Low Gradient Areas Research Articles

Multi-Scenario Prediction of Landscape Ecological Risk in the Sichuan-Yunnan Ecological Barrier Based on Terrain Gradients

Land use changes induced by human activities change landscape patterns and ecological processes, threatening regional and global ecosystems. Terrain gradient and anthropogenic multi-policy regulation can have a pronounced effect on landscape components. Forecasting the changing trend of landscape ecological risk (LER) is important for national ecological security and regional sustainability. The present study assessed changes in LER in the Sichuan-Yunnan Ecological Barrier over a 20-year period using land use data from 2000, 2010, and 2020. The enhanced Markov-PLUS (patch-generating land use simulation) model was used to predict and analyze the spatial distribution pattern of LER under the following three scenarios. These were business-as-usual (BAU), urban development and construction (UDC), and ecological development priority (EDP) in 2030. The influence of terrain conditions on LER was also explored. The results showed that over the past 20 years, the LER index increased and then decreased and was dominated by medium and low risk, accounting for more than 70% of the total risk-rated area. The highest and higher risk areas for the three future scenarios have increased in spatial extent. The UDC scenario showed the largest increase of 3341.13 km2 and 2684.85 km2, respectively. The highest-risk level has a strong selectivity for low gradients, with high-level risks more likely to occur at low gradients. The response of ecological risk to gradient changes shows a positive correlation distribution for high-gradient areas and a negative correlation distribution for low-gradient areas. The influence of future topographic gradient changes on LER remains significant. The value of multiscale geographically weighted regression (MGWR) for identifying the spatial heterogeneity of terrain gradient and LER is highlighted. It can play an important role in the formulation of scientific solutions for LER prevention and of an ecological conservation policy for mountainous areas with complex terrain.

Performance Evaluation Criteria for Triangular Microchannels with Smoothed Corners

The manufacturing capabilities available today for microchannels make it possible to produce in a comparatively easy, quick and cheap way several cross-sections, possibly allowing modifications of the macro-geometry. Another way is smoothing the corners of polygona cross-sections: this eliminates low-gradient areas and increases transport phenomena, i.e. frictional losses and heat transfer. Several ways of assessing the relative performance of the new shape in comparison to the original have been suggested over the years, among which are Performance Evaluation Criteria (PEC), as proposed by Bergles and Webb. PEC are based on a first-law analysis and aim at extremising the thermal power, heat exchange area, inlet temperature difference or pumping power under varying constraints. In this work equilateral triangular microchannels with uniform wall temperature are considered, through which a liquid flows in fully-developed, steady laminar regime. The cross-sectional area has its corners progressively rounded, and the velocity and temperature profiles are determined, in order to compute the Poiseuille, Nusselt and Stanton numbers, which are then employed in computing the objective functions for some PEC.

A mesh optimization method using machine learning technique and variational mesh adaptation

Computational mesh is an important ingredient that affects the accuracy and efficiency of CFD numerical simulation. In light of the introduced large amount of computational costs for many adaptive mesh methods, moving mesh methods keep the number of nodes and topology of a mesh unchanged and do not increase CFD computational expense. As the state-of-the-art moving mesh method, the variational mesh adaptation approach has been introduced to CFD calculation. However, quickly estimating the flow field on the updated meshes during the iterative algorithm is challenging. A mesh optimization method, which embeds a machine learning regression model into the variational mesh adaptation, is proposed. The regression model captures the mapping between the initial mesh nodes and the flow field, so that the variational method could move mesh nodes iteratively by solving the mesh functional which is built from the estimated flow field on the updated mesh via the regression model. After the optimization, the density of the nodes in the high gradient area increases while the density in the low gradient area decreases. Benchmark examples are first used to verify the feasibility and effectiveness of the proposed method. And then we use the steady subsonic and transonic flows over cylinder and NACA0012 airfoil on unstructured triangular meshes to test our method. Results show that the proposed method significantly improves the accuracy of the local flow features on the adaptive meshes. Our work indicates that the proposed mesh optimization approach is promising for improving the accuracy and efficiency of CFD computation.

