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971.
This study aimed at analysing the effects of biological and meteorological factors on stemflow generation in a temperate mixed oak (Quercus petraea Liebl.) and beech (Fagus sylvatica L.) stand. A statistical model was developed to predict single‐event individual stemflow volume from trunk circumference and rainfall depth allowing mechanistic stemflow parameters to be deduced from the model, namely stemflow rates (SFrate), storage of water on tree organs (St) and rainfall thresholds for stemflow (RFmin). SFrate and St increased with increasing trunk circumference while RFmin was not significantly influenced by tree size. RFmin and, for a given tree size, St were higher for oak than for beech, and inversely for SFrate. For each species, RFmin was higher for the leaved season than for the leafless period, while the opposite was found for SFrate, and St was not significantly affected by the season. Increasing wind speed during rain increased SFrate, lowered RFmin and did not influence St. In contrast, St and RFmin tended, respectively, to decrease and to increase with increasing values of the ratio between the cumulated potential evaporation during the dry period preceding the rain event and the volume of the preceding rainfall (Eva pADP/Rprevious). Stemflow volume, which results from the combined effects of the previous parameters, was higher for beech than for oak and also higher during the leafless period than during the leaved period; these differences were large for the smallest events but decreased rapidly as rainfall depth increased. In addition, an enhancing and a depressing effect on stemflow volume were shown for the average wind speed during rain and for the ratio Eva pADP/Rprevious, respectively. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
972.
François Fripiat Anne-Julie Cavagna Nicolas Savoye Frank Dehairs Luc André Damien Cardinal 《Marine Chemistry》2011,123(1-4):11-22
Estimation of the silicon (Si) mass balance in the ocean from direct measurements (Si uptake-dissolution rates …) is plagued by the strong temporal and spatial variability of the surface ocean as well as methodological artifacts. Tracers with different sensitivities toward physical and biological processes would be of great complementary use. Silicon isotopic composition is a promising proxy to improve constraints on the Si-biogeochemical cycle, since it integrates over longer timescales in comparison with direct measurements and since the isotopic balance allows to resolve the processes involved, i.e. uptake, dissolution, mixing. Si-isotopic signatures of seawater Si(OH)4 and biogenic silica (bSiO2) were investigated in late summer 2005 during the KEOPS experiment, focusing on two contrasting biogeochemical areas in the Antarctic Zone: a natural iron-fertilized area above the Kerguelen Plateau (< 500 m water depth) and the High Nutrient Low Chlorophyll area (HNLC) east of the plateau (> 1000 m water depth). For the HNLC area the Si-isotopic constraint identified Upper Circumpolar Deep Water as being the ultimate Si-source. The latter supplies summer mixed layer with 4.0 ± 0.7 mol Si m? 2 yr? 1. This supply must be equivalent to the net annual bSiO2 production and exceeds the seasonal depletion as estimated from a simple mixed layer mass balance (2.5 ± 0.2 mol Si m? 2 yr? 1). This discrepancy reveals that some 1.5 ± 0.7 mol Si m? 2 yr? 1 must be supplied to the mixed layer during the stratification period. For the fertilized plateau bloom area, a low apparent mixed layer isotopic fractionation value (?30Si) probably reflects (1) a significant impact of bSiO2 dissolution, enriching the bSiO2 pool in heavy isotope; and/or (2) a high Si uptake over supply ratio in mixed layer at the beginning of the bloom, following an initial closed system operating mode, which, however, becomes supplied toward the end of the bloom (low Si uptake over supply ratio) with isotopically light Si(OH)4 from below when the surface Si(OH)4 pool is significantly depleted. We estimated a net integrated bSiO2 production of 10.5 ± 1.4 mol Si m? 2 yr? 1 in the AASW above the plateau, which includes a significant contribution of bSiO2 production below the euphotic layer. However, advection which could be significant for this area has not been taken into account in the latter estimation based on a 1D approach of the plateau system. Finally, combining the KEOPS Si-isotopic data with those from previous studies, we refined the average Si-isotopic fractionation factor to ? 1.2 ± 0.2‰ for the Antarctic Circumpolar Current. 相似文献
973.
