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991.
Data on riverine fluxes are essential for calculating element cycles (carbon, nutrients, pollutants) and erosion rates from regional to global scales. At most water‐quality stations throughout the world, riverine fluxes are calculated from continuous flow data (q) and discrete concentration data (C), the latter being the main cause of sometimes large uncertainties. This article offers a comprehensive approach for predicting the magnitude of these uncertainties for water‐quality stations in medium to large basins (drainage basin area > 1000 km²) based on the commonly used discharge‐weighted method. Uncertainty levels – biases and imprecisions – for sampling intervals of 3 to 60 days are correlated first through a nomograph with a flux variability indicator, the quantity of riverine material discharged in 2% of time (M2%). In turn, M2% is estimated from the combination of a hydrological reactivity index, W2% (the cumulative flow volume discharged during the upper 2% of highest daily flow) and the truncated b50sup exponent, quantifying the concentration versus discharge relationship for the upper half of flow values (C = a q b50sup, for q > q50, where q50 is the median flow): M2% = W2% + 27.6b50sup. W2% can be calculated from continuous flow measurements, and the b50sup indicator can be calculated from infrequent sampling, which makes it possible to predict a priori the level of uncertainty at any station, for any type of riverine material either concentrated (b50sup > 0) or diluted (b50sup > 0) with flow. A large data base of daily surveys, 125 station variables of suspended particulate matter (SPM), total dissolved solids (TDS) and dissolved and particulate nutrients, was used to determine uncertainties from simulated discrete surveys and to establish relationships between indicators. Results show, for example, that for the same relatively reactive basin (W2% > 25%), calculated fluxes from monthly sampling would yield uncertainties approaching ±100% for SPM (b50sup > 1.4) fluxes and ±10% for TDS (b50sup = ?0.2). The application to the nitrate survey of the river Seine shows significant trends for the 1972–2009 records. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
992.
993.
994.
Dynamics of Turbidity in the Tweed Estuary   总被引:1,自引:0,他引:1  
Results are presented of turbidity measurements made in the Tweed Estuary during 14–29 September 1993. The observations covered a spring-neap period of very strong and relatively weak tides, which included strong wave activity at the mouth of the estuary, the aftermath of a strong freshwater inflow event and a minor inflow event which coincided with neap tides. Turbidity levels between the mouth and the limit of saline intrusion during this period were observed to lie in the range 2–30 ppm. Temperature–salinity relationships, based on rapid sampling throughout the estuary, often indicated conservative mixing between riverine and coastal waters. Turbidity–salinity relationships throughout the estuary were approximately linear for most of the lower salinity range (<30) although the relationships varied throughout a tidal cycle. At the highest salinities, in the lower 2·5 km of estuary, significantly enhanced turbidities occurred during strong, onshore swell-wave conditions. Near-mouth turbidities were very low (<5 ppm) at high-water (HW) when the swell-wave height was small (<0·3 m). There was no correlation between near-mouth turbidity at HW and tidal range, whereas the correlation between near-mouth turbidity and swell-wave height explained over 90% of the variance in near-mouth turbidity. The temporal trends in freshwater turbidity and freshwater inflow during the fieldwork period were fairly similar. A statistically significant relationship existed between these variables when freshwater turbidity was correlated against inflow 30 h earlier. A ‘ model ’ of turbidity, based on 5 days of inflows and used to hindcast turbidity from 14 days of inflows between 16–29 September, explained over 80% of the variance in freshwater turbidity. Therefore, inflow exerted a significant control on the fluvial turbidity. It appears that very fine-grained particles were responsible for the observed turbidity in the central and upper reaches of the Tweed. During the flood, in the presence of strong wave activity in the coastal zone, larger sediment was rapidly winnowed in the lower reaches, close to the mouth, whereas fine silt particles remained in suspension and reached the limit of saline intrusion.  相似文献   
995.
    
