首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
In Fayetteville Green Lake, past sedimentation rates can be accurately and precisely estimated by separating annual couplets or varves in dried sediment samples. Two measures were used, which serve as upper and lower limits on estimated sedimentation rate. They agree within 5 % with average annual sedimentation rate in couplets for recent years. Between 3 and 5 replicate samples are needed to reduce the half-width of 95 % confidence intervals on individual couplet sedimentation rates to 30 g m?2 yr?1 about 5 % of average recent rates. In the late 1800s sedimentation rate averaged 392 g m?2 yr?1 and ranged between 324 and 466 g m?2 yr?1, while in the 1970s the rate averaged 581 g m?2 yr?1 by the same measure, and ranged between 384 and 646 g m?2 yr?1. Sedimentation rate averaged for 13 years does not vary over short distances in the profundal zone, but lateral variation in sedimentation rate can be detected for individual years over the same distance. Not all this variation was associated with the non-uniform distribution of dark sublaminae and thin turbidites which cannot be separated from the annual layers. This indicates that although precise estimates of sedimentation rates can be made at different points in the lake, estimates will have to be made at numerous points before annual sedimentation rates for the lake as a whole can be accurately assessed.  相似文献   

2.
Benthic fluxes in two southern California borderland basins have been estimated by modeling water column property gradients, by modeling pore water gradients and by measuring changes in concentration in a benthic chamber. Results have been used to compare the different methods, to establish budgets for biogenic silica and carbon and to estimate rate constants for models of CaCO3 dissolution. In San Pedro Basin, a low oxygen, high sedimentation rate area, fluxes of radon-222 (86 ± 8 atoms m−2 s−1), SiO2 (0.7 ± 0.1 mmol m−2 d−1), alkalinity (1.7 ± 0.3 meq m−2 d−1), TCO2 (1.9 ± 0.3 mmol m−2 d−1) and nitrate (−0.8 ± 0.1 mmol m−2 d−1) measured in a benthic chamber agree within the measurement uncertainty with fluxes estimated from modeling profiles of nutrients and radon obtained in the water column. The diffusive fluxes of radon, SiO2 and TCO2 determined from modeling the sediment and pore water also agree with the other approaches. Approximately 33 ± 13% of the organic carbon and 37 ± 47% of the CaCO3 arriving at the sea floor are recycled. In San Nicolas Basin, which has larger oxygen concentrations and lower sedimentation rates than San Pedro, the fluxes of radon (490 ± 16 atoms m−2 s−1), SiO2 (0.7 ± 0.1 mmol m−2 d−1), alkalinity (1.7 ± 0.3 meq m−2 d−1), TCO2 (1.7 ± 0.2 mmol m−2 d−1), oxygen (−0.7 ± 0.1 mmol m−2 d−1) and nitrate (-0.4 ± 0.1 mmol m−2 d−1) determined from chamber measurements agree with the water column estimates given the uncertainty of the measurements and model estimates. Diffusion from the sediments matches the lander-measured SiO2 and PO43− (0.017 ± 0.002 mmol m−2 d−1) fluxes, but is not sufficient to supply the radon or TCO2 fluxes observed with the lander. In San Nicolas Basin 38 ± 9% of the organic carbon and 43 ± 22% of the CaCO3 are recycled. Approximately 90% of the biogenic silica arriving at the sea floor in each basin is recycled. The rates of CaCO3 dissolution determined from chamber flux measurements and material balances for protons and electrons are compared to those predicted by previously published models of CaCO3 dissolution and this comparison indicates that in situ rates are comparable to those observed in laboratory studies of bulk sediments, but orders of magnitude less than those observed in experiments done with suspended sediments.  相似文献   