A New Early Stage Diabetic Retinopathy Diagnosis Model Using Deep Convolutional Neural Networks and Principal Component Analysis

Diabetic retinopathy (DR) is a disease of the retina, which leads over time to vision problems such retinal detachment, vitreous hemorrhage, glaucoma, and in worse cases leads to blindness, which can initially be controlled by periodic DR-screening. Early diagnosis will lead to greater control of the disease, whereas performing retinal examinations on all diabetic patients is an unattainable need, as diabetes is a chronic disease and its global prevalence has been steadily increasing over the past few decades. According to recent World Health Organization statistics, about 422 million people worldwide have diabetes, the majority living in low-and middle-income countries. This paper proposes a new strategy that brings the strength of convolutional neural networks (CNNs) to the diagnosis of DR. Coupled with using principal component analysis (PCA) that performs dimension reduction to improve the diagnostic accuracy, the proposed model exploiting edge-preserving guided image filtering (E-GIF) that performs as a contrast enhancement mechanism, and in addition to smoothing low gradient areas, it also accentuates strong edges. Diabetic retinopathy causes progressive damage to the blood vessels in the retina to the extent that it leaves traces and lesions in the tissues of the retina. These lesions appear in the form of edges and when processing retinal images, we seek to accentuate these edges to enable better diagnosis of diabetic retinopathy symptoms. A new CNN architecture with residual connections is used, which performs very well in diagnosing DR. The proposed model is named with RUnet-PCA: Residual U-net Deep CNN with Principal Component Analysis. The well-known AlexNet, VggNet-s, VggNet-16, VggNet-19, GoogleNet, and ResNet models were adopted for comparison with the proposed model. Publicly available Kaggle dataset was employed for training exploring the DR diagnosis accuracy. Experimental results show that the proposed RUnet-PCA model achieved a diagnosis accuracy of 98.44% and it was extremely robust and promising in comparison to other diagnosis methods.

Efficiency of a siphon drainage system under conditions in a low gradient area

The drainage system designs and their function principles at the experimental production plot for the siphon- and vacuum-assisted drainage in the Mugan-Salyan massif characterized by the nongradient area have been described. The surveys are carried out in the natural environments during the vegetation period for the irrigation of cotton occupying the entire area of the experimental plot. A high efficiency of the siphonand vacuum-assisted drainage system has been calculated. In addition, the drain discharge and the discharge module are closely connected to the existing water pressure value in the interdrain. In the vegetation period, the maximum and minimum water discharges comprised 11.1 L/sec and 2.8 L/sec, respectively. The drainage module value varied from 0.14 to 0.55 L/sec hectare. It has been ascertained that the drainage system operation provides the opportunity to lower the depth of the groundwater table within the interdrain up to 250–280 cm (the initial depth is 120–150 cm). The possibilities of significant increases in the flushing intensity and the reduction of the terms of the regulated ameliorative activities within the certain hydrogeological regime have been proven.

Spatial patterns of vegetation, soils, and microtopography from terrestrial laser scanning on two semiarid hillslopes of contrasting lithology

