Evidence of gully formation by regional groundwater flow in the Gorgonum-Newton region (Mars)     

Alvaro Márquez  Miguel Ángel de Pablo  Cristóbal Viedma 《Icarus》2005,179(2):398-414

The discovery of gullies on Mars suggests liquid water activity near the surface of the planet in recent times. Since liquid water is unstable under the present-day P-T martian conditions, the formation mechanisms of the gullies, and the source of the putative water, have been a matter of debate for the last years. To provide new insights into these matters, we have approached the problem studying the gullies in relation to their regional setting. A major point in our study relates to the geographic orientation of gullies, an aspect that has been previously regarded as a crucial matter in different models, and has profound implications regarding their origin. We present a comprehensive and detailed survey of the Gorgonum-Newton region, and a study of the Dao and Nirgal Vallis regions. The survey was carried out with the aid of 965 high-resolution MOC images (752 for Gorgonum-Newton, 102 for Nirgal Vallis and 111 for Dao Vallis regions), and MOLA-derived DEMs. We found that gullies display a clear regional pattern, geographically and topographically consistent with a decreasing regional slope. We interpret the results in terms of the existence of several groundwater flow systems operating at different scales, which ultimately may have led to gully formation by seepage at the slopes of craters and canyons. We suggest that aquifers discharging at gully systems may have recharged from the surface, in response to the melting of young partially eroded ice-rich deposits.  相似文献   

Photogrammetric and lidar data integration using the centroid of a rectangular roof as a control point     

Edson  Mitishita  Ayman  Habib  Jorge  Centeno  Alvaro  Machado  Jennifer  Lay  Carmen  Wong 《The Photogrammetric Record》2008,23(121):19-35

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Effects of the 1997–1999 El Niño and La Niña events on zooplankton abundance and euphausiid community composition within the Monterey Bay coastal upwelling system     

B. B. Marinovic  D. A. Croll  N. Gong  S. R. Benson  F. P. Chavez 《Progress in Oceanography》2002,54(1-4)

Zooplankton abundance and euphausiid community composition were sampled seasonally (spring, summer, fall) within Monterey Bay, California, between 1997 and 1999. Measurements of sea surface temperature (SST), mixed layer depth, and upwelling indices provided concurrent data on physical oceanographic parameters. Both total zooplankton and krill abundance dramatically declined in the summer of 1997 coincident with a rapid increase in SST and mixed layer depth. Changes in euphausiid community composition occurred in concert with the decline in overall abundance. The relative abundance of the southern neritic Nyctiphanes simplex increased from August to November in 1997, the abundance of cold temperate Euphausia pacifica decreased significantly, and that of the northern neritic Thysanoessa spinifera declined dramatically. The sudden appearance of an adult cohort of N. simplex in July 1997 suggests that rapid poleward flow characteristic of coastally trapped Kelvin waves occurred between June and July of 1997. The persistent presence of warm temperate and subtropical taxa in samples collected between August 1997 and October 1998 indicates that this poleward flow continued in 1998. Zooplankton abundance, euphausiid community composition, and physical oceanographic parameters gradually returned to a more typical upwelling-dominated state in the spring and summer of 1998. E. pacifica and T. spinifera abundances gradually increased during the summer and fall of 1998, while N. simplex abundance abruptly declined in the spring of 1998. However, this recovery was confined to a narrow coastal band as a result of the onshore movement of the oceanic waters of the California Current. This was reflected by higher than normal numbers of the oceanic Nematoscelis difficilis within samples collected during the spring and summer of 1998. By the spring and summer of 1999, both zooplankton and euphausiid abundance had increased to the highest levels recorded during the 3-year study. Both E. pacifica and T. spinifera abundance increased relative to 1998 while N. simplex was completely absent in all samples. These changes reflected the cooler, highly productive environmental conditions associated with the 1998/1999 La Niña.  相似文献   

Calibration and Validation of Direction-Finding High-Frequency Radar Ocean Surface Current Observations   总被引：1，自引：0，他引：1  

Jeffrey D. Paduan Kyung Cheol Kim Michael S. Cook Francisco P. Chavez 《Oceanic Engineering, IEEE Journal of》2006,31(4):862-875

