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61.
Manuel?R.?Palacios-FestEmail author Simone?R.?Alin Andrew?S.?Cohen Brandon?Tanner Heather?Heuser 《Journal of Paleolimnology》2005,34(1):51-71
Fossil invertebrates from cores collected in Lake Tanganyika provide a record of probable nearshore ecological response to recent watershed deforestation and sediment erosion in several East African watersheds. We compared paleoecological profiles (primarily from ostracodes) from watersheds spanning a range of sizes and present-day deforestation disturbance levels to understand the timing and magnitude of faunal changes, and their relationship in time to terrestrially-derived disturbance indicators. Profiles from the Lubulungu and Nyasanga/Kahama Rivers (Tanzania) provide a record of faunal variability in watersheds that are currently undisturbed with respect to deforestation. These records indicate continuous faunal turnover through time. However, this pattern of turnover is accompanied by relatively high levels of diversity throughout the record, with no wholesale extinction events. Ostracode taphonomic data and other fossil abundance data from the Lubulungu area provide strong evidence in support of at least two episodes of lower lake levels, associated with episodes of Late Holocene aridity. Records from deltas of disturbed watersheds at the Kabesi River (Tanzania) and those of Northern Burundi all show a combination of profound and abrupt faunal turnover, in some cases accompanied by local extinction and establishment of a few dominant taxa. At the Mwamgongo River delta, fed from a very small, disturbed watershed, species turnover was subtler. In disturbed watershed cores showing abrupt faunal changes the transitions mostly occurred in the late 19th to early 20th centuries, predating the major mid-20th century increase in sediment mass accumulation rates, with the latter only correlated with changing fossil abundance and flux. However, the earlier faunal community changes are contemporaneous with both palynological and geochemical changes in the core profiles indicative of changing land-use patterns. This suggests that lacustrine ecosystem response to deforestation was a two-stage process, with an earlier phase of response to changing quality of sediments or dissolved matter being discharged from the watershed, and a subsequent phase responding to increased quantity of sediment. 相似文献
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Caroline Aubry-Wake Lauren D. Somers Haley Alcock Aspen M. Anderson Amin Azarkhish Samuel Bansah Nicole M. Bell Kelly Biagi Mariana Castaneda-Gonzalez Olivier Champagne Anna Chesnokova Devin Coone Tasha-Leigh J. Gauthier Uttam Ghimire Nathan Glas Dylan M. Hrach Oi Yin Lai Pierrick Lamontagne-Hallé Nicolas R. Leroux Laura Lyon Sohom Mandal Bouchra R. Nasri Nataša Popović Tracy E. Rankin Kabir Rasouli Alexis Robinson Palash Sanyal Nadine J. Shatilla Brandon Van Huizen Sophie Wilkinson Jessica Williamson Majid Zaremehrjardy 《水文研究》2020,34(8):2001-2006
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Michael DeMers Anna Klimaszewski‐Patterson Rebecca Richman Sean Ahearn Brandon Plewe André Skupin 《Transactions in GIS》2013,17(3):335-352
The UCGIS GIS&T Body of Knowledge document provided an opportunity for the GIS educational community to link course content and curricular sequencing to a catalog of subject matter. Focusing on learning objectives, a selection of relevant citations, and basic background for each topic, it has been used to a limited degree for both course topic selection and curriculum development. However, the static format of the document, lack of an index, and dated nature of the material limit its utility for education. Based on the success of our research team in developing a virtual platform for a new, more interactive, and collaborative environment to catalogue and interactively add to the body knowledge (Ahearn et al. 2013), this article describes efforts to develop a multi‐user virtual user environment that will add social presence to the experience. It describes the successes and failures of using Second Life as the initial platform for this work, illustrates the available interactions and limitations, and depicts ongoing efforts to move beyond Second Life for this development. Finally it discusses a possible methodology to leverage the power of virtual crowd sourcing within competitive gaming environments such as Unity to allow for the creation of on‐demand virtual 3‐D visualizations of GIS&T concepts in a digital Exploratorium. 相似文献
67.
