A field sampling strategy for semivariogram inference of fractures in rock outcrops     

Ryan M. Pollyea  Jerry P. Fairley  Robert K. Podgorney  Travis L. McLing 《Stochastic Environmental Research and Risk Assessment (SERRA)》2013,27(7):1735-1740

The stochastic continuum (SC) representation is one common approach for simulating the effects of fracture heterogeneity in groundwater flow and transport models. These SC reservoir models are generally developed using geostatistical methods (e.g., kriging or sequential simulation) that rely on the model semivariogram to describe the spatial variability of each continuum. Although a number of strategies for sampling spatial distributions have been published in the literature, little attention has been paid to the optimization of sampling in resource- or access-limited environments. Here we present a strategy for estimating the minimum sample spacing needed to define the spatial distribution of fractures on a vertical outcrop of basalt, located in the Box Canyon, east Snake River Plain, Idaho. We used fracture maps of similar basalts from the published literature to test experimentally the effects of different sample spacings on the resulting semivariogram model. Our final field sampling strategy was based on the lowest sample density that reproduced the semivariogram of the exhaustively sampled fracture map. Application of the derived sampling strategy to an outcrop in our field area gave excellent results, and illustrates the utility of this type of sample optimization. The method will work for developing a sampling plan for any intensive property, provided prior information for a similar domain is available; for example, fracture maps or ortho-rectified photographs from analogous rock types could be used to plan for sampling of a fractured rock outcrop.  相似文献   

Impacts of projected climate change on sediment yield and dredging costs          下载免费PDF全文

Travis A. Dahl  Anthony D. Kendall  David W. Hyndman 《水文研究》2018,32(9):1223-1234

Changes in climate may significantly affect how sediment moves through watersheds into harbours and channels that are dredged for navigation or flood control. Here, we applied a hydrologic model driven by a large suite of climate change scenarios to simulate both historical and future sediment yield and transport in two large, adjacent watersheds in the Great Lakes region. Using historical dredging expenditure data from the U.S. Army Corps of Engineers, we then developed a pair of statistical models that link sediment discharge from each river to dredging costs at the watershed outlet. Although both watersheds show similar slight decreases in streamflow and sediment yield in the near‐term, by Mid‐Century, they diverge substantially. Dredging costs are projected to change in opposite directions for the two watersheds; we estimate that future dredging costs will decline in the St. Joseph River by 8–16% by Mid‐Century but increase by 1–6% in the Maumee River. Our results show that the impacts of climate change on sediment yield and dredging may vary significantly by watershed even within a region and that agricultural practices will play a large role in determining future streamflow and sediment loads. We also show that there are large variations in responses across climate projections that cause significant uncertainty in sediment and dredging projections.  相似文献   

Increasing temperature and flow management alter mercury dynamics in East Fork Poplar Creek     

Scott C. Brooks  Carrie L. Miller  Ami L. Riscassi  Kenneth A. Lowe  Johnbull O. Dickson  Grace E. Schwartz 《水文研究》2021,35(8):e14344

East Fork Poplar Creek (EFPC) is a mercury (Hg) contaminated creek in east Tennessee, USA. Stream restoration activities included the initiation of a flow management programme in 1996 in which water from a nearby lake was pumped to the head of the creek. We conducted regular water sampling for 2 years along the length of EFPC during active flow management and for 5 years after flow management stopped. Total Hg and total monomethylmercury (MMHg) concentration and flux decreased in the uppermost reaches of EFPC that were closest to the point of water addition. Most water quality parameters, including DOC concentration, remained unchanged after flow management termination. Nevertheless, SUVA₂₅₄, a measure of dissolved organic matter (DOM) composition, increased and coincided with increased dissolved Hg (Hg_D) concentration and flux and decreased Hg solid-water partitioning coefficients throughout EFPC. Higher SUVA₂₅₄ and Hg_D concentration have potential implications for bioavailability and MMHg production. Total and dissolved MMHg concentrations increased in lower reaches of EFPC after the end of flow management and these increases were most pronounced during spring and early summer when biota are more susceptible to exposure and uptake. A general warming trend in the creek after active flow management ended likely acted in concert with higher Hg_D concentration to promote higher MMHg concentration. Total and dissolved MMHg concentrations were positively correlated with water temperature above a threshold value of 10°C. Concentration changes for Hg and MMHg could not be accounted for by changes in creek discharge that accompanied the cessation of flow management. In addition to the changing DOM composition in-stream, other watershed-scale factors likely contributed to the observed patterns, as these changes occurred over months rather than instantaneously after flow management stopped. Nevertheless, similar changes in MMHg have not been observed in a tributary to EFPC.  相似文献   