Shrublands in semiarid regions are heterogeneous landscapes consisting of infertile bare areas separated by nutrient rich vegetated areas known as resource islands. Spatial patterns in these landscapes are structured by feedbacks driven by the transport of water and nutrient resources from the intershrub space to areas below shrubs, and the retention of these resources to locally drive productivity and tight biogeochemical cycles. Most understanding of plant‐soil feedbacks is based predominantly on studies of low topographic gradient landscapes, and it is unclear whether the patterns of association between soils and vegetation, and the autogenic processes that create them, also occur on more steeply sloping terrain. Here we analyze the spatial patterns of soils, vegetation, and microtopography on hillslopes of contrasting lithology (one granite at 16°, one schist at 27°) in the Sonoran desert foothills of the Catalina Mountains. We also describe a method of extracting vegetation density from terrestrial laser scanning point cloud data at 5 cm × 5 cm scales and find that it correlates well with soil organic carbon measurements. Vegetation was associated with microtopographic mounds (relative to the mean slope) extending 0.3 m downslope and 1.8 m (schist) and 0.9 m (granite) upslope on the study hillslopes. Soils below the shrub canopies exhibited 2–3 times more soil organic matter and 2–4 times higher hydraulic conductivity than the interspaces. Soils enriched with organic matter were found to extend at least two canopy radii downslope of woody shrubs, but not upslope. These plumes were clearest in the lower gradient granite site where vegetation mounds created distinct patterns of microtopographic convergence and divergence. At the steeper schist site, microtopography appeared to have a weaker control on topographic flow accumulation. Collectively, our findings suggest that the spatial structure of association between soils and microtopography and vegetation on these slopes exhibit many of the features observed in lower gradient areas. However, microtopography and soils are more asymmetric along the downslope axis of the hillslopes than lower gradient areas and vary with lithology. Alluvial and colluvial processes are likely more important in shaping vegetation and soil dynamics on hillslopes, and these factors need further consideration in scaling results to the landscape level.

Alternative Approach for Satellite Cloud Classification: Edge Gradient Application

Global atmospheric heat exchanges are highly dependent on the variation of cloud types and amounts. For a better understanding of these exchanges, an appropriate cloud type classification method is necessary. The present study proposes an alternative approach to the often used cloud optical and thermodynamic properties based classifications. This approach relies on the application of edge detection techniques on cloud top temperature (CTT) derived from global satellite maps. The gradient map obtained through these techniques is then used to distinguish various types of clouds. The edge detection techniques used are based on the idea that a pixel’s neighborhood contains information about its intensity. The variation of this intensity (gradient) offers the possibility to decompose the image into different cloud morphological features. High gradient areas would correspond to cumulus-like clouds, while low gradient areas would be associated with stratus-like clouds. Following the application of these principles, the results of the cloud classification obtained are evaluated against a common cloud classification method based on cloud optical properties’ variations. Relatively good matches between the two approaches are obtained. The best results are observed with high gradient clouds and the worst with low gradient clouds.

Development and commissioning of a Monte Carlo photon beam model for the forthcoming clinical trials in microbeam radiation therapy

A new radiotherapy technique, named microbeam radiation therapy (MRT), is under development at the ID17 Biomedical Beamline of the European Synchrotron Radiation Facility (ESRF). This innovative method is based on the fact that normal tissue can withstand high radiation doses in small volumes without any significant damage. The promising results obtained in the preclinical studies have paved the way to forthcoming clinical trials, which are currently in preparation. Highly accurate dose calculations at the treatment planning stage are required in this context. The aims of this study are the development and experimental benchmarking of a photon beam source model, which will be the core of the future MRT treatment planning system (TPS). The ID17 x-ray source was modeled by the synchrotron ray tracing code SHADOW. The Monte Carlo (MC) simulation code PENELOPE/PENEASY was employed to transport the photon beam from the source to the patient position through all the beamline components. The phase-space state variables of the particles reaching the patient position were used as an input to generate a photon beam model. Computed dose distributions in a homogeneous media were experimentally verified by using Gafchromic(®) films in a solid-water phantom. Benchmarking was split into two phases. First, the lateral dose profiles and the percentage depth-dose (PDD) curves in the broad beam configuration were considered. The acceptability criteria for radiotherapy dose computations recommended by international protocols such as the Technical Reports Series 430 (TRS 430) of the International Atomic Energy Agency (IAEA) were used. Second, the analogous dosimetric magnitudes in MRT irradiations, i.e., PDD of the central microbeam and the corresponding peak-to-valley dose ratios (PVDR) were evaluated and compared with MC calculations. A full characterization of the ID17 Biomedical Beamline (ESRF) synchrotron x-ray source and the development of an accurate photon beam model were achieved in this work. Calculated and experimental dose distributions agreed to within the recommended acceptability criteria described in international codes of practice (TRS 430) for broad beam irradiations. The overall deviation in low gradient areas amounted to 2%-3%. The maximum distance-to-agreement in high gradient regions was lower than 0.7 mm. MC calculations also reproduced MRT experimental results within uncertainty bars. These results validate the photon beam model for its use in MRT radiation therapy calculations. The first MC synchrotron photon beam model for MRT irradiations that reproduces experimental dose distributions in homogeneous media has been developed. This beam model will constitute an essential component of the TPS calculation engine for patient dose computation in forthcoming MRT clinical trials.

