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291.
Daniel H. Doctor Carol Kendall Stephen D. Sebestyen James B. Shanley Nobuhito Ohte Elizabeth W. Boyer 《水文研究》2008,22(14):2410-2423
The stable isotopic composition of dissolved inorganic carbon (δ13C‐DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed δ13C‐DIC increased between 3–5‰ from the stream source to the outlet weir approximately 0·5 km downstream, concomitant with increasing pH and decreasing PCO2. An increase in δ13C‐DIC of 2·4 ± 0·1‰ per log unit decrease of excess PCO2 (stream PCO2 normalized to atmospheric PCO2) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO2 outgassing rather than exchange with atmospheric CO2 may be the primary cause of increased δ13C‐DIC values downstream when PCO2 of surface freshwater exceeds twice the atmospheric CO2 concentration. Although CO2 outgassing caused a general increase in stream δ13C‐DIC values, points of localized groundwater seepage into the stream were identified by decreases in δ13C‐DIC and increases in DIC concentration of the stream water superimposed upon the general downstream trend. In addition, comparison between snowmelt, early spring and summer seasons showed that DIC is flushed from shallow groundwater flowpaths during snowmelt and is replaced by a greater proportion of DIC derived from soil CO2 during the early spring growing season. Thus, in spite of effects from CO2 outgassing, δ13C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
292.
Sediment community metabolism associated with continental shelf hypoxia, Northern Gulf of Mexico 总被引:1,自引:0,他引:1
Gilbert T. Rowe Marta Elizabeth Cruz Kaegi John W. Morse Gregory S. Boland Elva G. Escobar Briones 《Estuaries and Coasts》2002,25(6):1097-1106
Net fluxes of respiratory metabolites (O2, dissolved inorganic carbon (DIC), NH4 +, NO3 ?, and NO2 ?) across the sediment-water interface were measured using in-situ benthic incubation chambers in the area of intermittent seasonal hypoxia associated with the Mississippi River plume. Sulfate reduction was measured in sediments incubated with trace levels of35S-labeled sulfate. Heterotrophic remineralization, measured as nutrient regeneration, sediment community oxygen consumption (SOC), sulfate reduction, or DIC production, varied positively as a function of temperature. SOC was inversely related to oxygen concentration of the bottom water. The DIC fluxes were more than 2 times higher than SOC alone, under hypoxic conditions, suggesting that oxygen uptake alone cannot be used to estimate total community remineralization under conditions of low oxygen concentration in the water column. A carbon budget is constructed that compares sources, stocks, transformations, and sinks of carbon in the top meter of sediment. A comparison of remineralization processes within the sediments implicates sulfate reduction as most important, followed by aerobic respiration and denitrification. Bacteria accounted for more than 90% of the total community biomass, compared to the metazoan invertebrates, due presumably to hypoxic stress. 相似文献
293.
In this article, we investigate the factors that lead to changes in the socioeconomic complexion of urban neighborhoods along four critical dimensions: crime, youth social distress, home ownership, and economic conditions. We argue that the dynamics of these dimensions are better apprehended simultaneously considering their potential mutual relationships and we propose a cross-lagged panel model approach within a structural equation modeling framework. Neighborhoods in Charlotte, North Carolina, are used as a case study, and change is modeled at several time lags throughout the 2000–2010 decade. Findings indicate that the model performs well and that it offers a very promising avenue for modeling the socioeconomic changes of neighborhoods that accounts for complex longitudinal effects as well as spatial dependencies. Specifically, it shows that lower human capital manifested by a decline in youth indicators is significant in explaining subsequent increases in crime and declines in economic indicators. Also, the predominance of housing stock constructed in the 1950s and 1960s is a significant trigger of declines across all neighborhood indicators. Finally, spatial spillover effects between neighborhoods are found to be short-lived and dissipate after a few years. 相似文献
294.
Elizabeth E Watson 《Area》2004,36(1):59-68
This paper discusses aspects of the theory and practice of foreign language learning for fieldwork in geography, a rarely discussed set of issues despite the personal investment that language learning requires from the researcher. In some cases of cross-cultural research, it may be particularly desirable to learn a vernacular language for which there are few (or no) study aids. However, there is little guidance about how learning such a language should be approached, or discussion of its significance for the subsequent research. Through a discussion of personal experience in Ethiopia, the paper describes one way of learning a vernacular language, and explores the advantages that this brought to the fieldwork and to the resulting research understandings. 相似文献
295.
