Late Holocene development of a floodplain along a small meandering stream, northern Québec, Canada     

tienne Boucher  Dominique Arseneault  Bernard Htu 《Geomorphology》2006,80(3-4):267-281

This paper describes the activity of a small meandering stream and the development of its floodplain during the last 4600 years (calendar years BP) in the northern boreal zone of Québec. Three trenches were excavated across the floodplain's full width and permitted the interpretation of morphosedimentary units in relation to modern analogs. Chronological controls within trenches was provided by the dendrochronological and radiocarbon dating of buried tree trunks. From 4600 to 2900 cal. BP and from 1000 to 120 cal. BP, the channel migrated and constructed its floodplain at very slow rates, mostly because of low flow velocities, vegetated streambanks and the cohesive texture of marine sediments reworked by the channel. Vertical accretion rates were extremely variable on the floodplain, with high rates proximal to the modern channel and low rates over distal (also older) portions of the floodplain. Following a major channel shift (meander cut-off or avulsion) around 2900 cal. BP, channel migration appears to have been constrained to a narrow zone adjacent to the modern channel. Within this constrained zone, the migrating channel has reworked its own sediments leading to a marked unconformity between 2900–1000 cal. BP. It is thought that underlying marine sediments protrusions, and perhaps the forested banks, protected older alluvial sediments from being eroded during the last three millenniums. Our study shows that small boreal floodplains may contain, in a very small area, abundant and diversified archives of their evolution.  相似文献   

Rainfall interception by an isolated evergreen oak tree in a Mediterranean savannah   总被引：1，自引：0，他引：1  

T. S. David  J. H. C. Gash  F. Valente  J. S. Pereira  M. I. Ferreira  J. S. David 《水文研究》2006,20(13):2713-2726

Redistribution of ground‐level rainfall and interception loss by an isolated Quercus ilex tree were measured over 2 years in a Mediterranean oak savannah. Stemflow, meteorological variables and sap flow were also monitored. Rainfall at ground level was measured by a set of rain‐gauges located in a radial layout centred on the tree trunk and extending beyond the crown limits. Interception loss was computed as the difference between the volume of rainwater that would reach the ground in the absence of the tree and the volume of water that actually fell on the ground sampling area (stemflow included). This procedure provided correct interception loss estimates, irrespective of rainfall inclination. Results have shown a clear non‐random spatial distribution of ground‐level rainfall, with rainwater concentrations upwind beneath the crown and rain‐shadows downwind. Interception loss amounted to 22% of gross rainfall, per unit of crown‐projected area. Stand interception loss, per unit of ground area, was only 8% of gross rainfall and 28% of tree evapotranspiration. These values reflect the low crown cover fraction of the stand (0·39) and the specific features of the Mediterranean rainfall regime (predominantly with few large storms). Nevertheless, it still is an important component of the water balance of these Mediterranean ecosystems. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

广州城市行道树植物区系的初步研究   总被引：6，自引：0，他引：6  

覃朝锋  管东生 《热带地理》1998,18(2):177-181

通过调查统计,对广州城市洗道树的区系成分和地理分布进行了分析。结果表明,广州城市行道树共１２３种,隶属４５科,８９属;其植物沤系成分以热带的属和种占绝对优势;其与罗浮山森林乔木植物区系有一定的联系,它们的科、属、种相似系数分别为５３％、２６％和１３％。指出了广州城市行道树目前存在优势种过于突出的问题,认为应增加其物种的多样性  相似文献   

Water uptake by two river red gum (Eucalyptus camaldulensis) clones in a discharge site plantation in the Western Australian wheatbelt     

John K. Marshall  Anne L. Morgan  Kandia Akilan  Richard C. C. Farrell  David T. Bell 《Journal of Hydrology》1997,200(1-4):136-148

The heat-pulse technique was used to estimate year-long water uptake in a discharge zone plantation of 9-year-old clonal Eucalyptus camaldulensis Dehnh. near Wubin, Western Australia. Water uptake matched rainfall closely during weter months but exceeded rainfall as the dry season progressed. Average annual water uptake (1148 mm) exceeded rainfall (432 mm) by about 2.7 fold and approached 56% of pan evaporation for the area. The data suggest that at least 37% (i.e. (1/2.7) × 100) of the lower catchment discharge zone should be planted to prevent the rise of groundwater.

Water uptake varied with soil environment, season and genotype. Upslope trees used more water than did downslope trees. Water uptake was higher in E. camaldulensis clone M80 than in clone M66 until late spring. The difference reversed as summer progressed. Both clones, however, have the potential to dry out the landscape when potential evapotranspiration exceeds rainfall. This variation in water uptake within the species indicates the potential for manipulating plantation uptake by matching tree characteristics to site characteristics.