3.
Short-term sedimentation patterns were evaluated from August 1992 to May 1993 in different wetland habitats characteristic of the Rhône Delta, including impounded and seasonally-dry saline Arthrocnemum marshes, brackish Juncus, Phragmites, and Scirpus riverine wetlands directly connected to the Rhône River, and Arthrocnemum-dominated marine marshes influenced by the Mediterranean. Short-term sedimentation was measured as sediment accumulation on paper filters which had been placed on the soil surface for several weeks. Total sedimentation and material lost on ignition was significantly related to individual sampling periods, reflecting the importance of short-term processes. High sedimentation at riverine sites (up to 22 g m?2 d?1) was related to a combination of river stage and wind events. Marine and impounded wetlands of the Rhône Delta experienced low sedimentation throughout the period of study. Sedimentation rates averaged over the study period were 0.8 g m?2 d?1, 1.8 g m?2 d?1, and 5.4 g m?2 d?1 for marine, impounded, and riverine sites, respectively. Percent material lost on ignition was low in all habitats (average 15%) and followed a seasonal trend with a minimum in late fall and winter (1%). Soil percent organic matter was also low in the top several centimeters (13%), suggesting that inorganic sedimentation is very important for accretion on these wetland surfaces. Coastal flooding was not a significant mechanism for sedimentation in the marine sites during the period of study. Sedimentation is an important factor in elevation change, and this study shows that impounded habitats, the most common “natural environment“ left in the delta, may become vulnerable to sea-level rise in the future if management practices continue to isolate these wetlands from riverine sources of sediment.  相似文献   

4.
Heat flow and thermal modeling of the Yinggehai Basin, South China Sea   总被引：9，自引：0，他引：9  
Geothermal gradients are estimated to vary from 31 to 43 °C/km in the Yinggehai Basin based on 99 temperature data sets compiled from oil well data. Thirty-seven thermal conductivity measurements on core samples were made and the effects of porosity and water saturation were corrected. Thermal conductivities of mudstone and sandstone range from 1.2 to 2.7 W/m K, with a mean of 2.0±0.5 W/m K after approximate correction. Heat flow at six sites in the Yinggehai Basin range from 69 to 86 mW/m2, with a mean value of 79±7 mW/m2. Thick sediments and high sedimentation rates resulted in a considerable radiogenic contribution, but also depressed the heat flow. Measurements indicate the radiogenic heat production in the sediment is 1.28 μW/m3, which contributes 20% to the surface heat flow. After subtracting radiogenic heat contribution of the sediment, and sedimentation correction, the average basal heat flow from basement is about 86 mW/m2.Three stages of extension are recognized in the subsidence history, and a kinematic model is used to study the thermal evolution of the basin since the Cenozoic era. Model results show that the peak value of basal heat flow was getting higher and higher through the Cenozoic. The maximum basal heat flow increased from 65 mW/m2 in the first stage to 75 mW/m2 in the second stage, and then 90 mW/m2 in the third stage. The present temperature field of the lithosphere of the Yinggehai Basin, which is still transient, is the result of the multistage extension, but was primarily associated with the Pliocene extension.  相似文献   

5.
Watershed degradation due to soil erosion and sedimentation is considered to be one of the major environmental problems in Iran. In order to address the critical conditions of watershed degradation in arid and semiarid regions, a study based on the Modified Pacific Southwest Inter-Agency Committee (MPSIAC) model was carried out at Golestan watershed, northeast of Iran. The model information layers comprising nine effective factors in erosion and sedimentation at the watershed site were obtained by digitalization and spatial interpolation of the basic information data in a GIS program. These factors are geology, soil, climate, runoff, topography, land cover, land use, channel, and upland erosion. The source data for the model were obtained from available records on rainfall and river discharge and sediment, topography, land use, geology, and soil maps as well as field surveys and laboratory analysis. The results of the MPSIAC model indicated that 60.75 % (194.4 km2) and 54.97 % (175.9 km2) of the total watershed area were classified in the heavy sedimentation and erosion classes, and the total basin sediment yield and erosion were calculated as 4,171.1 and 17,813.4 m3 km?2 year?1, respectively. In the sensitivity analysis, it was found that the most sensitive parameters of the model in order of importance were topography (slope), land cover and use, runoff, and channel erosion (R 2?=?0.92–0.94), while geology, climate (rainfall), soil, and upland erosion factors were found to have moderate effect to the model output (R 2?=?0.74–0.59).  相似文献   

6.
A variety of methids have been used to document contemporary rates of overbank sedimentation along an 11 km reach of the floodplain of the River Culm in Devon, UK. These have included measurements of the conveyance loss of suspended sediment between upstream and downstream measuring stations, the use of sediment traps, and the application of caesium-137 measurements to estimate the rate of accumulation of fine sediment over the past 30 years. The resultant data point to sedimentation rates typically of the order of 1500 g m–2 year–1 or 1.5 mm year–1. Values in excess of 15000 g m–2 year–1 or 15 mm year–1 have been documented in closed depressions and in backwater embayments close to the channel, whereas minimal deposition and even scour may occur along those areas that function as secondary channels during floodplain inundation.  相似文献   