Fernando F. Sodré Danielle C. Schnitzler Elizabeth W. O. Scheffer Marco T. Grassi 《Aquatic Geochemistry》2012,18(5):389-405
The goal of this work was to investigate the changes in copper behavior in Igua?u River, a body of water strongly affected by urban inputs. This work was carried out in a subtropical Brazilian watershed suffering with high loads of raw sewage discharges from the Metropolitan Region of Curitiba. A comparison between sampling sites located upstream and downstream from the urban region revealed that human inputs are able to modify the water chemistry of the river in a short distance basis, that is, approximately nine miles. Probably, the most important alterations were the creation of an anaerobic environment as well as the enhancement of humic-coated suspended solids. These two aspects were determinant to explain the high concentrations observed for particulate copper (57% of total recoverable copper) and dissolved copper sulfide species (13%) in the water column. Copper in the sediment was also higher in the downstream site, probably due to the sedimentation of the Cu-enriched particles. However, copper sulfides at the bottom sediment may also be a potential source for the metal in the water column due to the creation of anaerobic conditions in both compartments. Labile copper concentration was not affected by the changes in water chemistry. Despite the fact that sewage discharges motivate the enhancement of organic matter, but not the increase in potential complexing agents, additional ligands such as chloride, carbonates, and anthropogenic dissolved organic ligands can be now computed as a part of the labile fraction. 相似文献
296.
This article explores the dimensions of rapid urbanization in the Phoenix Standard Metropolitan Statistical Area (SMSA) over the past thirty years with respect to land use change. We devote primary attention to developing an understanding of what land transformations took place, the extent to which they occurred, and where they occurred. Our findings indicate that 32 percent of the Phoenix SMSA changed between 1970 and 2000. More than half of the overall change was from agriculture to some form of urban land use, and although a large percentage of the region remains open desert the parcels of desert are increasingly fragmented. This has significant implications for urban ecology and biodiversity. The growth indicates that rather than a pattern that reflects the agglomeration effects of a polycentric metropolis, the central business district of the City of Phoenix dominates the region. This has implications with respect to employment patterns, traffic congestion, and urban air quality and climate. 相似文献
297.
Zhenxue Dai Elizabeth Keating Carl Gable Daniel Levitt Jeff Heikoop Ardyth Simmons 《Hydrogeology Journal》2010,18(3):607-624
Based on the regional hydrogeology and the stratigraphy beneath the Los Alamos National Laboratory (LANL) site, New Mexico (USA), a site-scale groundwater model has been built with more than 20 stratified hydrofacies. A stepwise inverse method was developed to estimate permeabilities for these hydrofacies by coupling observation data from different sources and at various spatial scales including single-well test, multiple-well pumping test and regional aquifer monitoring data. Statistical analyses of outcrop permeability measurements and single-well test results were used to define the prior distributions of the parameters. These distributions were used to define the parameter initial values and the lower and upper bounds for inverse modeling. A number of inverse modeling steps were performed including the use of drawdown data from the pump tests at two wells (PM-2 and PM-4) separately, and a joint inversion coupling PM-2 and PM-4 pump test data and head data from regional aquifer monitoring. Parameter sensitivity coefficients for different data sets were computed to analyze if the model parameters can be estimated accurately with the data provided at different steps. The joint inversion offers a reasonable fit to all data sets. The uncertainty of estimated parameters for the hydrofacies is addressed with the parameter confidence intervals. 相似文献
298.
299.
Fundamental differences between SPH and grid methods 总被引:1,自引:0,他引:1
Oscar Agertz Ben Moore Joachim Stadel Doug Potter Francesco Miniati Justin Read Lucio Mayer Artur Gawryszczak rey Kravtsov Åke Nordlund Frazer Pearce Vicent Quilis Douglas Rudd Volker Springel James Stone Elizabeth Tasker Romain Teyssier James Wadsley Rolf Walder 《Monthly notices of the Royal Astronomical Society》2007,380(3):963-978
300.