Multiple Residence and Cyclical Migration: A Life Course Perspective*   总被引：1，自引：0，他引：1  

Kevin E. McHugh  Timothy D. Hogan  Stephen K. Happel 《The Professional geographer》1995,47(3):251-267

As a result of restrictive time-space bounds in viewing migration, surprisingly little is known about the tempos and rhythms of geographical mobility in America. We discuss limitations of the conventional definition of migration and develop a life course framework of multiple residence and cyclical migration. Results of an Arizona-based case study reveal that multiple residence is common and more diverse than the annual influx of elderly snowbirds. Coming to grips with multiple residence and recurrent mobility in the United States represents a fundamental challenge in population and migration studies.  相似文献   

Two Approaches to Design of Monitoring Networks   总被引：5，自引：0，他引：5  

Timothy B. Spruill  Lucila Candela 《Ground water》1990,28(3):430-442

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Atrazine Used as a Tracer of Induced Recharge     

Diana Duncan  Darryll T. Pederson  Timothy R. Shepherd  James D. Can 《Ground Water Monitoring & Remediation》1991,11(4):144-150

The city of Lincoln draws water from a well field along the banks of the Platte River in east-central Nebraska. We have been able to follow the infusion of atrazine into this well field under induced recharge. Samples of water from the river, several monitoring wells and a production well were analyzed by GC/MS and solid phase extraction and found to change in concentration over the range 0.1 and 5.0 ppb of atrazine through the spring, summer, and fall 1989. Increases in the concentration of atrazine relating to precipitation events in the Platte River Basin were observable in the well water samples and ultimately in Lincoln municipal tap water. Atrazine from the river was seen to move via induced recharge and into well field ground water and away from the river at an observable rate. The concentration of atrazine in the river is dependent upon time of year and precipitation in the river basin.  相似文献   

Memorial to Felix Chayes     

E. H. Timothy Whitten 《Mathematical Geology》1994,26(2):157-160

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Late Holocene history of Waldsea Lake,Saskatchewan, Canada     

William M. Last  Timothy H. Schweyen 《Quaternary Research》1985,24(2):219-234

The post-Hypsithermal history of Waldsea Lake, a saline meromictic lake located in south-central Saskatchewan, has been deduced from a study of the changes in physical, mineralogical, and paleobiological parameters in sediment cores from the basin. Six lithostratigraphic units and three palynological zones are identified in the most recent sediment. These units and zones indicate that a shallow hypersaline lake with extensive mudflats existed about 4000 yr B.P. In response to the subsequent trend toward a cooler and wetter climate, deeper water conditions ensued, and by about 3000 yr ago a relatively deep stratified lake occupied the Waldsea Basin. A short climatic reversal about 2500 yr B.P. again caused low-water and mudflat conditions, but by 2000 yr ago the lake had regained its higher levels. The past 2000 yr of Waldsea's history have been relatively uneventful, except for a minor lowering of the lake about 700 yr B.P.  相似文献   

Instrumental temperature records at two sites in Central Canada: 1768 to 1910     

Timothy F. Ball  Roger A. Kingsley 《Climatic change》1984,6(1):39-56

As a result of affiliation between the Hudson's Bay Company and the Royal Society a relatively large number of instrumental temperature records are available from York Factory and Churchill Factory on the southwest of Hudson Bay beginning in 1768. The nature of these records, details of the instruments and information about the observers are presented. The major difficulty with the records is that the number of observations and the time of observation varied considerably. Adjustment factors were calculated for all of the combinations using a modern record maintained at the Churchill airport. By combining the Hudson's Bay Company record with data recorded by members of the Royal Canadian Mounted Police after 1852, and up to 1910, a long and relatively continuous record of daily and monthly average temperatures has been created for Central Canada.  相似文献   

The nursery role of seagrass meadows in the upper and lower reaches of the Chesapeake Bay     

Kenneth L. Heck  Timothy A. Thoman 《Estuaries and Coasts》1984,7(1):70-92

A two-year trawling and gill-netting study of vegetated and unvegetated bottoms near Parson’s Island, Maryland and near the mouth of the York River, Virginia was carried out to assess the nursery function of submerged vegetation for populations of fishes and decapod crustaceans in the Chesapeake Bay. Results revealed that vegetated bottoms supported substantially larger numbers of decapods, but not fishes, than unvegetated substrates. The lower Bay grassbed was an important nursery area for juvenile blue crabs, although neither of the grassbeds functioned as a nursery for commercially or recreationally valuable fishes. Our results suggest that: (1) further decreases in lower Bay Seagrass biomass would result in reduced numbers of adult blue crabs, but should not substantially affect populations of valuable fish species; (2) additional decreases in Upper Bay submerged vegetation should not produce dramatic change in the population sizes of either adult blue crabs or fishes.  相似文献   

