Appearance of Chelophyes appendiculata and Abylopsis tetragona (Cnidaria, Siphonophora) in the Bay of Villefranche, northwestern Mediterranean     

Emmanuelle Buecher 《Journal of Sea Research》1999,41(4):95

The vertical and temporal distribution of two calycophoran siphonophores, Chelophyes appendiculata (Eschscholtz, 1829) and Abylopsis tetragona (Otto, 1823) in the Bay of Villefranche (northwestern Mediterranean) was investigated by an analysis of three different planktonic time series. A daily series (1993–1995) showed seasonal peaks of the nectophores of C. appendiculata during spring and particularly in late summer, while the abundance of A. tetragona remained similar throughout the year. A weekly series (1994–1995) showed that C. appendiculata (nectophores and eudoxids) became concentrated above the thermal discontinuity, in the most stratified and warm waters, whereas A. tetragona was collected in large numbers below this discontinuity. A 27-year survey (1966–1993) showed long-term fluctuations of these siphonophore populations, which became abundant in the Bay starting from 1980 and especially after 1984, when the water column grew warm and hypersaline, corresponding to a less rainy period. Temporal (seasonal and long-term) and bathymetric (between 10 and 60 m depth) successions of these two siphonophores were noted in this shallow coastal bay.  相似文献   

Modelling long-term groundwater fluctuations by extreme learning machine using hydro-climatic data   总被引：1，自引：1，他引：0  

Meysam Alizamir  Mohammad Zounemat-Kermani 《水文科学杂志》2018,63(1):63-73

The ability of the extreme learning machine (ELM) is investigated in modelling groundwater level (GWL) fluctuations using hydro-climatic data obtained for Hormozgan Province, southern Iran. Monthly precipitation, evaporation and previous GWL data were used as model inputs. Developed ELM models were compared with the artificial neural networks (ANN) and radial basis function (RBF) models. The models were also compared with the autoregressive moving average (ARMA), and evaluated using mean square errors, mean absolute error, Nash-Sutcliffe efficiency and determination coefficient statistics. All the data-driven models had better accuracy than the ARMA, and the ELM model’s performance was superior to that of the ANN and RBF models in modelling 1-, 2- and 3-month-ahead GWL. The RMSE accuracy of the ANN model was increased by 37, 34 and 52% using ELM for the 1-, 2- and 3-month-ahead forecasts, respectively. The accuracy of the ELM models was found to be less sensitive to increasing lead time.  相似文献   

A regional water balance indicator inferred from satellite images of an Andean endorheic basin in central-western Argentina     

Lucas O. Bianchi  Juan Antonio Rivera  Facundo Rojas  Mauro Britos Navarro  Ricardo Villalba 《水文科学杂志》2017,62(4):533-545

In the Central Andes of Argentina (30–37°S), snowmelt is the main source of freshwater, an essential natural resource for ~2.2 million people in the adjacent arid lowlands. In this region, Laguna Llancanelo collects the water inputs from the Malargüe endorheic basin. Previous studies concerning the annual and intra-annual variations of this lagoon and its relationship with regional climate are rare. We obtained a monthly record for the Laguna Llancanelo area (LLA, 1984–2013) using the modified normalized difference water index derived from Landsat images. Monthly LLA ranges between 35 km² and 411 km² and is significantly related to variations of the Río Malargüe, the main snow-fed tributary to the lagoon. There is no long-term relationship between LLA and local rainfall, but rapid increases in LLA result from heavy rainfall around the lagoon. Conversely, rapid reductions in LLA encompass periods with both reduced discharge from the Río Malargüe and low local rainfall. The LLA integrates moisture of both Pacific (snowfall in the upper Andes) and Atlantic (lowland rainfall) origins; therefore, we propose using LLA as an indicator of regional water balance.  相似文献   

Investigation of long waves in the tsunami frequency band on the southwestern shelf of Kamchatka     

