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81.
Computer flow simulations using the HEC-2 step-backwater routine are used to demonstrate the effect of systematically varying river channel width, riffle spacing and channel roughness on the shear velocity, section-mean velocity and energy slope in fixed-bed pool-riffle sequences. Initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the River Severn. Subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored. The purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence ‘reversal’, such that pools scour at high flow whilst deposition occurs on riffles. It is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal. For prescribed conditions, the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width. Downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool-riffle topography. 相似文献
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天山头屯河水沙情势变化分析 总被引:5,自引:1,他引:5
近三十年水文、气象资料表明,头屯河水量的90%由制材厂水文站以上流域汇入,河道泥沙主要来自水库实验以上中游河段,河道来水量与输沙量变化不完全一致,年径流量的多寡与年降水量密切相关,河流输沙量则与春、夏季节的暴雨或阵发性降水有关。头屯河最大月径流量出现在7月份,而最大月输沙量则出现在6月份,与最大降水月(6月)出现时间一致。汛期河道来水量占全年总数的60-65%,而来沙量占全年总数的80-85%,后 相似文献
84.
The determination of the bottom of a subfossil meandering palaeochannel is important for investigations of the palaeohydrology of valley bottoms, and crucial for the quantitative estimation of past river discharges. The authors present a procedure on the basis of which the bottom of a palaeochannel can be precisely determined. It is based on an analysis of the stratigraphic variability of such features of deposits as: mean diameter, standard deviation of the grain-size composition, the percentage of sand, and ignition loss. 相似文献
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本文根据安太堡露天煤矿排土场基底黄土的粘土矿物组成的微结构及其在分级荷载下微结构变化的研究,论述了排土场基底黄土内弱层的形成机理,提出弱层是在上覆排土压力下由于黄土的微结构变化而形成的演化弱层。并指出弱层的形成是导致排土场失稳滑坡的主要控制因素,这种弱层在新排土场设计时期并不存在,极具隐蔽性,但在设计时必须考虑在排土增至一定高度后,弱层的形成对排土场稳定性的影响,否则必将会存在重大滑坡隐患。 相似文献
87.
Dissolved (dialysis in situ) and total concentrations ofCu, Zn, Cd and Al in eight mining polluted rivers in the Røros area, central Norway, were determinedby atomic absorption spectrometry (flame and graphite furnace) and compared to pH, Caconcentration and alkalinity through seasonal variations in river discharge. Totalconcentrations of the metals were highest during early spring flood and during summer andautumn rain episodes. Dissolved concentrations also increased as the spring floodproceeded, but small discharge peaks within this 2 month period as well as a considerableautumn flood episode appeared to lower rather than to raise the dissolved metal concentrations.Consequently the dissolved fractions of Zn, Cd and Al showed a significant negative correlationwith river discharge, and were low at the discharge peaks. Possibly high sediment concentrationsoccurring at high flood conditions more than counteracted desorption induced by pHdecrease, and led to decreased dissolved fractions through adsorption. Cu speciationon the other hand seemed to be more closely linked to pH. Alkalinity and Ca concentration,both assumed to protect aquatic life from metal pollution, were significantly lowerduring episodes with high Cu and Al total concentrations. 相似文献
88.
D. Wang N. Takagi T. Watanabe V.A. Rakov M.A. Uman K.J. Rambo M.V. Stapleton 《Atmospheric Research》2005,76(1-4):412
A comparative analysis has been performed of the channel-base current and light waveforms for four rocket-triggered lightning strokes. It has been found that the current and light signals at the bottom of the channel exhibit a linear relationship (direct proportionality) in their rising portions. However, just after the peaks the linearity disappears, and the light signals usually decrease faster than the currents during the next several microseconds. Later, this trend is reversed and in some cases the light signals show another rising trend, even when the currents continue to decrease. The linear light/current relationship for the rising portions of the waveforms appears to be the same for different strokes. The findings support the idea of evaluating the variation of return stroke current along the lightning return stroke channel using light signals, provided that evaluation is limited to the rising portions of those signals and assuming that the light/current relationship observed at the bottom of the channel holds at other heights. 相似文献
89.
Channel morphology and its impact on flood passage, the Tianjiazhen reach of the middle Yangtze River 总被引:1,自引:0,他引:1
The Tianjiazhen reach of the middle Yangtze is about 8 km long, and characterized by a narrow river width of 650 m and local water depth of > 90 m in deep inner troughs, of which about 60 m is below the mean sea level. The troughs in the channel of such a large river are associated with regional tectonics and local lithology. The channel configuration plays a critical role in modifying the height and duration of river floods and erosion of the riverbed. The formation of the troughs in the bed of the Yangtze is considered to be controlled by sets of NW–SE-oriented neotectonic fault zones, in which some segments consist of highly folded thick Triassic limestone crossed by the Yangtze River. Several limestone hills, currently located next to the river channel, serve as nodes that create large vortices in the river, thereby accelerating downcutting on the riverbed composed of limestone highly susceptible to physical corrosion and chemical dissolution. Hydrological records indicate that the nodal hills and channel configuration at Tianjiazhen do not impact on normal flow discharges but discharges > 50,000 m3s− 1 are slowed down for 2–3 days. Catastrophic floods are held up for even longer periods. These inevitably result in elevated flood stages upstream of prolonged duration, affecting large cities such as Wuhan and a very large number of people. 相似文献
90.