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排序方式: 共有175条查询结果,搜索用时 140 毫秒
21.
Mark E. Perry Daniel S. Kahan Olivier S. Barnouin Carolyn M. Ernst Sean C. Solomon Maria T. Zuber David E. Smith Roger J. Phillips Dipak K. Srinivasan Jürgen Oberst Sami W. Asmar 《Planetary and Space Science》2011,59(15):1925-1931
The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft completed three flybys of Mercury in 2008–2009. During the first and third of those flybys, MESSENGER passed behind the planet from the perspective of Earth, occulting the radio-frequency (RF) transmissions. The occultation start and end times, recovered with 0.1 s accuracy or better by fitting edge-diffraction patterns to the RF power history, are used to estimate Mercury's radius at the tangent point of the RF path. To relate the measured radius to the planet shape, we evaluate local topography using images to identify the high-elevation feature that defines the RF path or using altimeter data to quantify surface roughness. Radius measurements are accurate to 150 m, and uncertainty in the average radius of the surrounding terrain, after adjustments are made from the local high at the tangent point of the RF path, is 350 m. The results are consistent with Mercury's equatorial shape as inferred from observations by the Mercury Laser Altimeter and ground-based radar. The three independent estimates of radius from occultation events collectively yield a mean radius for Mercury of 2439.2±0.5 km. 相似文献
22.
Impact of model development,calibration and validation decisions on hydrological simulations in West Lake Erie Basin 总被引:1,自引:0,他引:1
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Prasad Daggupati Haw Yen Michael J. White Raghavan Srinivasan Jeffrey G. Arnold Conor S. Keitzer Scott P. Sowa 《水文研究》2015,29(26):5307-5320
Watershed simulation models are used extensively to investigate hydrologic processes, landuse and climate change impacts, pollutant load assessments and best management practices (BMPs). Developing, calibrating and validating these models require a number of critical decisions that will influence the ability of the model to represent real world conditions. Understanding how these decisions influence model performance is crucial, especially when making science‐based policy decisions. This study used the Soil and Water Assessment Tool (SWAT) model in West Lake Erie Basin (WLEB) to examine the influence of several of these decisions on hydrological processes and streamflow simulations. Specifically, this study addressed the following objectives (1) demonstrate the importance of considering intra‐watershed processes during model development, (2) compare and evaluated spatial calibration versus calibration at outlet and (3) evaluate parameter transfers across temporal and spatial scales. A coarser resolution (HUC‐12) model and a finer resolution model (NHDPlus model) were used to support the objectives. Results showed that knowledge of watershed characteristics and intra‐watershed processes are critical to produced accurate and realistic hydrologic simulations. The spatial calibration strategy produced better results compared to outlet calibration strategy and provided more confidence. Transferring parameter values across spatial scales (i.e. from coarser resolution model to finer resolution model) needs additional fine tuning to produce realistic results. Transferring parameters across temporal scales (i.e. from monthly to yearly and daily time‐steps) performed well with a similar spatial resolution model. Furthermore, this study shows that relying solely on quantitative statistics without considering additional information can produce good but unrealistic simulations. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
23.
SWAT model application and prediction uncertainty analysis in the Lake Tana Basin,Ethiopia 总被引:4,自引:0,他引:4
Lake Tana Basin is of significant importance to Ethiopia concerning water resources aspects and the ecological balance of the area. Many years of mismanagement, wetland losses due to urban encroachment and population growth, and droughts are causing its rapid deterioration. The main objective of this study was to assess the performance and applicability of the soil water assessment tool (SWAT) model for prediction of streamflow in the Lake Tana Basin, so that the influence of topography, land use, soil and climatic condition on the hydrology of Lake Tana Basin can be well examined. The physically based SWAT model was calibrated and validated for four tributaries of Lake Tana. Sequential uncertainty fitting (SUFI‐2), parameter solution (ParaSol) and generalized likelihood uncertainty estimation (GLUE) calibration and uncertainty analysis methods were compared and used for the set‐up of the SWAT model. The model evaluation statistics for streamflows prediction shows that there is a good agreement between the measured and simulated flows that was verified by coefficients of determination and Nash Sutcliffe efficiency greater than 0·5. The hydrological water balance analysis of the basin indicated that baseflow is an important component of the total discharge within the study area that contributes more than the surface runoff. More than 60% of losses in the watershed are through evapotranspiration. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
24.