SU-E-T-741: The Effect of Grid Size on Dose Calculation Accuracy in Head & Neck and Prostate Cancers

Purpose: In this study, we evaluated the effect of grid size on dose calculation accuracy using 2 head & neck and 2 prostate IMRT cases. Methods: Dose distribution of four IMRT plan data were calculated at four calculation grid sizes (1.25, 2.5, 5, and 10 mm) and the calculated dose distributions were compared with measured dose distribution using 2D diode array detector. Results: there was no obvious difference in pass rate of gamma analysis with 3 mm/3% acceptance criteria except 10 mm grid size, we found that the pass rates of 2.5, 5 and 10 mm grid size were decreased 5%, 20% and 31.53% respectively. The calculation time were about 11.5 min, 4.77 min, 2.95 min, and 11.5 min at 1.25, 2.5, 5, and 10 mm, respectively and as the grid size increased to double, the calculation time decreased about one‐half. The grid size effect was observed more clearly in the high gradient area than the low gradient area. Conclusions: In conclusion, 2.5 mm grid size is considered acceptable for most IMRT plans but at least in the high gradient area, 1.25 mm grid size is required to accurately predict the dose distribution.

Differences in longitudinal distribution patterns along a Honshu stream of brown trout Salmo trutta, white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou

Brown trout Salmo trutta were first introduced into Japan in 1892, and they currently naturally reproduce in several rivers in Honshu and Hokkaido, Japan. Although negative impacts of brown trout introductions on native salmonid fishes have been documented in some Hokkaido rivers, studies of ecological interactions between brown trout and native salmonid fishes on Honshu are limited. In this study, we describe the longitudinal distribution patterns of introduced brown trout, white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou in a 4 km stretch of a stream in central Honshu. Underwater observations were conducted in all pools within upstream, middle and downstream sections (190–400 m in length) of this stretch in order to estimate the densities of these species. Only white-spotted charr was observed in the upstream section, while brown trout and masu salmon were observed in the middle and downstream sections. Masu salmon densities, however, were much lower than brown trout densities. In the downstream section, white-spotted charr was absent. These results are consistent with results from previous studies of Hokkaido rivers, where it was found that white-spotted charr in low-gradient areas tend to be displaced by brown trout.

