现代冲积河流的河型空间转化模式探讨   总被引：11，自引：2，他引：9  

倪晋仁  王随继  王光谦 《沉积学报》2000,18(1):1-6

论述了现代冲积河流系统中常见河型的自然分布及转化特征,归纳了河型从上游向下游发生空间转化的 5种主要模式。这些河型转化模式发生的相对频率是不同的,其中从辫状河流向曲流河流转化的发生频率最大。文中通过具体实例总结了这些转化模式分别发生时的主要影响因素。  相似文献   

Seasonal variations and mechanism for environmental control of NEE of CO_2 concerning the Potentilla fruticosa in alpine shrub meadow of Qinghai-Tibet Plateau     

《中国科学D辑(英文版)》2006,(Z2)

The study by the eddy covariance technique in the alpine shrub meadow of the Qing-hai-Tibet Plateau in 2003 and 2004 showed that the net ecosystem carbon dioxide exchange (NEE) exhibited noticeable diurnal and annual variations, with more distinct daily changes during the warmer seasons. The CO2 emission of the shrub ecosystem culminated in April and September while the CO2 absorption capacity reached a maximum in July and August. The absorbed carbon dioxide during the two consecutive years was 231.4 and 274.8 g CO2·m-2 respectively, yielding an average of 253.1 gCO2·m-2 per year: that accounts for a large proportion of absorbed CO2 in the region. Obviously, the diurnal carbon flux was negatively related to temperature, radiation and other atmospheric factors. Still, minute discrepancies in kurtosis and duration of carbon emission/absorption were detected between 2003 and 2004. It was found that the CO2 flux in the daytime was similarly affected by photosynthetic photon flux density in both years. Temperature appears to be the most important determinant of CO2 flux: specifically, the high temperature during the plant growing season inhibits the carbon absorption capacity. One potential explanation is that soil respiration is enhanced under such condition. Analysis of biomass revealed that the annual net carbon fixed capacity of aboveground and belowground biomass was 544.0 in 2003 and 559.4 g Cm"2 in 2004, which coincided with the NEE absorption capacity (63.1 g C·m-2 in 2003 and 74.9 g C·m-2 in 2004) in the corresponding plant growing season.  相似文献   

商丘暴雨日及分布特征研究          下载免费PDF全文

丁国超张艳玲  余卫东李春华 王慧娟 《气象与环境科学》2007,30(B09):52-55

对商丘国家观象台1954--2005年月报表中挑取的符合暴雨日条件的142个样本分析，结果表明：商丘暴雨日具有明显的季节性，频发于7、8月份；暴雨日年平均2．73个；日暴雨量最大（193．3mm）不超过200mm；最长连续暴雨日数不超过2日；连续暴雨日降水量累计（223．9mm）不超过250mm；1h最大降水量不超过70mm。暴雨目的年代际变化特征明显，20世纪80年代后暴雨日出现较晚，60--90年代的暴雨日数递减，90年代后有增加趋势；大暴雨日数自60年代起有逐步增加趋势。暴雨日对月、年降水量有显著贡献，4—9月暴雨日对月降水量的贡献很大，且从4月到7月暴雨目的贡献呈递增趋势。一年内暴雨日出现5次时，当年的年雨量为偏多年份。  相似文献   

Characteristics of landscape information Tupu of the Yellow River swings and its subdeltas during 1855―2000     

Ye QingHua  Chen ShenLiang  Huang Chong  Xue YunChuan  Tian GuoLiang  Chen ShuPeng  Shi YaNan  Liu QingSheng  Liu GaoHuan 《中国科学D辑(英文版)》2007,50(10):1566-1577

Based on the theories and methods of cartography on spatial pattern and temporal evolution in Geo-information Tupu (Carto-methodology in Geo-information, CMGI) and the quantitative measurements of pattern and process in landscape ecology, the methodology of landscape information Tupu in studying the swing process of the Yellow River (YR) over the Yellow River Delta (YRD) during 1855―2000 has been demonstrated. The main content includes (1) methodology of landscape information Tupu; (2) evolution process Tupu of YR rump channels; (3) evolution process Tupu of the 7 pieces of subdeltas in YRD during 1855―2000; (4) landscape information Tupu of the modern YRD and the current YRD; (5) landscape information Tupu of YR rump channels on the 7 pieces of subdeltas during 1855―2000; and (6) corridor-matrix landscape information Tupu by YR swings. The methodology of landscape information Tupu here is still primary but we hope that it could provide a new tool for exploring an integrated research on spatiotemporal approaches for landscape "pattern and process".  相似文献   