Sayantan Bhowmik Cesar A. Mantilla Sanjay Srinivasan 《Stochastic Environmental Research and Risk Assessment (SERRA)》2011,25(8):1085-1090
Tracking the migration of the CO2 plume is essential in order to better manage the operation of geologic sequestration of CO2. However, the large cost of most monitoring technologies, such as time-lapse seismic, limits their application. We investigated
the application of a probabilistic history matching methodology using routine measurements of injection data, which are affected
by the presence of large-scale heterogeneities, as an inexpensive alternative to track the migration of CO2 plume in an aquifer. The approach is demonstrated first through a synthetic example in which the ability to detect the presence
of flow barriers was tested. In a second example, we applied our method to the In Salah field, one of the largest geological
sequestration projects in the world, where the main direction of high permeability features was inferred. The accuracy and
reproducibility of the results show that our approach for assisted history matching is an economic and viable option for plume
monitoring during geologic CO2 sequestration. 相似文献
25.
Predicting the velocity within the ship’s propeller jet is the initial step to investigate the scouring made by the propeller jet. Albertson et al. (1950) suggested the investigation of a submerged jet can be undertaken through observation of the plain water jet from an orifice. The plain water jet investigation of Albertson et al. (1950) was based on the axial momentum theory. This has been the basis of all subsequent work with propeller jets. In reality, the velocity characteristic of a ship’s propeller jet is more complicated than a plain water jet. Fuehrer and Römisch (1977), Blaauw and van de Kaa (1978), Berger et al. (1981), Verhey (1983) and Hamill (1987) have carried out investigations using physical model. Current paper reviews the state-of-art of the equations used to predict the time-averaged axial, tangential and radial components of velocity within the zone of flow establishment and the zone of established flow of a ship’s propeller jet. 相似文献
26.
Jorge Villegas Corene Matyas Sivaramakrishnan Srinivasan Ignatius Cahyanto Brijesh Thapa Lori Pennington-Gray 《Natural Hazards》2013,66(1):97-116
Tourists are particularly vulnerable to natural disasters such as hurricanes since they might be less informed and prepared than residents of disaster-prone areas. Thus, understanding how the traits of a tropical cyclone as well as specific characteristics of tourists influence affective and cognitive responses to a hurricane warning message is a critical component in disaster planning. Using scenarios that presented tropical cyclones with different relevant characteristics (such as category at landfall), tourists’ knowledge, experience with hurricanes, trip traits, and the location of the survey (coastal or inland), this study contributes to the literature on sociological issues related to natural disasters. The findings suggest that risk perceptions and fear are influenced differently by the traits of the hurricanes and tourists’ knowledge and experience. Risk is strongly influenced by the projected category of the hurricane at landfall, while fear is not as sensitive to this extremely relevant trait of cyclones. The results also suggest that the influence of risk and fear on evacuation likelihood is strong and positive. This study shows the value of studying cognitive and affective responses to uncertain events. 相似文献
27.
The ENE-plunging macroscopic folds, traced by calc gneiss interbanded with marble and sillimanite schist within the Peninsular
Gneiss around Suganapuram in the ‘Palghat gap’ in southern India, represent structures of the second generation (D2). They have folded the axial planes of a set of D1 isoclinal folds on stratification coaxially, so that the mesoscopic D1 folds range from reclined in the hinge zones, through inclined to upright in the limb zones of the D2 folds. Orthogonal relation between stratification and axial planar cleavage, and ‘M’ shaped folds on layering locate the
hinge zones of the D1 folds, whereas folds on axial planar cleavage with ‘M’ shaped folds are the sites of the D2 fold hinges. Extreme variation in the shapes of the isoclinal D1 folds from class 1B through class 1C to nearly class 2 of Ramsay is a consequence of buckling followed by flattening on layers
of widely varying viscosity contrast.
