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121.
PhotoelectricB andV observations of the W UMa-type system OO Aql have been obtained for 11 nights during the period from 22 June–30 July, 1987. A total of 978 observations were obtained in eachB andV filter. All these observations were transformed to theB andV colours of theUBV standard system. Five light curves for primary and secondary eclipses were obtained, its times of minima were determined and a new linear ephemeris was given. The period changes of the system were discussed. 相似文献
122.
Hassan AE 《Ground water》2004,42(2):277-290
Many sites of ground water contamination rely heavily on complex numerical models of flow and transport to develop closure plans. This complexity has created a need for tools and approaches that can build confidence in model predictions and provide evidence that these predictions are sufficient for decision making. Confidence building is a long-term, iterative process and the author believes that this process should be termed model validation. Model validation is a process, not an end result. That is, the process of model validation cannot ensure acceptable prediction or quality of the model. Rather, it provides an important safeguard against faulty models or inadequately developed and tested models. If model results become the basis for decision making, then the validation process provides evidence that the model is valid for making decisions (not necessarily a true representation of reality). Validation, verification, and confirmation are concepts associated with ground water numerical models that not only do not represent established and generally accepted practices, but there is not even widespread agreement on the meaning of the terms as applied to models. This paper presents a review of model validation studies that pertain to ground water flow and transport modeling. Definitions, literature debates, previously proposed validation strategies, and conferences and symposia that focused on subsurface model validation are reviewed and discussed. The review is general and focuses on site-specific, predictive ground water models used for making decisions regarding remediation activities and site closure. The aim is to provide a reasonable starting point for hydrogeologists facing model validation for ground water systems, thus saving a significant amount of time, effort, and cost. This review is also aimed at reviving the issue of model validation in the hydrogeologic community and stimulating the thinking of researchers and practitioners to develop practical and efficient tools for evaluating and refining ground water predictive models. 相似文献
123.
Hassan AE 《Ground water》2006,44(5):710-722
A long-term monitoring well network is developed using complementary and simple approaches in conjunction with a stochastic ground water flow and transport model. The development is illustrated for a case study of a U.S. nuclear testing site (Shoal) that is undergoing environmental restoration. The network design builds on three different, yet complementary, tools for locating the monitoring wells with a main objective of detection monitoring. The first tool is applied to select potential siting horizons where monitoring wells could be located. The second tool is used to place monitoring wells in locations with high success probability. The success here is defined as the detection of migrating stochastic plumes before a certain mass percentage reaches a compliance boundary. The third tool is used to analyze detection efficiency of multiple combinations of three wells. Seventy-six different three-well networks are selected from 20 candidate locations and are evaluated for detection efficiency. From the 76 networks analyzed, 28 attain detection efficiency close to or above 70%. The results of the different analyses provide multiple alternatives for the locations of the three wells, which will become part of the long-term monitoring network at Shoal. A number of combinations are equally good, and the final choice will depend on practical considerations and future agreements between model sponsor and regulators. 相似文献
124.
Ahmed A. Balogun Jimmy O. Adegoke Sajith Vezhapparambu Matthias Mauder Joseph P. McFadden Kevin Gallo 《Boundary-Layer Meteorology》2009,133(3):299-321
Previous measurements of urban energy balances generally have been limited to densely built, central city sites and older
suburban locations with mature tree canopies that are higher than the height of the buildings. In contrast, few data are available
for the extensive, open vegetated types typical of low-density residential areas that have been newly converted from rural
land use. We made direct measurements of surface energy fluxes using the eddy-covariance technique at Greenwood, a recently
developed exurban neighbourhood near Kansas City, Missouri, USA, during an intensive field campaign in August 2004. Energy
partitioning was dominated by the latent heat flux under both cloudy and near clear-sky conditions. The mean daytime Bowen
ratio (β) values were 0.46, 0.48, and 0.47 respectively for the cloudy, near clear-sky and all-sky conditions. Net radiation (R
n
) increased rapidly from dawn (−34 and −58W m−2) during the night to reach a maximum (423 and 630W m−2) after midday for cloudy and near clear-sky conditions respectively. Mean daytime values were 253 and 370W m−2, respectively for the cloudy and near clear-sky conditions, while mean daily values were 114 for cloudy and 171W m−2 for near clear-sky conditions, respectively. Midday surface albedo values were 0.25 and 0.24 for the cloudy and near clear-sky
conditions, respectively. The site exhibited an angular dependence on the solar elevation angle, in contrast to previous observations
over urban and suburban areas, but similar to vegetated surfaces. The latent heat flux (Q
E
), sensible heat flux (Q
H
), and the residual heat storage ΔQ
s
terms accounted for between 46–58%, 21–23%, and 18–31% of R
n
, respectively, for all-sky conditions and time averages. The observed albedo, R
n
, and Q
E
values are higher than the values that have been reported for suburban areas with high summer evapotranspiration rates in
North America. These results suggest that the rapidly growing residential areas at the exurban fringe of large metropolitan
areas have a surface energy balance that is more similar to the rural areas from which they were developed than it is to the
older suburbs and city centres that make up the urban fabric to which they are being joined. 相似文献
125.