Discrepancy of IMRT Dose Delivery due to the Dose-Dynamic MLC Gravity Effect

The aim of the present study was to investigate the multi-leaf collimator (MLC) gravity e ect on intensity modulated radiation therapy (IMRT) dose-dynamic delivery at di erent gantry angles. The non-uniform uence for IMRT was generated by means of dose-dynamic MLC (DMLC) performances, which were multiple static segments (MSS) or fully dynamic sliding window (SW) delivery methods at di erent gantry angles. SW delivery is quite similar to MSS with regard to MLC sliding pattern through the X1 to X2 jaw direction, except that it is irradiated during DMLC movement. To determine how the DMLC gravity a ects the IMRT dose uence, we mounted a two-dimensional ion chamber matrix (MatriXX, Scanditronix-Wellhofer, Schwarzenbruck, Germany) to a linear accelerator (CL 21EX, Varian, Palo Alto, CA, USA). Individual IMRT dose-dynamic segments were applied by using MSS and SW for 6-MV photon beams at ve di erent gantry angles: 0 (neutral gravity), 45 (semi-along gravity), 90 (vertical-along gravity), 315 (semi-against gravity) and 270 (against gravity). To test the correlation of leaf speed and gravity, we delivered half monitor units (MUs) for planned doses for absolute dose comparison. Strict -index (dose di erence: 2 %; distance to agreement: 2 mm) histograms were used for quantitative analyses of the discrepancy. The dose distribution by MSS under neutral gravity (gantry 0 ) with 137 MUs at a prescribed dose (45 cGy) point, which was in the low gradient area, was used as a reference for the other results. The -index histograms showed an increased tendency of the dose discrepancy toward the gravity-along direction rather than toward the gravity-against direction. The acceptable proportional ranges below 1 of the -index were 96.2 { 99.6 % (mean: 97.6 %), 92.6 { 93.5 % (mean: 93.0 %), 90.4 { 92.6 % (mean: 91.3 %), 94.6 { 98.0 % (mean: 96.4 %) and 92.9 { 96.8 % (mean: 95.0 %) for neutral, semi-along, vertical-along, semi-against and vertical-against gravity of the positions of MLC segments, respectively. we noted that MSS deliveries were more stable than were the SW deliveries (unacceptable -index range 1.0 { 2.0: mean 3.6 % for MSS and 5.4 % for SW). For the dose measurement on the prescribed dose point, all measurements showed a good agreement within an average of 2 %. Our experiment conclusively demonstrated that the DMLC gravity a ects the

Eco-geomorphology of banded vegetation patterns in arid and semi-arid regions

Abstract. The interaction between vegetation and hydrologic processes is particularly tight in water-limited environments where a positive-feedback links soil moisture and vegetation. The vegetation of these systems is commonly patterned, that is, arranged in a two phase mosaic composed of patches with high biomass cover interspersed within a low-cover or bare soil component. These patterns are strongly linked to the redistribution of runoff and resources from source areas (bare patches) to sink areas (vegetation patches) and play an important role in controlling erosion. In this paper, the dynamics of these systems is investigated using a new modeling framework that couples landform and vegetation evolution, explicitly accounting for the dynamics of runon-runoff areas. The objective of this study is to analyze water-limited systems on hillslopes with mild slopes, in which overland flow occurs predominantly in only one direction and vegetation displays a banded pattern. Our simulations reproduce bands that can be either stationary or upstream migrating depending on the magnitude of the runoff-induced seed dispersal. We also found that stationary banded systems redistribute sediment so that a stepped microtopography is developed. The modelling results are the first to incorporate the effects of runoff redistribution and variable infiltration rates on the development of both the vegetation patterns and microtopography. The microtopography for stationary bands is characterized by bare soil on the lower gradient areas and vegetation on steeper gradients areas. For the case of migrating vegetation bands the model generates hillslope profiles with planar topography. The success at generating not only the observed patterns of vegetation, but also patterns of runoff and sediment redistribution suggests that the hydrologic and erosion mechanisms represented in the model are correctly capturing some of the key processes driving these ecosystems.

A New Implicit Method for Surface Segmentation by Minimal Paths in 3D Images

We introduce a novel implicit approach for single-object segmentation in 3D images. The boundary surface of this object is assumed to contain two known curves (the constraining curves), given by an expert. The aim of our method is to find the wanted surface by exploiting as much as possible the information given in the supplied curves and in the image. As for active surfaces, we use a cost potential that penalizes image regions of low interest (most likely areas of low gradient or too far from the surface to be extracted). In order to avoid local minima, we introduce a new partial differential equation and use its solution for segmentation. We show that the zero level set of this solution contains the constraining curves as well as a set of paths joining them. We present a fast implementation that has been successfully applied to 3D medical and synthetic images.

Fast Surface Segmentation Guided by User Input Using Implicit Extension of Minimal Paths

We introduce a novel implicit approach for single object segmentation in 3D images. The boundary surface of this object is assumed to contain two or more known curves (the constraining curves), given by an expert. The aim of our method is to find the desired surface by exploiting the information given in the supplied curves as much as possible. We use a cost potential which penalizes image regions of low interest (for example areas of low gradient). In order to avoid local minima, we introduce a new partial differential equation and use its solution for segmentation. We show that the zero level set of this solution contains the constraining curves as well as a set of paths joining them. These paths globally minimize an energy which is defined from the cost potential. Our approach, although conceptually different, can be seen as an implicit extension to 3D of the minimal path framework already known for 2D image segmentation. As for this previous approach, and unlike other variational methods, our method is not prone to local minima traps of the energy. We present a fast implementation which has been successfully applied to 3D medical and synthetic images.