Sedimentary processes were monitored in a varved lake in the Canadian High Arctic through three melt seasons and revealed that seasonal sediment deposition rates were highly dependent on short-lived inflow events driven by high suspended sediment concentrations that varied with runoff intensity. Our results illustrate that in accordance with the suspended sediment discharge into the lake, the rate of sediment accumulation changed over short distances down-lake, in a given year. This result indicates that there is a rate and accumulation dependence on short-lived, intense inflow conditions. In addition, there was strong evidence for substantial decoupling between deposition rate and mean grain size of sedimentary deposits. These results have important implications for paleoclimate interpretation of annually laminated sedimentary records from dynamic lake environments and suggest that grain size measures may not be representative proxies of inflow competence. Grain size indices based on a measure of the coarser fraction, rather than the bulk sediment, may be more appropriate to use as a link between contemporary runoff processes and sedimentary characteristics.  相似文献   
996.
气候因素对土壤有机质组成和性质的影响   总被引:1,自引:0,他引:1  
云南省哀牢山徐家坝自然保护区不同海拔的土壤有机质组成和性质与气候因素的相关分析结果表明,土壤有机质的组成(H/F),胡敏酸的光学特征(E_4/E_6)和CaCl_2的絮凝极限(FL值),除FL和E_4/E_6与年均降雨量不呈显著相关关系外,其它数值均与年均温、≥10℃积温和年均降雨量呈显著(p<0.01,n=7)的相关关系,并服从一元二次方程Y=A+BX+CX ̄2.由此证明了气候因素对土壤有机质组成和胡敏酸的性质有显著的影响。  相似文献   
997.
高邮湖的形成和发展   总被引:4,自引:1,他引:3  
廖高明 《地理学报》1992,47(2):139-145
高邮湖地区古为古潟湖浅洼平原,局部浅洼地段有小湖泊。金明昌五年(1194年)黄河夺淮以后,由于治河者多采用在黄河北岸筑堤南岸分流以保漕为主的政策,把大量黄水引泻到淮河流域的广大地区,使高邮湖诸小湖的湖面不断扩大,曾一度发展到五荡十二湖,到了明代后期,才基本上汇为一湖。  相似文献   
998.
    
The effects of dry summer ambient temperature on some physiological and productive responses were studied in an exotic European breed (Holstein–Friesian) and a local zebu breed (Butana ecotype) of cattle in Sudan. Rectal temperature and respiration rate were positively correlated, and dry matter intake and milk yield were negatively correlated with variation in ambient temperature. However, significant adverse effects on high ambient temperature were only observed in the exotic breed.  相似文献   
999.
    
Investigation of sediment samples from Sites 1039 and 1040 (ODP Leg 170) drilled off the Nicoya peninsula (Costa Rica) by organic geochemical and organic petrological methods has revealed that subduction has only little influence on the composition of the sedimentary organic matter. Organic carbon contents reached 1.5% in the Pleistocene samples but Miocene and Pliocene sediments had an average organic carbon content of less than 0.5%. Organic carbon/sulfur ratios are generally below 2.8, reflecting an intense sulfate reduction in the uppermost sediments which was enhanced by sulfate supply both from sea water and deeper strata. Microscopical examinations indicate that the organic matter is mainly derived from marine sources. A small amount of terrigenous organic matter is, however, present as well according to n-alkane and fatty acid distributions. The alkenone unsaturation index U37k′ shows only a slight decrease during the Miocene and Pliocene, and stronger variations in the Pleistocene, probably indicating more stable sea surface temperatures during the Miocene and Pliocene. Variations in the Pleistocene can possibly be related to glacial/interglacial changes.  相似文献   
1000.
烃源岩的非均质性、动力学模型中频率因子的不确定性、热模拟实验条件(开放/封闭程度、加水与否、升温速率)以及热模拟实验的再现性都将影响有机质成烃动力学模型参数, 进而影响油气资源潜力评价结果的可信度.对此, 从多个盆地不同类型有机质样品(18块)的开放体系(Rock-Eval)热模拟实验入手, 选取SFF模型和MFF模型来描述不同类型有机质样品成烃动力学特征, 对比地质外推结果的差异性.其中采用SFF模型低估了具有较低和较高活化能有机质的生烃潜力(反应分数), 而采用MFF模型可以避免这一问题, 同时频率因子随活化能的升高而升高, 能有效解决频率因子不确定性的问题.同时采用算术平均法和依据S2加权平均法对不同类型样品的动力学参数求取平均动力学参数, 并以3.3 ℃/Ma的升温速率进行外推, 对比分析了不同类型有机质外推结果的差异性.最后, 通过对两种模型依据S2加权平均法获得的动力学参数在松辽盆地黑鱼泡凹陷实际应用表明, 应用MFF模型计算得出的生烃门限深度为1 700 m, 与前人研究结论一致.表明采用MFF模型和S2加权平均法获得的动力学参数进行资源评价具有较高可信度.   相似文献   
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