P. D. Kovalev  A. B. Rabinovich  G. V. Shevchenko 《Natural Hazards》1991,4(2-3):141-159

Two inexpensive cable bottom pressure stations were installed on the southwestern shelf of Kamchatka (Okhotsk Sea) in 1987 and two more in 1988 to provide longwave measurements in the tsunami frequency band, to investigate the generating mechanism of these waves, and to test the instrumentation. Microfluctuations of atmospheric pressure were recorded simultaneously. Two cable lines were torn off by ship anchors in March 1989 but others are still working in spite of highly dynamic activity on beaches and in hard ice regimes. Careful data analysis of two months of observations (September–October, 1987) showed that: (1) the atmospheric spectra were very stable and monotonic in the period range 2–50 min and corresponded to a power law of ^–2.3, (2) the direct generation of long waves by atmospheric pressure fluctuations was negligible, (3) there was high correlation between background longwave oscillations and sea state, (4) the structure of the offshore longwave field was in good agreement with theoretical estimates of standing waves for a linear slope.  相似文献   

Oscillating peat surface levels in a restiad peatland,New Zealand—magnitude and spatiotemporal variability     

Christian Fritz  David I. Campbell  Louis A. Schipper 《水文研究》2008,22(17):3264-3274

Hydrology, particularly the water table position below the surface (relative water level, RWL), is an important control on biogeochemical and ecological processes in peatlands. The surface elevation (SE) in a peatland oscillates in response to changes in effective stress on the peat matrix mainly caused by water level fluctuations. This phenomenon is called peatland surface oscillation (PSO). To investigate the spatiotemporal variability of PSO, surface elevation and the water level above sea level (AWL) were measured monthly (23 sites) over one year in a warm‐temperate restiad peatland, New Zealand. At one site peat surface elevation was measured indirectly by monitoring AWL and RWL continuously with pressure transducers. Annual PSO (the difference between maximum and minimum surface elevation) ranged from 3·2 to 28 cm (mean = 14·9 cm). Surface elevation changes were caused by AWL fluctuations. Spatially homogenous AWL fluctuations (mean 40 cm among sites) translated into RWL fluctuations reduced 27–56% by PSO except for three sites with shallow and dense peat at the peatland margin (7–17%). The SE‐AWL relationship was linear for 15 sites. However, eight sites showed significantly higher rates of surface elevation changes during the wet season and thus a non‐linear behaviour. We suggest flotation of upper peat layers during the wet season causing this non‐linear behaviour. Surprisingly, PSO was subjected to hysteresis: the positive SE‐AWL relationship reversed after rainfall when the surface slowly rose despite rapidly receding AWL. Hysteresis was more prominent during the dry season than during the wet season. Total peat thickness and bulk density together could only explain 50% of the spatial variability of PSO based on manual measurements. However, we found three broad types of SE‐AWL relationships differing in shape and slope of SE‐AWL curves. These oscillation types reflected patterns in vegetation and flooding. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

海滩冲流带高频振动及碎波带波浪作用的模态分析   总被引：1，自引：0，他引：1       下载免费PDF全文

李志强  陈子燊 《海洋学报》2008,30(2):161-168

一般来说,研究者对海滩中长尺度变化的研究比较多,通过时间间隔为数小时、1d,1周、1个月甚至1a一次的重复测量来分析海滩的变化特征,但海滩冲流带是动力和地形发生高频率变化的一个区域,每次波浪引起的上冲和回流过程均要引起滩面泥沙的运动,造成滩面的局部堆积和侵蚀,而滩面地形的改变又反过来影响到下一次的冲流过程。这些现象对研究海滩碎波带动力和滩面地形相互作用机理、认识近岸泥沙的运动规律以及探讨岸滩的长期冲淤演变是十分有意义的。  相似文献   

Melt water driven stream and groundwater stage fluctuations on a glacier forefield (Dammagletscher,Switzerland)          下载免费PDF全文

Jan Magnusson  Florian Kobierska  Stephan Huxol  Masaki Hayashi  Tobias Jonas  James W. Kirchner 《水文研究》2014,28(3):823-836