Inverted-polarity electrical structures in thunderstorms in the Severe Thunderstorm Electrification and Precipitation Study (STEPS)     

W. David Rust  Donald R. MacGorman  Eric C. Bruning  Stephanie A. Weiss  Paul R. Krehbiel  Ronald J. Thomas  William Rison  Timothy Hamlin  Jeremiah Harlin 《Atmospheric Research》2005,76(1-4):247

Balloon-borne electric field soundings and lightning mapping data have been analyzed for three of the storms that occurred in the Severe Thunderstorm Electrification and Precipitation Study field program in 2000 to determine if the storms had inverted-polarity electrical structures. The polarities of all or some of the vertically stacked charge regions in such storms are opposite to the polarities observed at comparable heights in normal storms. Analyses compared the charge structures inferred from electric field soundings in the storms with charges inferred from three-dimensional lightning mapping data. Charge structures were inferred from electric field profiles by combining the one-dimensional approximation of Gauss's law with additional information from three-dimensional patterns in the electric field vectors. The three different ways of inferring the charge structure in the storms were found to complement each other and to be consistent overall. Charge deposition by lightning possibly occurred and increased the charge complexity of one of the storms.Many of the cloud flashes in each case were inverted-polarity flashes. Two storms produced ground flash activity comprised predominantly of positive ground flashes. One storm, which was an isolated thunderstorm, produced inverted-polarity cloud flashes, but no flashes to ground. The positive and negative thunderstorm charge regions were found at altitudes where, respectively, negative and positive charge would be found in normal-polarity storms. Thus, we conclude that these storms had anomalous and inverted-polarity electrical structures. Collectively, these three cases (along with the limited cases in the refereed literature) provide additional evidence that thunderstorms can have inverted-polarity electrical structures.  相似文献   

Bedform morphology of salmon spawning areas in a large gravel-bed river     

Timothy P. Hanrahan   《Geomorphology》2007,86(3-4):529-536

While the importance of river channel morphology to salmon spawning habitat is increasingly recognized, quantitative measures of the relationships between channel morphology and habitat use are lacking. Such quantitative measures are necessary as management and regulatory agencies within the Pacific Northwest region of the USA, and elsewhere, seek to quantify potential spawning habitat and develop recovery goals for declining salmon populations. The objective of this study was to determine if fall Chinook salmon (Oncorhynchus tshawytscha) spawning areas in the Snake River, Idaho, USA, were correlated with specific bedform types at the pool–riffle scale. A bedform differencing technique was used to objectively quantify the longitudinal riverbed profile into four distinct pool–riffle units that were independent of discharge. The vertical location of thalweg points within these units was quantified with a riffle proximity index. Chinook salmon spawning areas were mapped and correlated with the pool–riffle units through the use of cross-tabulation tables. The results indicate that 84% of fall Chinook salmon spawning areas were correlated with riffles (χ² = 57.5, df = 3, p < 0.001), with 53% of those areas located on the upstream side of riffle crests. The majority of Snake River fall Chinook salmon spawning occurred at elevations greater than 80% of the difference in elevation between the nearest riffle crest and pool bottom. The analyses of bedform morphology will assist regional fish managers in quantifying existing and potential fall Chinook salmon spawning habitat, and will provide a quantitative framework for evaluating general ecological implications of channel morphology in large gravel-bed rivers.  相似文献   

Synoptic circulation and stream discharge in the Great Lakes basin, USA     

R. V. Rohli  A. J. Vega  M. R. Binkley  S. D. Britton  H. E. Heckman  J. M. Jenkins  Y. Ono  D. E. Sheeler 《Applied geography (Sevenoaks, England)》2001,21(4)

Impacts on water resources caused by human activity, natural climate variation and long-term climate change are unclear in the US Great Lakes region. Improved understanding of the impact of atmospheric circulation on stream discharge variability into the Lakes is thus important. In this analysis, monthly surface and mid-tropospheric circulation patterns suggest that surface pressure variations over Missouri and Illinois are most strongly correlated to discharge. The mid-tropospheric patterns most directly related to discharge place the Great Lakes in a trough-to-ridge flow pattern. The analysis confirms that at this scale, lee shore advection resulting in ‘lake-effect’ precipitation is not very important to regional discharge, and neither are variations in the Pacific–North American teleconnection.  相似文献