Controlled experiments on the heat-pulse technique indicated accuracy errors of approximately 10%. This, combined with the ability to obtain long-term, continuous data and the superior logistics of use of the heat-pulse technique, suggests that results obtained by it would be much more reliable than those achieved by the ventilated chamber technique.  相似文献   

Sensitivity of seismic hazard evaluations to uncertainties determined from seismic source characterization     

Behrooz Tavakoli 《Journal of Seismology》2002,6(4):525-545

The sensitivity and overall uncertainty in peak ground acceleration (PGA)estimates have been calculated for the city of Tabriz, northwestern Iran byusing a specific randomized blocks design. Eight seismic hazard models andparameters with randomly selected uncertainties at two levels have beenconsidered and then a linear model between predicted PGA at a givenprobability level and the uncertainties has been performed. The inputmodels and parameters are those related to the attenuation, magnituderupture-length and recurrence relationships with their uncertainties.Application of this procedure to the studied area indicates that effects ofthe simultaneous variation of all eight input models and parameters on thesensitivity of the seismic hazard can be investigated with a decreasingnumber of computations for all possible combinations at a fixed annualprobability. The results show that the choice of a mathematical model ofthe source mechanism, attenuation relationships and the definition ofseismic parameters are most critical in estimating the sensitivity of seismichazard evaluation, in particular at low levels of probability of exceedance.The overall uncertainty in the expected PGA for an annual probability of0.0021 (10% exceedence in 50 yr) is expressed by a coefficient ofvariation (CV) of about 34% at 68% confidence level for a distance ofabout 5km from the field of the major faults. The CV will decrease withincreasing site-source distance and remains constant, CV = 15%, fordistances larger than 15 km. Finally, treating alternative models on theoverall uncertainty are investigated by additional outliers in input decision.  相似文献   

Late Quaternary western Mediterranean pollen records and African winds     

D. Magri  I. Parra   《Earth and Planetary Science Letters》2002,200(3-4):401-408

Late Quaternary vegetation changes of regional and global significance in the Mediterranean region are generally interpreted as being controlled by changes in the circulation patterns of North Atlantic air masses. However, the possibility cannot be excluded that they may also be related to winds blowing from Africa. Long pollen records from southwest Europe show that Cedrus pollen of northwest African provenance is found during the glacial periods, and occasionally during abrupt deforestation events in the forest-dominated periods of the interglacials. A pollen concentration record from central Italy shows that during the Holocene the presence of Cedrus pollen coincides with two abrupt deforestation events, around 8.1 and 4.2 cal kyr BP. These observations raise the question of a possible influence of eolian activity on arboreal vegetation during phases of climatic stress towards aridity, and suggest the use of modern pollen monitoring as a strategy for revealing tendencies towards aridification in southwestern Europe.  相似文献   

郑州极端高温天气成因分析   总被引：9，自引：2，他引：9  

贺哲  李平  乔春贵  王蕊  梁钰  谷秀杰 《气象》2007,33(3):68-75

采用NCEP／NCAR1°×1°格点资料对2005年6月22—23日郑州出现的40℃以上的极端高温天气成因进行了分析。指出，河套高压是初夏导致郑州乃至黄淮地区西部出现极端高温的重要天气系统。河套高压的形成与高低层暖平流存在着联系。同时高温的产生也与副热带急流有关，河套高压东南侧的强下沉气流是热力和动力共同作用结果，即当河套高压东南部处于高层副热带急流入口区左侧时，动力辐合机制使得此区域的下沉气流极为强盛，由此所产生的晴空辐射增温和下沉绝热增温使得极端高温天气的出现成为可能。另外，地形所产生的焚风效应对高温的形成具有增幅作用。  相似文献   

桂林靖江王城防雷方案初探     

方琼玉黎梓华  姚立宏 《广西气象》2007,28(3):59-61

根据靖江王城古建筑物的特点、分布情况及所处的地理环境特征,结合雷击风险评估结果,对不同的保护物划分防雷类别,提出了雷电保护方案。  相似文献   

Desiccation of a Clay by Grass,Bushes and Trees   总被引：1，自引：1，他引：0  

Geoffrey?E.?BlightEmail author 《Geotechnical and Geological Engineering》2005,23(6):697-720

After reviewing the past 60 years of research into the desiccation of clay soils by grass, bushes and trees as it affects shallow building foundations and roads pavements, it was concluded that most past research has been opportunistic and unplanned. Also, no attempt has been made to relate changes of soil water content to rainfall and rates of evapotranspiration. The paper reports the second stage of a sustained programme to observe and understand seasonal and annual changes of soil water content under grass and adjacent to bushes and trees. A 12 months set of two-monthly soil water content contours is presented together with the associated soil water balance, and a method is described for assessing evapotranspiration by bushes and trees by means of a solar energy balance.  相似文献   

Thermal infrared imaging of the temporal variability in stomatal conductance for fruit trees     

《International Journal of Applied Earth Observation and Geoinformation》2015

Repeated measurements using thermal infrared remote sensing were used to characterize the change in canopy temperature over time and factors that influenced this change on ‘Conference’ pear trees (Pyrus communis L.). Three different types of sensors were used, a leaf porometer to measure leaf stomatal conductance, a thermal infrared camera to measure the canopy temperature and a meteorological sensor to measure weather variables. Stomatal conductance of water stressed pear was significantly lower than in the control group 9 days after stress began. This decrease in stomatal conductance reduced transpiration, reducing evaporative cooling that increased canopy temperature. Using thermal infrared imaging with wavelengths between 7.5 and13 μm, the first significant difference was measured 18 days after stress began. A second order derivative described the average rate of change of the difference between the stress treatment and control group. The average rate of change for stomatal conductance was 0.06 (mmol m⁻² s⁻¹) and for canopy temperature was −0.04 (°C) with respect to days. Thermal infrared remote sensing and data analysis presented in this study demonstrated that the differences in canopy temperatures between the water stress and control treatment due to stomata regulation can be validated.  相似文献