Melting seasonal ground ice (SGI) in western Boreal Plains (WBP) peatlands can reduce the available energy at the surface by reducing potential evapotranspiration (PET). PET often exceeds annual precipitation in the WBP. Including this effect in hydrological models may be important in assessing water deficits. However, SGI melt and the timing of ice-free conditions vary spatially, which suggests PET spatial variability could be influenced by SGI. Understanding this potential linkage can help improve site scale PET in peatland hydrological models. The objectives of this paper were (a) to quantify the effect of ice thickness and melt rate on peatland PET; (b) quantify the spatial variability of SGI thickness and melt rate across spatial scales; and (c) assess how/if spatial variability in SGI thickness/melt rate affects site scale PET. Results from the sensitivity analysis indicated that SGI thickness had a bigger impact on reducing PET compared with the melt rate. Two SGI thickness values were used that were observed on site: 0.32 m, which was measured in a more treed area, and 0.18 m, which was in a more open area. The 0.32 m had an average PET reduction of 14 mm (±0.7), over the month of May, compared with 9 mm (±1 mm) when there was 0.18 m of SGI, which are 13.7 and 8.8% reductions, respectively. SGI thickness and melt rate, both exhibited large- and small-scale spatial variability. At the large scale, spatial patterns in SGI thickness appeared to be influenced by extensive shading from the adjacent hillslopes. Small scale, SGI thickness may be a function of tree proximity and the snowpack. Finally, net radiation, rather than SGI, appeared to be the main driver behind PET spatial variability. This work enhances our conceptual understanding of the role of SGI in WBP peatlands. Future work can use the findings to better inform peatland hydrological models, allowing for better representation of peatlands in regional-scale models. 相似文献
68.
Hydraulic fracturing has become an important technique for enhancing the permeability of hydrocarbon source rocks and increasing aquifer transmissivity in many hard rock environments where natural fractures are limited, yet little is known about the nature or behaviour of these hydraulically induced fractures as conduits to flow and transport. We propose that these fractures tend to be smooth based on observed hydraulic and transport behaviour. In this investigation a multi‐faceted approach was used to quantify the properties and characteristics of an isolated hydraulically induced fracture in crystalline rocks. Packers were used to isolate the fracture that is penetrated by two separate observation wells located approximately 33 m apart. A series of aquifer tests and an induced gradient tracer test were performed to better understand the nature of this fracture. Aquifer test results indicate that full recovery is slow because of the overall low permeability of the crystalline rocks. Drawdown tests indicate that the fracture has a transmissivity of 1–2 m2/day and a specific storage on the order of 2–9 × 10?7/m. Analysis of a potassium–bromide tracer test break through curve shows classic Fickian behaviour with minimal tailing analogous to parallel plate flow. Virtually all of the tracer was recovered, and the breakthrough curve dilution indicates that the swept area is only about 11% of a radial flow field and the estimated aperture is ≤0.5 mm, which implies a narrow linear flow region. These outcomes suggest that transport within these hydraulically induced ‘smooth’ fractures in crystalline rocks is rapid with minimal mixing, small local velocity fluctuations and no apparent diffusion into the host rock or secondary fractures. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
69.
The combination of tree canopy cover and a free water surface makes the subcanopy environment of flooded forested wetlands unlike other aquatic or terrestrial systems. Subcanopy vapour fluxes and energy budgets represent key controls on water level and understorey climate but are not well understood. In a permanently flooded forest in south‐eastern Louisiana, USA, an energy balance approach was used to address (a) whether evaporation from floodwater under a forest canopy is solely energy limited and (b) how energy availability was modulated by radiation and changes in floodwater heat storage. A 5‐month continuous measurement period (June–November) was used to sample across seasonal changes in canopy activity and temperature regimes. Over this period, the subcanopy airspace was humid, maintaining saturation vapour pressure for 28% of the total record. High humidity coupled with the thermal inertia of surface water altered both seasonal and diel energy exchanges, including atypical phenomena such as frequent day‐time vapour pressure gradients towards the water surface. Throughout the study period, nearly all available energy was partitioned to evaporation, with minimal sensible heat exchange. Monthly mean evaporation ranged from 0.7 to 1.7 mm/day, peaking in fall when canopy senescence allowed greater radiation transmission; contemporaneous seasonal temperature shifts and a net release of stored heat from the surface water resulted in energy availability exceeding net radiation by 30% in October and November. Relatively stable energy partitioning matches Priestley–Taylor assumptions for a general model of evaporation in this ecosystem. 相似文献
70.
The formation sequence of prominent ridges and other tectonic lineaments on the southern portion of the leading hemisphere of Europa is determined from cross-cutting relationships. These selected features formed fairly recently relative to most of the surface; older lineaments no longer retain clear evidence of cross-cutting. If we assume that this sequence represents the order of formation of cracks that underlie each lineament, and that the orientation of each crack was determined by tidal stress whose azimuth varies monotonically counter-clockwise with time, the azimuth must have rotated more than 740°, which would correspond to the change in tidal stress over two periods of nonsynchronous rotation (relative to the direction of Jupiter). However, that interpretation is not necessarily compelling, because the observed orientations of cross-cutting lineaments are not densely spaced over these cycles; in fact, the sequence would fit nearly as well into an arbitrary model with rotation in the opposite sense from that predicted by theory. This tectonic record may have formed over many more rotational cycles, such that typically only a few cracks form per cycle, which would be consistent with evidence from considerations of cycloidal crack patterns. Sets of cracks that cluster near certain azimuth orientations appear to be parts of globe-encircling lineament systems and may result from other effects, perhaps polar wander that occurred rapidly relative to nonsynchronous rotation. 相似文献