The large ENE-trending structures in this supracrustal belt within the Peninsular Gneiss in the ‘Palghat gap’ could not have
evolved by reorientation of NS-trending structures of the Dharwar tectonic province to the north by movement along the Moyar-Bhavani
shear zone which marks the boundary between the two provinces. This is because the Moyar and Bhavani faults are steep dipping
reverse faults with dominant dip-slip component.
Deceased 相似文献
28.
Stromatolites associated with cherty dolomites of the Vanivilaspura Formation of the Archaean Dharwar Supergroup show a morphology
indicative of the deposition of the latter in a intertidal to subtidal environment. The cherts are moderately high in their
Al/Al + Fe ratios but depleted in Fe2C3 and also most trace elements. Unlike most other Archaean cherts, the Vanivilaspur cherts exhibit significant negative Ce
anomaly, which is interpreted to have resulted from contemporary manganese deposition. The Rb/Sr ratios in the cherts show
a sufficient spread to define a linear correlation line in the Rb-Sr evolution diagram corresponding to an age of 2512 ± 159
Ma and initial Sr ratio of 0.7128 ± 0.0012 (2σ). While this age is strikingly close to that of regional metamorphism in the
Dharwar craton, the initial ratio is distinctly higher than that of the associated volcanics. Acid leaching experiments on
the cherts suggest that they may have been isotopically equilibrated on a mm to cm scale about 500 Ma later than the time
of regional metamorphism. 相似文献
29.
Neogene deep sea benthic foraminiferal diversity in the Indian Ocean: Paleoceanographic implications
The species diversity indices, as defined by the number of species,S; Shannon-Wiener index,H(S) and Buzas-Gibson index,É, of DSDP sites 219, 220, 237 and 238 were measured to determine the benthic foraminiferal diversity patterns in the Indian Ocean deep sea sequences during the Neogene. The Time-Stability hypothesis could satisfactorily explain the observed diversity patterns. The general patterns of diversity suggest environmental stability during the Neogene. However, few small fluctuations in diversity during the Middle Miocene (c.14·8 Ma), Late Miocene (c.6·0 Ma) and Late Pliocene (c.2·0 Ma) may possibly be the effects of Antarctic Bottom Water (AABW) activity in this region. The benthic foraminiferal diversity in the tropical Indian Ocean is more than the high latitudinal areas with comparable depths. 相似文献
30.
Y. J. Bhaskar Rao K. Naha R. Srinivasan K. Gopalan 《Journal of Earth System Science》1991,100(4):399-412
The Peninsular Gneiss around Gorur in the Dharwar craton, reported to be one of the oldest gneisses, shows nealy E-W striking
gneissosity parallel to the axial planes of a set of isoclinal folds (DhF1). These have been over printed by near-coaxial open folding (DhF12) and non-coaxial upright folding on almost N-S trend (DhF2). This structural sequence is remarkably similar to that in the Holenarasipur schist belt bordering the gneisses as well
as in the surpracrustal enclaves within the gneisses, suggesting that the Peninsular Gneiss has evolved by migmatization synkinematically
with DhF1 deformation.
The Gorur gneisses are high silica, low alumina trondhjemites enriched in REE (up to 100 times chondrite), with less fractionated
REE patterns (CeN/YbN < 7) and consistently negative Eu anomalies (Eu/Eu* = 0.5 to 0.7).
A whole rock Rb-Sr isochron of eight trondhjemitic gneisses sampled from two adjacent quarries yields an age of 3204 ± 30
Ma with Sr
i
of 0.7011 ± 6 (2σ). These are marginally different from the results of Beckinsale and coworkers (3315 ± 54 Ma, Sr
i
= 0.7006 ± 3) based on a much wider sampling. Our results indicate that the precursors of Gorur gneisses had a short crustal
residence history of less than a 100 Ma. 相似文献