N. S. Krishnamurthy V. Ananda Rao Dewashish Kumar K. K. K. Singh Shakeel Ahmed 《Journal of the Geological Society of India》2009,73(5):639-650
Exploration and exploitation of coal seams is one of the major resources for the energy sector in any country but at the same
time water filled voids/water logged areas in the old workings of these seams are very critical problems for the coal mining
industry. In such situations, disasters like inundation, landslides, collapsing of the old seams may occur. In this regard,
it is necessary to find out the water saturated/water filled voids and zones in the mining areas. Since no established technique
is available to find such zones, an experimental study using Electrical Resistivity Imaging (ERI) has been carried out in
one of the coal mining areas near Dhanbad, to find out the feasibility of finding the barrier thickness and the water logged
area in underground coal mines. The area under study forms part of Jharia coalfield in Dhanbad district, Jharkhand state.
The coal bearing rocks of Barakar Formation of Lower Permian age (Gondwana period) occur in the area under a thin cover (10
m to15 m) of soil and or alluvium. Coal bearing Barakar Formations consist mainly of sandstone of varying grain size, intercalation
of shale and sandstone, grey and carbonaceous-shale and coal seams.
Since the water saturation reduces the resistivity of a formation to a large extent, water filled voids and old coal workings
are expected to have significant resistivity contrast with the surrounding host rock. Hence, ERI technique was applied in
such an environment as this technique uses high-density data acquisition both laterally and vertically by using multiple number
of electrodes. Along with ERI, mise-à-la-masse (also called charged body) technique was also employed at one of the promising
sites to find out the connectivity of water logged areas and also detection of these old workings from the surface measurements
was analyzed. The interpreted 2D resistivity sections have clearly indicated the water bearing zone(s) along the profile which
was well confirmed with the existing water level in the nearby borewells. On the other hand, this technique did not identify
the size of the coal pillar and gallery (air filled voids), which might be due to the small size of the voids (i.e. about
2 m × 2 m) below a depth of 15m and more but have indicated altogether as a high resistive zone ranging from 600–1000 Ohm-m. 相似文献
126.
Stefano Urbini Iacopo Nicolosi Antonio Zeoli Sami El Khrepy Ahmed Lethy Mahfooz Hafez Mohamed El Gabry Ahmed El Barkooky Aly Barakat Mahomoud GOMAA Ali M. Radwan Mohamed El Sharkawi Massimo D’orazio Luigi Folco 《Meteoritics & planetary science》2012,47(11):1842-1868
Abstract– We detail the Kamil crater (Egypt) structure and refine the impact scenario, based on the geological and geophysical data collected during our first expedition in February 2010. Kamil Crater is a model for terrestrial small‐scale hypervelocity impact craters. It is an exceptionally well‐preserved, simple crater with a diameter of 45 m, depth of 10 m, and rayed pattern of bright ejecta. It occurs in a simple geological context: flat, rocky desert surface, and target rocks comprising subhorizontally layered sandstones. The high depth‐to‐diameter ratio of the transient crater, its concave, yet asymmetric, bottom, and the fact that Kamil Crater is not part of a crater field confirm that it formed by the impact of a single iron mass (or a tight cluster of fragments) that fragmented upon hypervelocity impact with the ground. The circular crater shape and asymmetries in ejecta and shrapnel distributions coherently indicate a direction of incidence from the NW and an impact angle of approximately 30 to 45°. Newly identified asymmetries, including the off‐center bottom of the transient crater floor downrange, maximum overturning of target rocks along the impact direction, and lower crater rim elevation downrange, may be diagnostic of oblique impacts in well‐preserved craters. Geomagnetic data reveal no buried individual impactor masses >100 kg and suggest that the total mass of the buried shrapnel >100 g is approximately 1050–1700 kg. Based on this mass value plus that of shrapnel >10 g identified earlier on the surface during systematic search, the new estimate of the minimum projectile mass is approximately 5 t. 相似文献
127.