Temporal and spatial variability of Chl-a and SST on the South Atlantic Bight: Revisiting with cloud-free reconstructions of MODIS satellite imagery     

Travis N. Miles  Ruoying He 《Continental Shelf Research》2010

Daily, cloud-free data interpolating empirical orthogonal function (DINEOF) reconstructions of sea-surface temperature (SST) and chlorophyll (Chl-a) satellite imagery are compiled into monthly mean images for a six-year period (2003–2008) and used to identify their spatial and temporal variability on the South Atlantic Bight. Monthly-mean SST has the highest variability on the inner-shelf, decreasing seaward approaching the more stable temperatures of the Gulf Stream (GS). Monthly-mean Chl-a concentrations are similarly highest on the inner shelf throughout the year and decrease cross-shelf toward the nutrient depleted open ocean. Empirical orthogonal function (EOF) analyses on SST and Chl-a show a clear seasonal cycle in their 1st mode of variability, with SST lagging behind Chl-a by approximately one month. The 1st EOF modes account for 95.8% and 46.4% variance of SST and Chl-a, respectively. Chl-a EOF mode 1 in particular shows a highly regionalized spatial pattern with values on the central SAB clearly out of phase with the southern and northern SAB. This regional difference is likely a result of shelf geometry and stratification, which modulate GS influence on the shelf. SST EOF mode 2 exhibits a seasonal cycle as well, which previous studies have shown to be a function of local wind. Chl-a EOF mode 2 is well correlated with the cumulative river transport onto the SAB, but accounts for a relatively small 10.8% of Chl-a variability.  相似文献   

The effect of submerged plateaux on Pleistocene gyral circulation and sea-surface temperatures in the Southwest Pacific     

Bruce W. Hayward  George H. Scott  Martin P. Crundwell  James P. Kennett  Lionel Carter  Helen L. Neil  Ashwaq T. Sabaa  Kate Wilson  J. Stuart Rodger  Grace Schaefer  Hugh R. Grenfell  Qianyu Li 《Global and Planetary Change》2008,63(4):309-316

Uniquely in the Southern Hemisphere the New Zealand micro-continent spans the interface between a subtropical gyre and the Subantarctic Circumpolar Current. Its 20° latitudinal extent includes a complex of submerged plateaux, ridges, saddles and basins which, in the present interglacial, are partial barriers to circulation and steer the Subtropical (STF) and Subantarctic (SAF) fronts. This configuration offers a singular opportunity to assess the influence of bottom topography on oceanic circulation through Pleistocene glacial – interglacial (G/I) cycles, its effect on the location and strength of the fronts, and its ability to generate significant differences in mixed layer thermal history over short distances.For this study we use new planktic foraminiferal based sea-surface temperature (SST) estimates spanning the past 1 million years from a latitudinal transect of four deep ocean drilling sites. We conclude that: 1. the effect of the New Zealand landmass was to deflect the water masses south around the bathymetric impediments; 2. the effect of a shallow submerged ridge on the down-current side (Chatham Rise), was to dynamically trap the STF along its crest, in stark contrast to the usual glacial–interglacial (G–I) meridional migration that occurs in the open ocean; 3. the effect of more deeply submerged, downstream plateaux (Campbell, Bounty) was to dynamically trap the SAF along its steep southeastern margin; 4. the effects of saddles across the submarine plateaux was to facilitate the development of jets of subtropical and subantarctic surface water through the fronts, forming localized downstream gyres or eddies during different phases in the G–I climate cycles; 5. the deep Pukaki Saddle across the Campbell-Bounty Plateaux guided a branch of the SAF to flow northwards during each glacial, to form a strong gyre of circumpolar surface water in the Bounty Trough, especially during the mid-Pleistocene Climate Transition (MIS 22-16) when exceptionally high SST gradients existed across the STF; 6. the shallower Mernoo Saddle, at the western end of the Chatham Rise, provided a conduit for subtropical water to jet southwards across the STF in the warmest interglacial peaks (MIS 11, 5.5) and for subantarctic water to flow northwards during glacials; 7. although subtropical or subantarctic drivers can prevail at a particular phase of a G–I cycles, it appears that the Antarctic Circumpolar Current is the main influence on the regional hydrography.Thus complex submarine topography can affect distinct differences in the climate records over short distances with implications for using such records in interpreting global or regional trends. Conversely, the local topography can amplify the paleoclimate record in different ways in different places, thus enhancing its value for the study of more minor paleoceanographic influences that elsewhere are more difficult to detect. Such sites include DSDP 594, which like some other Southern Ocean sites, has the typical late Pleistocene asymmetrical saw-tooth G–I climate pattern transformed to a gap-tooth pattern of quasi-symmetrical interglacial spikes that interrupt extended periods of minimum glacial temperatures.  相似文献   