7.
The present study describes variations in the vertical fluxes measured concurrently with sediment traps at both a shallow water (4 m) and a deeper water (7.5 m) position in a coastal lagoon in April 1995. A tripod equipped with five sediment traps (trap openings at 0.35 m, 0.75 m, 1.05 m, 1.40 m, and 1.80 m above the seabed) was placed at the shallow water position. This tripod was deployed three times during the study period and deployment periods varied between 2 d and 5 d. The second sediment trap, placed at the deep water position in the central part of the lagoon, measured vertical flux for intervals of 12 h at 1.4 m above the seabed. The horizontal distance between the sediment traps was 8 km. The average maximum vertical flux at the shallow water position reached 27.9 g m−2 d−1 during a period of high, westerly wind speeds, and a maximum vertical flux of 16.9 g m−2 d−1 was reached at the deep water position during a period of high, easterly wind speeds. Both strong resuspension events were closely related to increased wave shear stress derived from surface waves. Maximum wave-induced resuspension rate was 10 times higher at the shallow water position and 3.8 times higher at the deep water position compared with the net sedimentation rate in the lagoon. Small resuspension events occurred at the shallow water position during periods of increased current shear stress, Estimations of conditions for transport of sediment between shallow water and deep water showed that particles must be resuspended to a height between 3 m and 4 m and that current speeds must be higher than about 0.1 m s−1. An average sedimentation rate of 3.8 g m−2 d−1 was obtained at the shallow water position during a period without wave shear stress and low current shear stress. This rate measured by sediment traps is similar to a net sedimentation rate in the lagoon of 4.4 g m−2 d−1, which was determined by radiocarbon dating of a sediment core (Kristensen et al. 1995).  相似文献   

8.
Aeolian sand and dust in polar regions are transported offshore over sea ice and released to the ocean during summer melt. This process has long been considered an important contributor to polar sea floor sedimentation and as a source of bioavailable iron that triggers vast phytoplankton blooms. Reported here are aeolian sediment dispersal patterns and accumulation rates varying between 0·2 g m?2 yr?1 and 55 g m?2 yr?1 over 3000 km2 of sea ice in McMurdo Sound, south‐west Ross Sea, adjacent to the largest ice free area in Antarctica. Sediment distribution and the abundance of southern McMurdo Volcanic Group‐derived glass, show that most sediment originates from the McMurdo Ice Shelf and nearby coastal outcrops. Almost no sediment is derived from the extensive ice free areas of the McMurdo Dry Valleys due to winnowed surficial layers shielding sand‐sized and silt‐sized material from wind erosion and because of the imposing topographic barrier of the north‐south aligned piedmont glaciers. Southerly winds of intermediate strength (ca 20 m sec?1) are primarily responsible for transporting sediment northwards and offshore. The results presented here indicate that sand‐sized sediment does not travel more than ca 5 km offshore, but very‐fine sand and silt grains can travel >100 km from source. For sites >10 km from the coast, the mass accumulation rate is relatively uniform (1·14 ± 0·57 g m?2 yr?1), three orders of magnitude above estimated global atmospheric dust values for the region. This uniformity represents a sea floor sedimentation rate of only 0·2 cm kyr?1, well below the rates of >9 cm kyr?1 reported for biogenic‐dominated sedimentation measured over much of the Ross Sea. These results show that, even for this region of high‐windblown sediment flux, aeolian processes are only a minor contributor to sea floor sedimentation, excepting areas proximal to coastal sources.  相似文献   

9.
Modeling and simulation using GPS-X software for a packed bed up-flow anaerobic sludge blanket followed by a biological aerated filter were studied. Both treatment units were packed with a non-woven polyester fabric as a bio-bed. The system was operated at a hydraulic and organic loading rate of 9.65 m3/m2/d and 2.64 kg BOD5/m3/day. Verification of the experimental results and calibration of the model were carried out prior simulation and modeling. Variables under consideration were HLR, OLR, and surface area of the packing material. HLR and OLR are increased incrementally until the break through point has been achieved. The results obtained from modeling indicated that the treatment system has great potential to be used as an ideal and efficient option for high hydraulic and organic loading rates up to 19.29 m3/m2/d and 4.48 kg BOD5/m3/day. The model indicated that increasing the input HLR and OLR loads to the treatment system up to 50 % of the original values achieved removal efficiencies 98 % for TSS, 88 % for BOD5, and 85 % for COD. Moreover, increasing the HLR to four times the original value (38.59 m3/m2/d) reduced the efficiency of the treatment system to 50 % for COD and BOD5. However, the removal rates of TSS, TKN, and TP were not affected. Also, the modeling results indicated that increasing the surface area of the packing material increased the overall efficiency of the treatment system.  相似文献   