This paper focuses on the validation of remotely sensed ocean surface currents from SeaSonde-type high-frequency (HF) radar systems. Hourly observations during the period July 22, 2003 through September 9, 2003 are used from four separate radar sites deployed around the shores of Monterey Bay, CA. Calibration of direction-finding techniques is addressed through the comparisons of results obtained using measured and ideal (i.e., perfect) antenna patterns. Radial currents are compared with observations from a moored current meter and from 16 surface drifter trajectories. In addition, four overwater baselines are used for radar-to-radar comparisons. Use of measured antenna patterns improves system performance in almost all cases. Antenna-pattern measurements repeated one year later at three of the four radar locations exhibit only minor changes indicating that pattern distortions are stable. Calibrated results show root-mean-square (rms) radial velocity differences in the range of 9.8-13.0 cm/s, which suggest radar observation error levels in the range of 6.9-9.2 cm/s. In most cases, clear evidence of bearing errors can be seen, which range up to 30deg for uncalibrated radar-derived radial currents and up to 15deg for currents obtained using measured antenna patterns. Bearing errors are not, however, constant with angle. The results recommend use of measured antenna patterns in all SeaSonde-type applications. They also recommend an expanded simulation effort to better describe the effects of antenna-pattern distortions on bearing determination under a variety of ocean conditions  相似文献   

Changes in the cetacean assemblage of a coastal upwelling ecosystem during El Niño 1997–98 and La Niña 1999     

Scott R. Benson  Donald A. Croll  Baldo B. Marinovic  Francisco P. Chavez  James T. Harvey 《Progress in Oceanography》2002,54(1-4)

We report results of ecosystem studies in Monterey Bay, California, during the summer upwelling periods, 1996–99, including impacts of El Niño 1997–98 and La Niña 1999. Random-systematic line-transect surveys of marine mammals were conducted monthly from August to November 1996, and from May to November 1997–99. CTDs and zooplankton net tows were conducted opportunistically, and at 10 predetermined locations. Hydroacoustic backscatter was measured continuously while underway to estimate prevalence of zooplankton, with emphasis on euphausiids, a key trophic link between primary production and higher trophic level consumers.The occurrences of several of the California Current’s most common cetaceans varied among years. The assemblage of odontocetes became more diverse during the El Niño with a temporary influx of warm-water species. Densities of cold-temperate Dall’s porpoise, Phocoenoides dalli, were greatest before the onset of El Niño, whereas warm-temperate common dolphins, Delphinus spp., were present only during the warm-water period associated with El Niño. Rorqual densities decreased in August 1997 as euphausiid backscatter was reduced. In 1998, as euphausiid backscatter slowly increased, rorqual densities increased sharply to the greatest observed values. Euphausiid backscatter further increased in 1999, whereas rorqual densities were similar to those observed during 1998. We hypothesize that a dramatic reduction in zooplankton biomass offshore during El Niño 1997–98 led to the concentration of rorquals in the remaining productive coastal upwelling areas, including Monterey Bay. These patterns exemplify short-term responses of cetaceans to large-scale changes in oceanic conditions.  相似文献   

MINISAT 01 as a scientific mission and its future     

Alvaro Gimenez 《Astrophysics and Space Science》2001,276(1):13-19

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Thermoelastic behavior and dehydration process of cancrinite     

G. D. Gatta  D. Comboni  M. Alvaro  P. Lotti  F. Cámara  M. C. Domeneghetti 《Physics and Chemistry of Minerals》2014,41(5):373-386

The high-temperature thermoelastic behavior of a natural cancrinite has been investigated by in situ single-crystal X-ray diffraction. The unit-cell volume variation as a function of temperature (T) exhibits a continuous trend up to 748 K (hydrous expansion regime). The unit-cell edges expansion clearly shows an anisotropic expansion scheme (α _a  < α _c ). At 748 K, a dehydration process takes place, and a series of unit-cell parameter measurements at constant temperature (748 K) for a period of 12 days indicate that the dehydration process continued for the entire period of time, until the cell parameters were found to be constant. After the dehydration process is completed, the structure expands almost linearly with increasing temperature up to 823 K, where a sudden broadening of the diffraction peaks, likely due to the impending decomposition, did not allow the collection of further data points. Even with a very limited temperature range for the anhydrous regime, we observed that the behavior of the two (i.e., hydrous and anhydrous) high-temperature structures is similar in terms of (1) volume thermal expansion coefficient and (2) thermoelastic anisotropy. The structure refinements based on the data collected at 303, 478 and 748 K (after the dehydration), respectively, showed a change in the mechanism of tilting of the quasi-rigid (Si,Al)O₄ tetrahedra, following the loss of H₂O molecules, ascribable to the high-temperature Na⁺ coordination environment within the cages.  相似文献   