Public health in the Canadian Arctic: contributions from International Polar Year research     

Sandra Owens  Philippe De Wals  Grace Egeland  Christopher Furgal  Yang Mao  Gerald Y. Minuk  Paul A. Peters  Manon Simard  éric Dewailly 《Climatic change》2012,115(1):259-281

The contributions of several IPY health projects are highlighted in the context of population health indicators for Inuit Nunangat. Food and housing are two critical social determinants of health contributing to health issues on many levels. The nutritional deficit associated with food insecurity and the transition away from traditional and towards market foods represents a dual risk with profound consequences. Knowledge of the physiological benefits associated with traditional food consumption is increasing, including for mental health and chronic disease. Ensuring the safety of traditional foods in terms of zoonotic diseases is thus highly valuable and efforts to institute adequate monitoring and address knowledge gaps are underway. Acute respiratory disease among the young remains a significant public health issue with potential long term effects. The human papilloma virus is manifesting itself among women across northern Canada with high risk types that are more similar to profiles observed in Europe than in North America with possible implications for immunization programs. Despite a high prevalence of Hepatitis B virus infections among residents of Inuit Nunangat, the outcomes appear to be relatively benign. Communication of new knowledge on the manifestation of this virus among northern populations is provided to health care providers in the North through modern technology.  相似文献   

Historical Land Use,Nitrogen, and Coastal Eutrophication: A Paleoecological Perspective     

Grace S. Brush 《Estuaries and Coasts》2009,32(1):18-28

Organisms and chemicals preserved in sediment cores from the Chesapeake estuary in mid-Atlantic USA are consistent with a precolonial landscape covered with a diversity of forests and marshes, large and small. During the past 300 years, many of the wetlands have been drained, and the landscape was converted to agricultural fields and urban and suburban development. During this time, sources of nitrogen have diversified, and loadings have increased. Since precolonial time, the mesohaline estuary has become increasingly eutrophic and anoxic. Estuaries and coastal regions throughout the world have experienced similar conditions in their recent history. These changes are recorded in Chesapeake sediment cores by increases in ragweed pollen, dry taxa, sedimentation rates, nitrogen influxes, and a major change in estuarine autotrophs from benthic to planktonic. In many areas, attempts to reverse estuarine eutrophication and anoxia have centered on restoring streams and riparian areas and reducing fertilizer use on agricultural lands. However, data from soils and historical reports and the paleoecological record suggest that to reduce the effects of modern nitrogen inputs, it may be necessary to locate and enhance denitrifying areas throughout the watershed.  相似文献   

Short-Term Response of a Downstream Marine System to the Partial Opening of a Tidal-River Causeway     

Travis G. Gerwing  Diana J. Hamilton  Myriam A. Barbeau  Katy Haralampides  Gordon Yamazaki 《Estuaries and Coasts》2017,40(3):717-725