影响中国的台风频数年代际变化趋势:1951-2004年     

王小玲  王咏梅  任福民  Li Weijing 《气候变化研究进展》2007,3(Z1):66-69

Interdecadal variations in the frequency of typhoon affecting China (TAC) during 1951-2004 were investigated in this study. The yearly frequency of TACs showed a slightly increasing trend from the 1950s to the early 1970s and a significant decreasing trend afterwards. There were more TACs during the 1960s while the most inactive period was observed in recent 10 years. A decreasing trend was observed in the monthly frequency of TACs in June, August and September throughout the 54 years. Large interannual variabilities were apparent in yearly and monthly frequencies of TACs.  相似文献   

The wavelet method as an alternative for reducing ionospheric effects from single-frequency GPS receivers     

E. M. de Souza  J. F. G. Monico 《Journal of Geodesy》2007,81(12):799-804

The ionospheric effect is one of the major errors in GPS data processing over long baselines. As a dispersive medium, it is possible to compute its influence on the GPS signal with the ionosphere-free linear combination of L1 and L2 observables, requiring dual-frequency receivers. In the case of single-frequency receivers, ionospheric effects are either neglected or reduced by using a model. In this paper, an alternative for single-frequency users is proposed. It involves multiresolution analysis (MRA) using a wavelet analysis of the double-difference observations to remove the short- and medium-scale ionosphere variations and disturbances, as well as some minor tropospheric effects. Experiments were carried out over three baseline lengths from 50 to 450 km, and the results provided by the proposed method were better than those from dual-frequency receivers. The horizontal root mean square was of about 0.28 m (1σ).  相似文献   

网络中移动对象的二维时空索引     

陈碧宇  陈晓玲  陈慧萍  吴玮 《武汉大学学报(信息科学版)》2007,32(10):919-923

基于2DSTMON(2-Dimensional spatio-temporal indexfor moving objects in network)二维时空数据模型,提出了一种新的二维网络中移动对象的时空索引2DSTI及其时空查询算法。这种二维时空索引机制简单且易于实现,支持当前轨迹数据和历史轨迹数据的大量时空查询操作。在此基础上,通过实验实现并验证了二维时空索引机制及其时空查询算法。  相似文献   

南极中国长城站空气微生物含量监测(英)     

CHEN Hao-Wen 《冰川冻土》1998,(4)

The outdoor air borne microbial content over Great Wall Station, Antarctica has been monitored, including its diurnal variations and states in fair or foul weather conditions.The results obtained show: the concentration averaged 161. 9 CFU. m-3. When the weather condition was fair, its range of variation is 0～1 336. 2 CFU. m-3, the average value was 1488. 3 CFU. m-3,when the weather was foul, the range of variation was 471. 4-4 296. 8 CFU. m-3. The average value of air borne microbial number in either fair or foul weather was 383. 0 CFU. m-3. This value is over than 21 times that obtained during 1986/1987.The results seem to show that the influence of human activities have been increasing on Antarctic ecosystem. The diurnal variation of the outdoor air borne microbes shows that the peak of content appeared at about 01: 00, and the trough at about 13: 00 in a whole day. Analyses were made of the relationships between the microbial concentration with its related indexes, i. e. relevant air temperature relative humidity or wind force.The results showed some particularities as compared with those from the areas outside Antarctica.  相似文献   

SSBUV and NOAA-11 SBUV/2 Solar Variability Measurements     

Matthew T. DeLand  Richard P. Cebula  Ernest Hilsenrath 《Studia Geophysica et Geodaetica》1998,42(2):83-91

The Shuttle SBUV (SSBUV) and NOAA-11 SBUV/2 instruments measured solar spectral UV irradiance during the maximum and declining phase of solar cycle 22. The SSBUV data accurately represent the absolute solar UV irradiance between 200–405 nm, and also show the long-term variations during eight flights between October 1989 and January 1996. These data have been used to correct long-term sensitivity changes in the NOAA-11 SBUV/2 data, which provide a near-daily record of solar UV variations over the 170–400 nm region between December 1988 and October 1994. The NOAA-11 data demonstrate the evolution of short-term solar UV activity during solar cycle 22.  相似文献   

A Three-year Study of Nonmethane Hydrocarbons in Surface Air over Saxony (Germany)     

Thomas Gnauk  Wolfgang Rolle 《Journal of Atmospheric Chemistry》1998,30(3):371-395

Measurements of the nonmethane hydrocarbon (NMHC) mixing ratio over a period of 42 months were carried out for the first time in the air of a research station situated in the former East Germany during the SANA project. Apart from four species, all other species analysed showed a statistically significant downward trend at the 95% significance level. The decrease of the hydrocarbon concentrations was superimposed by seasonal variations. A drop of about 40% of the annual mean values from 1993 to 1995 was observed. This development reflects the dramatic changes in traffic, industry, power economy, and agriculture in Saxony after the reunification of Germany. The remove of two-stroke engined cars is reflected in NMHC mixing ratio changes, as is the removal of obsolete chemical plants. Generally it was not possible to relate causes and effects of a single event, but in some cases major changes in concentrations and NMHC ratios occurred coincidentally with the disappearance of a specific emission source.  相似文献