In many mountain regions, large land areas with heterogeneous soils have become ice‐free with the ongoing glacier retreat. On these recently formed proglacial fields, the melt of the remaining glaciers typically drives pronounced diurnal stream level fluctuations that propagate into the riparian zone. This behaviour was measured on the Damma glacier forefield in central Switzerland with stage recorders in the stream and groundwater monitoring wells along four transects. In spite of the large groundwater stage variations, radon measurements in the near‐stream riparian zone indicate that there is little mixing between stream water and groundwater on daily time scales. At all four transects, including both losing and gaining reaches, the groundwater level fluctuations lagged the stream stage variations and were often damped with distance from the stream. Similar behaviours have been modelled using the diffusion equation in coastal regions influenced by tidal sea level variations. We thus tested the ability of such a model to predict groundwater level fluctuations in proglacial fields. The model reproduced several key features of the observed fluctuations at three of four locations, although discrepancies also arise due to non representative input data and model simplifications. Nevertheless, calibration of the model for the individual transects yielded realistic estimates of hydraulic diffusivities between the stream and groundwater monitoring wells. We conclude that studying diurnal groundwater fluctuations can provide important information about the subsurface hydrology of alpine watersheds dominated by glacier melt. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2008～2009年云台等高仪时纬残差的异常波动   总被引：1，自引：0，他引：1  

胡辉  韩延本  苏有锦  付虹  尹志强  李语强  王锐 《天文研究与技术》2011,8(1)

分析了2008～2009年间云南天文台光电等高仪的时纬资料的残差值的变化,讨论了残差异常变化与此期间云南天文台周围发生的2008年5月12日四川汶川8.0级地震、8月30日攀枝花6.1级地震以及12月26日昆明4.3级地震的可能的关系,并讨论了与该研究有关的问题。  相似文献   

New Zealand glacier response to climate change of the past 2 decades   总被引：1，自引：0，他引：1  

T. J. Chinn   《Global and Planetary Change》1999,22(1-4)

Oblique aerial photography of 111 glaciers during the past 2 decades has recorded a reversal of the past century glacier-recession trend. Cirque glaciers show little response to the recent mass balance increase; mountain glaciers show visible advances. Some valley glaciers have advanced, some have thickened in the upper trunk, and the larger ones and those with proglacial lakes continue to recede. The shift to advance is driven by an average lowering of snowlines of 67 m, equivalent to a cooling of 0.47°C if other factors are held constant.  相似文献   

Climate changes and recent glacier behaviour in the Chilean Lake District   总被引：1，自引：1，他引：0  

Francisca Bown  Andrs Rivera 《Global and Planetary Change》2007,59(1-4):79

Atmospheric temperatures measured at the Chilean Lake District (38°–42°S) showed contrasting trends during the second half of the 20th century. The surface cooling detected at several meteorological stations ranged from − 0.014 to − 0.021 °C a^− 1, whilst upper troposphere (850–300 gpm) records at radiosonde of Puerto Montt (41°26′S/73°07′W) revealed warming between 0.019 and 0.031 °C a^− 1. Regional rainfall data collected from 1961 to 2000 showed the overall decrease with a maximum rate of − 15 mm a^− 2 at Valdivia st. (39°38′S/73°05′W). These ongoing climatic changes, especially the precipitation reduction, seem to be related to El Niño–Southern Oscillation (ENSO) phenomena which has been more frequent after 1976. Glaciers within the Chilean Lake District have significantly retreated during recent decades, in an apparent out-of-phase response to the regional surface cooling. Moreover, very little is known about upper troposphere changes and how they can enhance the glacier responses. In order to analyse their behaviour in the context of the observed climate changes, Casa Pangue glacier (41°08′S/71°52′W) has been selected and studied by comparing Digital Elevation Models (DEMs) computed at three different dates throughout the last four decades. This approach allowed the determination of ice elevation changes between 1961 and 1998, yielding a mean thinning rate of − 2.3 ± 0.6 m a^− 1. Strikingly, when ice thinning is computed for the period between 1981 and 1998, the resulting rate is 50% higher (− 3.6 ± 0.6 m a^− 1). This enhanced trend and the related area loss and frontal retreat suggests that Casa Pangue might currently be suffering negative mass balances in response to the upper troposphere warming and decreased precipitation of the last 25–30 yr, as well as debris cover would not prevent the glacier from a fast reaction to climate forcing. Most of recent glaciological studies regarding Andean glaciers have concentrated on low altitude changes, namely frontal variations, however, in order to better understand the regional glacier changes, new data are necessary, especially from the accumulation areas.  相似文献