Analysis and Tectonic Interpretation to the Horizontal‐Gradient Map Calculated from Bouguer Gravity Data in the China Mainland

AbstractWe make a careful analysis and comparison to the colored map of horizontal gradient calculated from Bouguer gravity data of the China mainland, and obtain some new insights into tectonics. Taking roughly 105°E as the boundary in the map, the eastern part of the China mainland is characterized by low gradients (blue color) background, appearing belt‐shape structure of medium and high gradient as well as linear structure of gradient difference borders. The west part has a framework of belt‐shape structure with medium and high gradient (red color), inlaid by low gradient areas which contain shade‐line structure. In general, structures exhibited by horizontal gradient of gravity anomalies can be classified into three major sets. The first set includes high‐gradient zones along the NNE trending Da hinggan ling, Taihangshan, and Wulingshan Mountains, the Tanlu great fault zone which extends obliquely through East China, and three NNW trending great fault zones through western China. The second set presents an intersection pattern of NWW and NEE direction gradient zones in the west, as well as three NWW discontinuous shade zones with roughly equal spacing, and a single tectonic line in the east, plus NE trending low gradient lines in South China. And the third set consists of several EW tectonic zones through the whole mainland and a wide complex NS striking zone in the middle of the mainland, of which on the both sides spread NS direction shade lines which are short, discontinuous and of large intervals. These three sets of tectonic zones delineated by gravity gradient are associated with variations of topography and crustal thickness on a large scale. The inner structures and formation times as well as evolution histories of each set are likely different. They have overlapped each other to shape the current tectonic patterns of the China mainland. Each tectonic zone has varied response to the recent tectonic stress fields of large scales, leading to regional change of directions of principle stresses. The ubiquitous segmentation caused by conjunction between tectonic zones of different directions is an important issue for theoretical and applied research in structural geology.

An automated phantom-film QA procedure for intensity-modulated radiation therapy

To verify that the calculated dose distribution is delivered accurately during intensity-modulated radiation therapy (IMRT), we have implemented an automated plan/film validation protocol. The cubic polystyrene film phantom provided with the Peacock IMRT system and the Radiation Imaging Technology (RIT) film dosimetry system were used to compare planned and delivered dose distributions. The calculated dose matrix from CORVUS was transferred to RIT and analyzed. The analysis included dose-difference histograms, dose comparison in low-gradient areas, distance to agreement in high-gradient areas, dose profiles, and isodose comparisons. Dose differences of up to 5% were commonly observed in the high-dose and low-gradient areas between verification films and treatment plans for prostate patients. The most prominent discrepancies were detected in the high-gradient areas of dose distributions. The automated protocol is an efficient technique that provides information about spatial differences between calculated and delivered doses.

Environmental gradients and identification of wetlands in north-central Florida

Vegetation composition, soil morphology, and hydrology were characterized along wetland-to-upland gradients at six forested sites in north-central Florida to compare results of Federal wetland delineation methods with 3–5 yr of hydrologic data. Wetland and non-wetland identifications were supported by hydrology data in eight of nine plant communities. Lack of hydric soil indicators and hydrophytic vegetation in two upland communities (scrub and mixed mesic hardwoods) agreed with a deep water table. Six wetland communities (cypress dome, cypress strand, bayhead, cypress/bayhead, red maple/oak swamp, and cedar swamp) with field indicators of wetland hydrology, hydrophytic vegetation, and hydric soils were inundated or had water tables at or near the ground surface at least 5% of the growing season in most years., Flatwoods communities, however, occurred at intermediate positions on the moisture gradient and could not be consistently identified as wetland or upland communities. Identification of flatwoods as wetlands depended on wetland delineation method and was not usually supported by hydrologic measurements. In the flatwoods community, soil properties and vegetation composition were correlated with the mean and standard deviation of water-table depths, as well as the depth continuously exceeded by the water table at least 5% of the growing season in most years. Various hydrologic parameters need to be considered in addition to the 5% exceedence level currently used in Federal wetland delineation guidance when characterizing wetland conditions in low-gradient areas such as flatwoods.

