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991.
Abstract This paper describes briefly some findings and investigations on the problem of channel processes conducted in the USSR. It was stimulated by the publication of River Channel Changes (Gregory, 1977). 相似文献
992.
Abstract A physically based model of moisture transfer in the soil-canopy-atmosphere system, fitted to measured data in wheat fields, was applied to calculate the sensitivity of evapotranspiration to the combined effect of possible CO2-induced climate changes and the direct impact of doubling CO2 content in the atmosphere. The impact of carbon dioxide was taken into account by changing the stomatal resistance and the leaf area index. Several climate change scenarios were used. Simulation studies showed that when the changes of a number of climatic and plant factors are considered, evapotranspiration responses can differ greatly from those that consider only temperature change. The temperature effect of the increase in CO2 content in the atmosphere can be compensated by changes in the physiological parameters of vegetation. Thus, under certain conditions, one can expect a decrease in evapotranspiration instead of an increase such as is predicted by a number of models which neglect the direct effects of CO2 on plant behaviour. 相似文献
993.
The warming of the Earth's atmosphere system is likely to change temperature and precipitation, which may affect the climate, hydrology and water resources at the river basins over the world. The importance of temperature change becomes even greater in snow or glacier dominated basins where it controls the snowmelt processes during the late‐winter, spring and summer months. In this study hydrologic responses of streamflow in the Pyanj and Vaksh River basins to climate change are analysed with a watershed hydrology model, based on the downscaled atmospheric data as input, in order to assess the regional climate change impact for the snowfed and glacierfed river basins in the Republic of Tajikistan. As a result of this analysis, it was found that the annual mean river discharge is increasing in the future at snow and glacier dominated areas due to the air temperature increase and the consequent increase in snow/ice melt rates until about 2060. Then the annual mean flow discharge starts to decrease from about 2080 onward because the small glaciers start to disappear in the glacier areas. It was also found that there is a gradual change in the hydrologic flow regime throughout a year, with the high flows occuring earlier in the hydrologic year, due to the warmer climate in the future. Furthermore, significant increases in annual maximum daily flows, including the 100‐year return period flows, at the Pyanj and Vaksh River basins toward the end of the 21st century can be inferred from flood frequency analysis results. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
994.
Doklady Earth Sciences - The data obtained for 2015–2017 on the content and composition of aliphatic hydrocarbons (HCs) in particulate matter at the river–sea geochemical barriers (the... 相似文献
995.
Medium‐temperature ultrahigh pressure (MT‐UHP) eclogites from the south Dabie orogen, as represented by samples from the Jinheqiao, Shuanghe and Bixiling areas, consist of garnet, omphacite, phengite, epidote, hornblendic amphibole, quartz/coesite and rutile with or without kyanite and talc. Garnet is mostly anhedral and unzoned, but a few porphyroblasts are weakly zoned with core–mantle increasing grossular (Xgr) and decreasing pyrope (Xpy) contents. Garnet compositions are closely correlated with the bulk compositions. For instance, the Xpy and Xgr contents are positively correlated with the bulk MgO and CaO contents. Phengite is occasionally zoned with core–rim deceasing Si content, and phengite grains as inclusions in garnet show higher Si than in the matrix, suggesting differently resetting during post‐peak stages. The maximum Si contents are mostly 3.60–3.63 p.f.u. for the three areas. Pseudosections calculated using THERMOCALC suggest that the MT‐UHP eclogites should have a peak assemblage of garnet + omphacite + lawsonite + phengite + coesite in most rocks of higher MgO content. In this assemblage, the Xpy in garnet mostly depends on bulk compositions, whereas the Xgr in garnet and the Si contents in phengite regularly increase, respectively, as temperature and as pressure rise, and thus, can provide robust thermobarometric constraints. Using the Xgr and Si isopleths in pseudosections, the peak P–T conditions were estimated to be 40 kbar/730 °C for the Jinheqiao, 41 kbar/726 °C for the Shuanghe, and 37–52 kbar and 700–830 °C for the Bixiling eclogites. Some eclogites with higher FeO are predicted to have a peak assemblage of garnet + omphacite + coesite ± phengite without lawsonite, where the garnet and phengite compositions highly depend on bulk compositions and generally cannot give available thermobarometric constraints. Decompression of the eclogites with lawsonite in the peak stage is inferred to be accompanied with cooling and involves two stages: an early‐stage decompression is dominated by lawsonite dehydration, resulting in increase in the mode of anhydrous minerals, or further eclogitization, and formation of epidote porphyroblasts and kyanite‐bearing quartz veins in eclogite. As lawsonite dehydration can facilitate evolution of assemblages under fluid‐present conditions, it is difficult to recover real peak P–T conditions for UHP eclogites with lawsonite. This may be a reason why the P–T conditions estimated for eclogites using thermobarometers are mostly lower than those estimated for the coherent ultramafic rocks, and lower than those suggested from the inclusion assemblages in zircon from marble. A late‐stage decompression is dominated by formation of hornblendic amphibole and plagioclase with fluid infiltration. The lawsonite‐absent MT‐UHP eclogites have only experienced a decompression metamorphism corresponding to the later stage and generally lack the epidote overprinting. 相似文献
996.