Long-term vertical land motion from double-differenced tide gauge and satellite altimetry data     

Alvaro Santamaría-Gómez  Médéric Gravelle  Guy Wöppelmann 《Journal of Geodesy》2014,88(3):207-222

We present a new approach to estimate precise long-term vertical land motion (VLM) based on double-differences of long tide gauge (TG) and short altimetry data. We identify and difference rates of pairs of highly correlated sea level records providing relative VLM estimates that are less dependent on record length and benefit from reduced uncertainty and mitigated biases (e.g. altimeter drift). This approach also overcomes the key limitation of previous techniques in that it is not geographically limited to semi-enclosed seas and can thus be applied to estimate VLM at TGs along any coast, provided data of sufficient quality are available. Using this approach, we have estimated VLM at a global set of 86 TGs with a median precision of 0.7 mm/year in a conventional reference frame. These estimates were compared to previous VLM estimates at TGs in the Baltic Sea and to estimates from co-located Global Positioning System (GPS) stations and Glacial Isostatic Adjustment (GIA) predictions. Differences with respect to the GPS and VLM estimates from previous studies resulted in a scatter of around 0.6 mm/year. Differences with respect to GIA predictions had a larger scatter in excess of 1 mm/year. Until satellite altimetry records reach enough length to estimate precise VLM at each TG, this new approach constitutes a substantial advance in the geodetic monitoring of TGs with major applications in long-term sea level change and climate change studies.  相似文献   

Efficiently implementable algebra for distributed in-network spatial analysis     

Farhana Jabeen  Alvaro A.A. Fernandes 《International journal of geographical information science》2013,27(4):815-844

Existing sensor network query processors (SNQPs) have demonstrated that in-network processing is an effective and efficient means of interacting with wireless sensor networks (WSNs) for data collection tasks. Inspired by these findings, this article investigates the question as to whether spatial analysis over WSNs can be built upon established distributed query processing techniques, but, here, emphasis is on the spatial aspects of sensed data, which are not adequately addressed in the existing SNQPs. By spatial analysis, we mean the ability to detect topological relationships between spatially referenced entities (e.g. whether mist intersects a vineyard or is disjoint from it) and to derive representations grounded on such relationships (e.g. the geometrical extent of that part of a vineyard that is covered by mist). To support the efficient representation, querying and manipulation of spatial data, we use an algebraic approach. We revisit a previously proposed centralized spatial algebra comprising a set of spatial data types and a comprehensive collection of operations. We have redefined and re-conceptualized the algebra for distributed evaluation and shown that it can be efficiently implemented for in-network execution. This article provides rigorous, formal definitions of the spatial data types, points, lines and regions, together with spatial-valued and topological operations over them. The article shows how the algebra can be used to characterize complex and expressive topological relationships between spatial entities and spatial phenomena that, due to their dynamic, evolving nature, cannot be represented a priori.  相似文献   

Water use efficiency in an arid watershed: a case study     

Damaris Orphanopoulos  Koen Verbist  Alvaro Chavez  Guido Soto 《寒旱区科学》2013,5(1):0016-0026

Integrated Water Resource Management (IWRM) becomes increasingly important in arid watersheds, where water resources are scarce and demands are rising under increased population pressure. In this case study, we present the results from the Elqui Valley in Chile, where water resources are shared between agriculture, mining operations, hydropower generation and drinking water for nearby cities. An IWRM model was applied to visualize water resources distribution in the catchment and determine irrigation and water use efficiencies. After calibration with observed surface and groundwater measurements, the model showed large differences in efficiencies between different irrigation sectors. Most notably, irrigation sectors with the highest irrigation security, i.e., higher water rights per hectare, were characterized with lower water use efficiencies. Frequent water deficits were present in sectors downstream of the main reservoir, which was related to limited water rights per hectare, lower irrigation efficiencies and higher water demands due to larger agricultural areas. In subsequent scenario analyses, the model was used to evaluate the impact of improved irrigation and water use efficiencies in these downstream sectors, to confirm an important reduction in water deficits, except for drought years when surface water resources are insufficient. In a third scenario, groundwater resources were allowed to compensate for water deficits, effectively reducing these in most irrigation sectors, especially during droughts. Expansion of the current agricultural area by 37% was evaluated, as a future scenario, and was found to be unsustainable, as aquifer levels are lowered to such levels that make annual recharge insufficient.  相似文献