10.
The objective of this study was to examine the mineralogy and geochemical stability of ochreous sediments accumulated in a compost wetland constructed in 1990 for acid mine drainage treatment. Intact sediment cores were collected in 1996 and 2000 from an area that had accumulated 33 cm of ochre. Solids and pore waters were subsequently separated by centrifugation and analyzed using conventional methods, including X-ray diffraction, infrared spectroscopy, scanning electron microscopy, and wet chemical techniques. The solid phase had an average Fe content of 585 g/kg and was predominantly schwertmannite [Fe 8O8(OH)4.8(SO4)1.6] in the upper portion of the sediment column, but transformed to goethite (α-FeOOH) with depth. The rate of transformation was calculated to be 30 mol/m3/yr in the initial 6 yr of sedimentation as compared to 10 mol/m3/yr for the 4-yr period from 1996 to 2000. Pore water composition was affected by this mineral transformation through production of acidity and the release of Fe and SO4. These results demonstrate that the sediment column was not a static environment. In addition, the transformation of schwertmannite to goethite, which has been observed under laboratory conditions, also occurs in natural systems.  相似文献   

11.
Heat flow has been determined by combining temperature measurements in 7 boreholes with thermal conductivity measurements in the Upper Vindhyan sedimentary rocks of Shivpuri area, central India. The boreholes are distributed at 5 sites within an area of 15 × 10 km2; their depths range from 174 to 268 m. Geothermal gradients estimated from borehole temperature profiles vary from 8.0–12.7 mK m−1 in the sandstone-rich formations to 25.5–27.5 mK m−1 in the shale-rich formations, consistent with the contrast in thermal conductivities of the two rock types. Heat flow in the area ranges between 45 and 61 mW m−2, with a mean of 52±6 mW m−2. The heat flow values are similar to the >50 mW m−2 heat flow observed in other parts of the northern Indian shield. The heat flow determinations represent the steady-state heat flow because, the thermal transients associated with the initial rifting, convergence and sedimentation in the basin as well as the more recent Deccan volcanism that affected the region to the south of the basin would have decayed, and therefore, the heat flow is in equilibrium with the radiogenic heat production of the crust and the heat flow from the mantle. The present study reports the heat flow measurements from the western part of the Vindhyan basin and provides heat flow information for the Bundhelkhand craton for the first time. Radioelement (Th, U and K) abundances have been measured both in the sedimentary rocks exposed in the area as well as in the underlying basement granite-gneiss of Bundelkhand massif exposed in the adjacent area. Radioactive heat production, estimated from those abundances, indicate mean values of 0.3 μW m−3 for sandstone with inter-bands of shale and siltstone, 0.25 μW m−3 for sandstone with inter-bands siltstone, 0.6 μW m−3 for quartzose sandstone, and 2.7 μW m−3 for the basement granitoids. With a total sedimentary thickness not exceeding a few hundred metres in the area, the heat production of the sedimentary cover would be insignificant. The radioactive heat contribution from the basement granitoids in the upper crust is expected to be large, and together with the heat flow component from the mantle, would control the crustal thermal structure in the region.  相似文献   

12.
Flow parameters (velocity and density) for turbidity currents in the Northwest Atlantic Mid-Ocean Channel (NAMOC) have been determined based on two different approaches, channel geometry and grain-size distributions of turbidites. Channel geometry has been obtained by a quantitative morphological analysis of the NAMOC which shows three genetically different segments in the upper 2000 km: (1) an upper 350 km-long ‘equilibrium channel’, (2) a middle 700 km-long ‘modified equilibrium channel’and (3) a lower ‘basement-controlled channel’which is more than 1000 km-long. In contrast to other meandering submarine channels the NAMOC has very low sinuosities and gradients. A consistently higher right-hand levee limits mean flow velocities to 3ms?1 and channel geometry indicates mean flow velocities of 0·86 m s?1 that decrease within the equilibrium channel to 0·05 m s?1. Grain-size distributions on the levees and in the channel suggest strong vertical velocity and density gradients for bank-full flows with velocities of up to 8 m s?1 and excess densities up to 87 kg m?3 at the base, and 0·45 m s?1 and 4 kg m?3 at the top. The internal shear produced by these strong vertical gradients results in a decoupling of the current head and body. Channel geometry appears to be mainly the result of the slowly moving dilute body of the current.  相似文献   