Spatial and temporal variability of temperature,precipitation, and streamflow in upper Sang‐kan basin,China
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Temporal streamflow variability in an inland hydrologic station and temporal trends and frequency changes at three weather stations in a semiarid river basin located in Loess Plateau, China, were detected by using linear regression, Mann–Kendall analysis, and wavelet transform methods. Double cumulative curve and ordered clustering were used to identify the hydrological periods of upper Sang‐kan (USK) basin between 1957 and 2012. The results indicate that (1) precipitation in the USK basin over the study period did not show any trend, while the temperature showed a significant increase; (2) streamflow flowing out of the USK basin indicated a significant decrease; (3) two distinct hydrological periods – the ‘natural period’ from 1957 to 1984 and the ‘human impact period’ from 1985 to 2012 – were present; and (4) the contributions of climate change and human activities to reduce the streamflow were 36.9% and 63.1% respectively. The results indicate that human activities may be contributing to a decrease in streamflow in the USK basin. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
997.
Mohamed Ashour Gary M. Norris Sherif Elfass Ashraf Z. Al-Hamdan 《Computers and Geotechnics》2010,37(7-8):858-866
This paper presents a procedure to assess the mobilized pile side and tip resistance versus pile head and tip settlement under axial load in clay soil. The load transfer (t–z) curve is evaluated at any point on the loaded pile based on the combined pile tip/side resistance–displacement mechanisms along the length of the pile. Unlike current methods that assume the pile settlements as a percentage of the pile/shaft diameter, the presented technique determines the side and tip resistance of the pile and the associated pile settlement under existing load based on the current stress/strain level in the surrounding soil up to failure (excessive settlement). The technique employs the concepts of the elastic theory and Ramberg–Osgood characterization of the stress–strain behavior of the clay soil. Case studies are also presented to exhibit the capabilities of the proposed procedure. The good agreement between measured and calculated load transfer curves along the pile and pile tip resistance versus pile head, side, and tip displacements shows the consistency of the proposed procedure. A computer code is developed to employ the presented technique. 相似文献
998.
By performing Monte-Carlo calculations of the spin-down of young pulsars surrounded by a supernova fallback disk, we present
possible constraints on the propeller torques exerted by the disks on neutron stars.
This work was supported by the National Natural Science Foundation of China. 相似文献
999.
Hedgerow is one of the most important rural landscapes in the world, especially in Europe. Knowledge about the hydrological role of hedgerows is useful in many fields of study, such as hydrological modelling and rural landscape management. The aim of this study was to investigate the impact of a hedgerow on rainfall distribution, soil-water potential gradient, lateral water transfer and water balance. A hillslope with a hedgerow perpendicular to the slope was monitored. To evaluate hedgerow rainfall interception, rainfall was measured (hourly, daily, and by rainfall event) both next to and up to 16 m upslope and 12 m downslope perpendicularly away from the hedgerow. The strongest correlation between rainfall next to the hedgerow and rainfall at more distant points was obtained using data measured per rainfall event. The average percentage of rainfall intercepted equalled 28% for the leafed period and 12% for the leafless period. The impact of the hedgerow on spatial rainfall distribution was related to distance from the hedgerow and rainfall amount. Annual distribution of soil-water potential showed that the hedgerow influenced it up to 9 m upslope and 6 m downslope, the area in which most of the hedgerow's roots were observed. The soil was driest at the end of summer, which delayed soil rewetting during autumn in areas surrounding the hedgerow. Annual groundwater dynamics exhibited three distinct periods due to temporal rainfall distribution and, especially at the end of summer, root-water uptake. In addition, the total potential gradient showed that unsaturated flow was directed towards the hedgerow in summer and autumn. These results indicate that at the local scale hedgerows influences (1) spatial rainfall distribution, (2) soil rewetting, and (3) groundwater recharge, often at distances well beyond the hedgerow's drip line. Consequently, the processes involved in soil-water dynamics around hedgerows should be integrated into relevant hydrological models, especially for catchments with a dense hedgerow network. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
1000.
In extensive air shower (EAS) experiments, the primary direction is reconstructed by the space–time pattern of secondary particles. Thus the equalization of the transit time of signals coming from different parts of the detector is crucial in order to get the best angular resolution and pointing accuracy allowed by the detector. In this paper an off-line calibration method is proposed and studied by means of proper simulations. It allows to calibrate the array repeatedly just using the collected data without disturbing the standard acquisition. The calibration method is based on the definition of a Characteristic Plane introduced to analyze the effects of the time systematic offsets, such as the quasi-sinusoidal modulation on azimuth angle distribution. This calibration procedure works also when a pre-modulation on the primary azimuthal distribution is present. 相似文献