Rehman Sufia Sahana Mehebub Hong Haoyuan Sajjad Haroon Ahmed Baharin Bin 《Natural Hazards》2019,96(2):975-998
Natural Hazards - Floods have always been associated with widespread devastation and destruction since the emergence of human civilization. The intensity of this disaster has been increasing due to... 相似文献
128.
Station-keeping for a translunar communication station 总被引:1,自引:0,他引:1
John V. Breakwell Ahmed A. Kamel Martin J. Ratner 《Celestial Mechanics and Dynamical Astronomy》1974,10(3):357-373
A translunar communication station is to be kept close to a nominal unstable periodic ‘Halo’ orbit, visible at all times from Earth. The analytically computed nominal orbit is not perfect, requiring an average control acceleration of about 10?6 g's for tight control. An adjustable quadratic combination of position deviation and control acceleration is minimized to provide an (adjustable) control law with period feedback gains and a periodic bias. The average control acceleration can be reduced to less than 10?8 g's with an error settling time of less than 21/2 months. The resulting limiting motion provides, in turn, an improved nominal, permitting the same low control cost with much tighter control, corresponding to settling times of the order of one day. 相似文献
129.
Luís André Magaia Tada-nori Goto Alaa Ahmed Masoud Katsuaki Koike 《Natural Resources Research》2018,27(3):275-298
Exploring for groundwater in crystalline rocks in semiarid areas is a challenge because of their complex hydrogeology and low potential yields. An integrated approach was applied in central western Mozambique, in an area covered by Precambrian crystalline basement rocks. The approach combined a digital elevation model (DEM), remote sensing, and a ground-based geophysical survey. The aim was to identify groundwater zones with high potential and to identify geological structures controlling that potential. Lineaments were extracted from the DEM that had been enhanced using an adaptive-tilt, multi-directional, shading technique and a non-filtering technique to characterize the regional fracture system. The shallowness and amount of stored groundwater in the fracture zones was assessed using vegetation indices derived from Landsat 8 OLI images. Then, 14 transient electromagnetic (TEM) survey profiles were taken in different geological settings across continuous lineaments that were considered to be aligned along inferred faults. In the central lineament zones, the TEM soundings gave resistivity values of less than 300 Ωm at a depth of 20–80 m. The values varied with location. Conversely, values greater than 400 Ωm were observed at the sites away from the central zones. This contrast is probably caused by the differences in permeability and degree of weathering along the fractured zones. These differences could be key factors in determining groundwater occurrence. By integrating five water-related factors (lineament density, slope, geology, vegetation index, and proximity to lineaments), high groundwater potential zones were located in the vicinity of the lineaments. In these zones, vegetation remains active regardless of the season. 相似文献
130.
Zeinab SALAH Ahmed SHALABY Allison L.STEINER Ashraf S.ZAKEY Ritesh GAUTAM Mohamed M.ABDEL WAHAB 《大气科学进展》2018,35(2):182-194
This study assesses the direct and indirect effects of natural and anthropogenic aerosols(e.g., black carbon and sulfate)over West and Central Africa during the West African monsoon(WAM) period(June–July–August). We investigate the impacts of aerosols on the amount of cloudiness, the influences on the precipitation efficiency of clouds, and the associated radiative forcing(direct and indirect). Our study includes the implementation of three new formulations of auto-conversion parameterization [namely, the Beheng(BH), Tripoli and Cotton(TC) and Liu and Daum(R6) schemes] in Reg CM4.4.1,besides the default model's auto-conversion scheme(Kessler). Among the new schemes, BH reduces the precipitation wet bias by more than 50% over West Africa and achieves a bias reduction of around 25% over Central Africa. Results from detailed sensitivity experiments suggest a significant path forward in terms of addressing the long-standing issue of the characteristic wet bias in Reg CM. In terms of aerosol-induced radiative forcing, the impact of the various schemes is found to vary considerably(ranging from-5 to-25 W m~(-2)). 相似文献