13.
The Outardes Estuary (Quebec) is a regressive sandy system developed in a sub-boreal climatic setting. The Outardes River drains an area of 18 780 km2 and was dammed for hydroelectric purposes in the 1960s and‘70s. By reconstructing the hydrographs and mapping the morphology of the estuary from aerial photographs over a period of 21 years, we have documented the changes in the estuary resulting from the modification of the hydrologic regime. Until the mid-1960s, a catastrophic spring freshet (1800–2800 m33s-1) controlled the morphology of the estuary. The mouth was dominated by a disorganized, braided channel pattern, and large fluvial discharge-generated transverse bars. Peripheral channel areas in the upper estuary were zones of transport at flood stages. For 18 months in 1968 and‘69 the river discharge was drastically reduced (50 m3s-1) to accomplish the main reservoir filling. Tidal currents and waves filled and remodelled the morphology of the mouth of the estuary, while the peripheral zones in the upper estuary experienced low-energy sedimentation. Since the generating plants were brought on-line in 1969, the fluvial discharge has been unable to modify substantially the sand body geometry in the mouth of the estuary. In the upper estuary the peripheral regions continue to be filled in, but the thalweg is being eroded. Based on the results of our coring programme, we suggest that major environmental changes may leave a sedimentary signature (addition or subtraction of size modes) which can be identified by the detailed analysis of grain size distributions.  相似文献   

14.
Soil radon (222Rn) has been monitored during winter months under cool-temperate deciduous stands of different surface geology in Tomakomai and in Sapporo, Hokkaido, Japan. Radon level was lower in Tomakomai of immature soil of porous volcanic ash emitted from an active volcano (Mt. Tarumae), compared with those in Sapporo of alluvial sediments. In Tomakomai, mean value of the 222Rn activity concentration was higher in winter (570 Bq m?3) than in summer (350 Bq m?3) at a depth of 1 m, which is consistent with the results in cold and dry winter reported in the literature. In contrast, soil radon decreasing with decreasing soil temperature from mid-September (5.0 kBq m?3) remained low (2.6 kBq m?3) under persistent snow in Sapporo, which had already been observed in the same location. Measurements of the activity concentrations of 222Rn in snow and in snow air as well as in soil air indicate that the small amount of 222Rn is released from the ground surface to the overlying snowpack with a 222Rn flux density of 0.4 mBq m?2 s?1 under thick snow cover in Sapporo.  相似文献   

15.
The goal of this paper is to find out whether suspended mussel culture affects the vertical fluxes of biogenic particles in the Ría de Vigo on a seasonal scale. With this aim, vertical fluxes of particulate organic carbon (POC) and the magnitude and composition of vertical export of phytoplankton carbon (Cphyto) collected in sediment traps were examined by comparing data obtained inside a mussel farming area (RaS) with those found at a reference station (ReS) not affected by mussels. Our results indicate that mussel farming has a strong impact on sedimentation fluxes under the rafts, not only increasing POC flux but also altering the magnitude and composition of Cphyto fluxes. Average POC flux at RaS (2564?±?1936 mg m?2 day?1) was four times higher than at ReS (731?±?276 mg m?2 day?1), and much of this increase was due to biodeposit fluxes (Cbiodep) which accounted for large proportion of POC flux (35–60 %). Indeed, because of this high Cbiodep flux, only a small proportion of the POC flux was due to Cphyto flux (3–12 %). At the same time, we observed an increased sedimentation of phytoplankton cells at RaS that could be explained by a combination of mechanisms: less energetic hydrodynamic conditions under mussel rafts, ballast effect by sinking mussel feces, and diatom aggregates. Moreover, mussel farming also altered the quality of the Cphyto flux by removing part of the predatory pressure of zooplankton and thus matching diatom composition in water column and sediment traps.  相似文献   

