A J. Broccoli, Steve Frolking, J N. Galloway, M Heimann, C le Quere, S Levitus, and V Ramaswamy
Log-Exponential Reservoir Operating Rules for Global And Regional Hydrological Modeling, A Prousevitch, A Shiklomanov, Steve Frolking, Stanley Glidden, Richard B. Lammers, and Dominik Wisser
Sobol' sensitivity analysis of the Holocene Peat Model: What drives carbon accumulation in peatlands?, Anne Quillet, Michelle Garneau, and Steve Frolking
Response of vegetation and carbon accumulation to changes in precipitation and water table depths in two bogs during the Holocene: a modelling exercise, Anne Quillet, Michelle Garneau, Simon Van Bellen, Steve Frolking, and Eeva-Stiina Tuittila
Evaluation of continental carbon cycle simulations with North American flux tower observations, Brett M. Raczka, Kenneth J. Davis, Deborah N. Huntzinger, Ronald P. Neilson, Benjamin Poulter, Andrew D. Richardson, Jingfeng Xiao, Ian Baker, Philippe Ciais, Trevor F. Keenan, Beverly Law, Wilfred M. Post, Daniel Ricciuto, Kevin Schaefer, Hanqin Tian, Enrico Tomelleri, Hans Verbeeck, and Nicolas Viovy
Recovery from disturbance requires resynchronization of ecosystem nutrient cycles, Edward B. Rastetter, Ruth D. Yanai, R Quinn Thomas, Matthew A. Vadeboncoeur, Timothy J. Fahey, Melany C. Fisk, Bonnie L. Kwiatkowski, and Steven P. Hamburg
Seasonal, not annual precipitation drives community productivity across ecosystems, Todd M.P. Robinson, Kimberly J. La Pierre, Matthew A. Vadeboncoeur, Kerry M. Byrne, Michell L. Thomey, and Samantha E. Colby
Changing Discharge Patterns of High-Latitude Rivers, Alexander I. Shiklomanov and Richard B. Lammers
Horizontal cooling towers: riverine ecosystem services and the fate of thermoelectric heat in the contemporary Northeast US, Robert J. Stewart, Wilfred M. Wollheim, Ariel Miara, Charles J. Vorosmarty, Balazs M. Fekete, Richard B. Lammers, and Bernice Rosenzweig
Impacts of global change on water resources in Dryland East Asia, Ge Sun, Xiaoming Feng, Jingfeng Xiao, Alex Shiklomanov, Shengping Wang, Zhiqiang Zhang, Nan Lu, Shuai Wang, Liding Chen, Bojie Fu, Yaning Chen, and Jiquan Chen
How are pools influencing the long-term rate of peat accumulation at the ecosystem scale in northern peatlands?, Julie Talbot, L Pelletier, Oliver Sonnentag, and Steve Frolking
Ground-based imaging spectrometry of canopy phenology and chemistry in a deciduous forest, Michael Toomey, Mark A. Friedl, Steve Frolking, T Hilker, Timothy O'Keefe, and A D. Richardson
A permafrost carbon bomb? The fate of permafrost soil carbon following thaw depends on hydrology, C C. Treat and Steve Frolking
Modelling the effects of climate change and disturbance on permafrost stability in northern organic soils, C C. Treat, Dominik Wisser, S Marchenko, and Steve Frolking
Extreme rainfall, vulnerability and risk: a continental-scale assessment for South America, Charles J. Vorosmarty, Lelys Bravo de Guenni, Wilfred M. Wollheim, Brian Pellerin, David Bjerklie, Manoel Cardoso, Cassiano D'Almeida, Pamela Green, and Lilybeth Colon
Assessing the Risk of 100-year Freshwater Floods in the Lamprey River Watershed of New Hampshire Resulting from Changes in Climate and Land Use, Cameron P. Wake, Steve Miller, Robert M. Roseen, Ann Scholz, Fay Rubin, Michael Simpson, Cliff Sinnott, Julia Peterson, and Lisa Townson
Assessing the Risk of 100-year Freshwater Floods in the Lamprey River Watershed of New Hampshire Resulting from Changes in Climate and Land Use: Review of Land Development (Build-out) and Climate Scenarios, Cameron P. Wake, Fay Rubin, Ann Scholz, Michael Simpson, and Cliff Sinnott
Cycling of Molecular Hydrogen in Subarctic Sweden, Victoria Ward
Multiyear measurements of ebullitive methane flux from three subarctic lakes, Martin Wik, Ruth K. Varner, and David Bastviken
Beyond peak reservoir storage? A global estimate of declining water storage capacity in large reservoirs, Dominik Wisser, Steve Frolking, Stephen Hagen, and Marc F. P. Bierkens
Carbon fluxes, evapotranspiration, and water use efficiency of terrestrial ecosystems in China, Jingfeng Xiao, Ge Sun, Jiquan Chen, Shiping Chen, Gang Dong, Shenghua Gao, Haiqiang Guo, Jixun Guo, Shijie Han, Tomomichi Kato, Yuelin Li, Guanghui Lin, Weizhi Lu, Mingguo Ma, Steven McNulty, Changliang Shao, Xufeng Wang, Xiao Xie, Xudong Zhang, and Zhiqiang Zhang
Dynamics of vegetation productivity in Dryland East Asia from 1982 to 2010, Jingfeng Xiao, Li Zhang, Jiquan Chen, and Ranjeet John
From missing source to missing sink: Long-term changes in the nitrogen budget of a northern hardwood forest, Ruth D. Yanai, Matthew A. Vadeboncoeur, Steven P. Hamburg, Mary A. Arthur, Colin B. Fuss, Peter M. Groffman, Thomas G. Siccama, and Charles T. Driscoll
Two-year simultaneous records of N2O and NO fluxes from a farmed cropland in the northern China plain with a reduced nitrogen addition rate by one-third, Guangxuan Yan, Xunhua Zheng, Feng Cui, Zhisheng Yao, Zaixing Zhou, Jia Deng, and Yu Xu