The spillway gates of the Petitcodiac Causeway, a hydraulic structure ~35 km upstream of the mouth of the Petitcodiac River in New Brunswick, Canada, were permanently opened in April 2010. The short-term effect opening the spillway gates had on downstream intertidal mudflats of the upper Bay of Fundy was investigated. Specifically, a multivariate before-after-control-impact design was used to determine if opening the spillway gates affected the invertebrate community (crustaceans, polychaetes, and molluscs), abiotic sediment conditions (sediment water content, mean particle size, penetrability, and depth of the apparent redox potential discontinuity), or resource availability (sediment chlorophyll a concentration and organic matter content) of five intertidal mudflats (two impacted sites, three reference sites) spanning Chignecto Bay, the northern arm of the upper Bay of Fundy, up to 5 months post-opening. No biologically or statistically meaningful differences were detected between impacted and reference sites for any of the measured variables. This suggests that opening the causeway did not have a quantifiable impact on these intertidal mudflats, at least within half a year of the opening. This is likely a result of the macrotidal nature of the Bay of Fundy that overwhelmed any immediate changes to hydrodynamics that occurred after the opening of the causeway gates.  相似文献   

Biogeomorphic controls on sedimentation and substrate on a vegetated tidal freshwater delta in upper Chesapeake Bay     

Gregory B. Pasternack  Grace S. Brush 《Geomorphology》2002,43(3-4)

Classic geomorphic theory on the dynamics of delta evolution posits a purely physical mechanism for spatial and temporal patterns of sediment accumulation over decades to centuries. Meanwhile, intertidal marsh vegetation that grows on deltas is well known to influence short-term fluid mechanics and sediment transport. This dichotomy points toward a large gap in the understanding of the role of vegetation in delta evolution as a function of spatial and temporal scale. In the research reported here, substrate characteristics and seasonal sedimentation rates in a tidal freshwater delta at the head of a Chesapeake Bay tributary were studied to assess the existence and extent of physical–ecological interactions on a delta over the seasonal to interannual time scale. Both vegetation data and sediment variables showed significant spatial variations at this time scale. When multiple regression analysis was used to compare vegetated versus nonvegetated conditions on the studied delta, 84% of the spatial variation in sedimentation with vegetation was explained by plant association and distance to the nearest distributary channel. In contrast, only 33% of the spatial variation in sedimentation could be explained when no vegetation was present, and in that case, the dominant variable was distance to the subtidal front. Spatial variability in organic content was less sensitive to vegetation and strongly influenced by the distance to the subtidal front. Substrate grain size parameters were explained by distance to the subtidal front and to the nearest distributary channel. This research demonstrates that sediment sequestration, and thus delta evolution, is highly predictable at the habitat scale and is driven by a strong interplay between abiotic and biotic variables.  相似文献   

Comparisons of ²¹⁰Pb and pollen methods for determining rates of estuarine sediment accumulation     

Grace S. Brush  E.Ann Martin  Ruth S. DeFries  Cynthia A. Rice 《Quaternary Research》1982,18(2):196-217

Comparisons of sedimentation rates obtained by ²¹⁰Pb and pollen analyses of 1-m cores collected throughout the Potomac Estuary show good agreement in the majority of cores that can be analyzed by both methods. Most of the discrepancy between the methods can be explained by the analytical precision of the ²¹⁰Pb method and by the exactness with which time horizons can be identified and dated for the pollen method. X-radiographs of the cores and the distinctness of the pollen horizons preclude significant displacement by reworking and/or mixing of sediments. Differences between the methods are greatest where uncertainties exist in assigning a rate by one or both methods (i.e., ²¹⁰Pb trends and/or “possible” horizon assignments). Both methods show the same relative rates, with greater sediment accumulation more common in the upper and middle estuary and less toward the mouth. The results indicate that geochronologic studies of estuarine sediments should be preceded by careful observation of sedimentary structures, preferably by X-radiography, to evaluate the extent of mixing of the sediments. Time horizons, whether paleontologic or isotopic, are generally blurred where mixing has occurred, precluding precise identification. Whenever possible, two methods should be used for dating sediments because a rate, albeit erroneous, can be obtained isotopically in sediments that are mixed; accurate sedimentation rates are also difficult to determine where the time boundary is a zone rather than a horizon, where the historical record does not provide a precise date for the pollen horizon, or where scouring has removed some of the sediment above a dated pollen horizon.  相似文献