16.
Surficial and core samples collected from the eastern Thessaloniki Gulf, located in the NW Aegean Sea, were analyzed for their sedimentation rate and inventories of 137Cs and 210Pb. The study of the spatial radionuclide dispersion in the specific region is essential for the assessment of marine pollution levels. The sedimentation rates were calculated from the vertical distribution of 137Cs and excess 210Pb in the sediment cores. The spatial distribution of 137Cs was studied with respect to sediment characteristics such as the grain size of the sample and the organic carbon content. The activity concentrations were measured by means of gamma ray spectrometry using HPGe detector for 137Cs and gross alpha counting using alpha counter for 210Pb. The average sedimentation rate along the sediment cores is characterized by a consistent pattern and varies from 0.18?±?0.02 to 0.22?±?0.03?cm?year?1. The 137Cs inventories varied from 205?±?15 to 602?±?39?Bq?m?2, while the 210Pb inventories of the studied cores exhibited average value of 140?±?9?Bq?m?2. Elevated 137Cs activities were observed compared to certain Mediterranean marine areas; however, they were lower than the reported values in the Black Sea.  相似文献   

17.
The Maknassy basin in central Tunisia receives insignificant precipitation (207 mm/y), but the hydrological system retain very small quantities of water due to the steep topography and surface water resource partially mobilised witch is evacuated toward the basin outlet. However, the Maknassy plain support agriculture based on ground water irrigation with increasing water demand last decades. These developments have boosted agricultural productivity in the region. While these problems are mainly due to poor surface water management strategies in the region, the groundwater resources in this basin should be properly assessed and suitable measures taken for uniform surface water mobilization. As a first step in this direction, groundwater resources have been assessed. Regional specific yield (0.017) and groundwater recharge have been estimated on the basis of water table fluctuation method. Groundwater recharge amounting to 61.5 106 m3 in a year takes place in the region through infiltration of rainwater (48.1*106 m3 for phreatic aquifer and 13.4*106 m3 for the deep one), and recharge due to the infiltration in the Leben quady bed (1.57*106 m3). Recharge to deeper aquifers has been estimated at 0.1*106 m3 during dry seasons. Assuming that at least 40 % (102.61*106 m3) of the total precipitation water (256.64*106 m3) makes the runoff water, this important resource can be mobilized in order to increase groundwater recharge. Subject to an arid climate, such region requires an integrated water resource management. It permits to keep aquiferous system equilibrium and participate to the sustainable development which integrates natural resource management.  相似文献   

18.
The paper presents the results of our investigations carried out to examine the behavior of native gold in the southern West Siberia during the fluvial transport of an auriferous terrigenous material at the mountain-to-plain transition geomorphological zone. The piedmont alluvium accumulates the small- and, to a lesser extent, fine-grained gold transported from the denudation zones by the main mountain-lowland rivers. The background Au content varies from n to 10n mg/m3. High contents (100n mg/m3 to n g/m3) are recorded in some sectors in the upper zones of channel alluvium (spit and shoal facies). Flatness coefficient of particles is an important characteristic of allochthonous native gold in the piedmont alluvium. This property shows direct correlation with the lateral dimension of particles. Composition is an essential typomorphic property of the small- and fine-grained gold in the studied sediments. This statement is primarily valid for the degree of fineness commonly related to the Ag content in gold. In some cases, a significant indicator role can be played by traces of Hg and Cu. For example, Hg is an important indicator for the gold-mercury mineralization, whereas Cu is an essential indicator in the case of gold mineralization associated with rhodingites and high-temperature Cu-Au mineralization. Abundance of the allochthonous native gold in different-age terrigenous sediments testifies to its high stability during the continental sedimentation.  相似文献   

19.
The future availability and sustainability of fresh groundwater resources in the South West district of the national capital territory (NCT) Delhi, India, have been projected. Due to a rapid decline in groundwater level and quality, the district has been required by the Government of India to regulate development of groundwater resources. Shallow groundwater is mostly saline and water resources in the area are limited. The methodology applied here involves microzonation of the district in terms of thickness of fresh groundwater and then quantification of present and future availability of freshwater in different freshwater zones, including tentative timescales. The calculation method has been aided by data on historic trends in water level at representative groundwater monitoring stations, located either in fresh groundwater zones or near to them. It is estimated that the presently available 481 million m3 of resources will be reduced to 374 million m3 by year 2007 and to 303 million m3 by the year 2012, and by the year 2022 the district will have only 176 million m3 of available fresh groundwater resources.  相似文献   

20.
The paper studies the propagation of Rayleigh waves in an incompressible layer with general variation of rigidity as μ=μ0(1+bz)m resting over a homogeneous incompressible elastic half-space. Instead of using the Whittaker function, the expansion formula proposed by Newlands has been used for better results at shallow depths. As a particular case for m=1, the results have been shown to coincide with those obtained by Newlands. The velocities have been computed for different values of m and the results are presented in graphs.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号