Why do some patients have high defibrillation thresholds at defibrillator implantation? Answers from basic research.

Implantable cardioverter defibrillators reduce the risk of sudden cardiac death in patients with ventricular tachyarrhythmias. However, for the few patients with unacceptably high defibrillation thresholds at implantation the risk of sudden death may remain high. If a small number of defibrillation attempts are used to determine a defibrillation threshold, then a high defibrillation threshold may occur in some patients due to the probabilistic nature of defibrillation: a small percentage of shocks will fail even at optimal shock strengths. Basic investigations have suggested mechanisms for high defibrillation thresholds in other patients. The extracellular potential gradients produced by a shock correlate with ability to defibrillate and may be used to classify mechanisms for high defibrillation thresholds. Computerized mapping studies have demonstrated that extracellular potential gradient fields produced by defibrillation shocks are uneven with high gradient areas close to the electrodes and low gradient areas distant from the electrodes. A high defibrillation threshold may occur because: (1) a shock creates a subthreshold potential gradient in the low gradient areas; (2) a patient has a higher minimum potential gradient threshold than other patients; or (3) a shock leads to refibrillation in the high gradient areas. This article reviews experimental evidence to support each of these three possibilities then suggests experimental and clinical investigations that may clarify the causes of high defibrillation thresholds in patients.

Evidence for Floating Ice Shelves in Franz Josef Land, Russian High Arctic

Examination of digital Landsat TM and MSS imagery of Franz Josef Land, Russian High Arctic, reveals a number of ice caps with apparently very low surface gradients at their seaward margins. The largest of these low gradient areas is 45 km2. The areas are dynamically a part of the parent ice mass, and have a marked break of slope at their inner margins. They generally occur in protected embayments and often have relatively deep water offshore. The presence of deep inter-island channels (up to 600 m) in the archipelago also suggests that deglaciation after the last glaciation may have proceeded rapidly due to enhanced iceberg calving. Tabular icebergs (maximum observed length 2.3 km) are produced from several of the low gradient ice cap margins today. Ice surface profiles, derived from analysis of vertical aerial photographs, show slopes of 0.50 on these features, as compared with 3.5 to 50 on other ice caps. At least some are likely to be floating ice shelves. They have similar ice surface gradients to a known ice shelf on Severnaya Zemlya. There is no requirement for deep water to occur beneath these features, but simply that they become buoyant over a significant part of their base. Glacier thinning, due to reduced mass balance since the termination of the Little Ice Age, may have contributed to the presence of these features. An origin for some of these low gradient margins by deformation of an unlithified substrate cannot be ruled out. Field radio-echo experiments could be used to test the interpretation of these features as ice shelves.

Winter Stream Conditions and Use of Habitat by Brook Trout in High-Elevation Wyoming Streams

Abstract Winter stream conditions at elevations between 2,280 and 3,205 m above mean sea level and the use of winter habitat by adult brook trout Salvelinus fontinalis above 2,990 m were evaluated in 1983–1984 and 1984–1985. Little surface ice was observed at elevations above 2,900 m, which was associated with high snow accumulation; moderate surface ice and anchor ice formation were observed at elevations from 2,550 to 2,900 m; extensive surface ice formation occurred at 2,550 m. Little snow accumulated at 2,550 m and surface ice physically excluded substantial brook trout habitat. In late fall, brook trout at elevations above 2,990 m tended to move into low-gradient areas where they remained active throughout the winter. During winter, brook trout appeared to select for areas with maximum velocities of 15 cm/s or less, measured during summer low flow, and for deeper water, but not for substrate type.