CCD BVI Johnson–Cousins photometry of the open cluster candidates Pismis 23 and BH 222 is presented. Both the analysis of the colour-magnitude diagrams and star counts performed in the regions of these two objects support their physical reality. For Pismis 23 we derive E(B?V) = 2.0 ± 0.1, E(V?I) = 2.6 ± 0.1, a distance from the Sun d= (2.6 ± 0.6) kpc and an age of (300 ± 100) Myr, while for BH 222 we obtain E(V?I) = 2. 4 ± 0.2, d= (6.0 ± 2.7) kpc and (60 ± 30) Myr. Both objects, located beyond the Sagittarius arm, are among the most reddened and distant open clusters known in the direction towards the Galactic centre. 相似文献
974.
Inés Crespo-Chacón David Montes María José Fernández-Figueroa Javier López-Santiago David GarcÍa-Alvarez Bernard H. Foing 《Astrophysics and Space Science》2004,292(1-4):697-703
We present the results of a high temporal resolution spectroscopic monitoring of the flare (UV Cet type) star V1054 Oph (Wolf 630AB), classified as a dM3.5e visual binary system. Intermediate resolution spectra have been taken during four nights (2–5 April 2001) using the IDS spectrograph of the 2.5 m Isaac Newton Telescope (INT) (La Palma, Canary Islands, Spain). The V1054 Oph spectra show very strong emission lines even in its quiescent state. The analysis of the temporal evolution of the observed emission lines (from Hβ to H11 and the Ca II H and K lines) reveals four strong flares and several weak flares. We have studied in detail the behaviour of the chromospheric lines during the different phases (pre-flare, impulsive and gradual decay) of these flares. The observed flares last from ~25 to 95 min. The equivalent width of the Hβ line changes by a factor up to ~2.3. Broad wings and asymmetric (red-shifted) lines are observed as well. 相似文献
975.
Thierry Montmerle Philippe Claeys Muriel Gargaud Purificatión López-García Hervé Martin Robert Pascal Jacques Reisse Franck Selsis 《Earth, Moon, and Planets》2006,98(1-4):299-312
This concluding chapter is divided into two main parts. The first part is a summary of the main facts and events which constitute the present body of knowledge of the chronology of life in the solar system, in the form of “highlights” in astronomy, geology, chemistry and biology. The second part raises the interrogation “Is life universal?”, and tries to provide answers based on these facts and events. These answers turn out to differ widely among the various disciplines, depending on how far they feel able to extrapolate their current knowledge. 相似文献
976.
Josep M. Trigo‐Rodríguez Jordi Llorca Mona Weyrauch Addi Bischoff Carles E. Moyano‐Cambero Klaus Keil Matthias Laubenstein Andreas Pack José María Madiedo Jacinto Alonso‐Azcárate My Riebe Rainer Wieler Uli Ott Mar Tapia Narcís Mestres 《Meteoritics & planetary science》2014,49(8):1475-1484
We report and describe an L6 ordinary chondrite fall that occurred in Ardón, León province, Spain (longitude 5.5605°W, latitude 42.4364°N) on July 9th, 1931. The 5.5 g single stone was kept hidden for 83 yr by Rosa González Pérez, at the time an 11 yr old who had observed the fall and had recovered the meteorite. According to various newspaper reports, the event was widely observed in Northern Spain. Ardón is a very well‐preserved, fresh, strongly metamorphosed (petrologic type 6), and weakly shocked (S3) ordinary chondrite with well‐equilibrated and recrystallized minerals. The mineral compositions (olivine Fa23.7±0.3, low‐Ca pyroxene Fs20.4±0.2Wo1.5±0.2, plagioclase An10.3±0.5Ab84.3±1.2), magnetic susceptibility (log χ = 4.95 ± 0.05 × 10?9 m3 kg?1), bulk density (3.49 ± 0.05 g cm?3), grain density (3.58 ± 0.05 g cm?3), and porosity (2.5 vol%) are typical for L6 chondrites. Short‐lived radionuclides confirm that the meteorite constitutes a recent fall. The 21Ne and 38Ar cosmic ray exposure ages are both about 20–30 Ma, similar to values for many other L chondrites. The cosmogenic 22Ne/21Ne ratio indicates that preatmospheric Ardón was a relatively large body. The fact that the meteorite was hidden in private hands for 83 yr makes one wonder if other meteorite falls may have experienced the same fate, thus possibly explaining the anomalously low number of falls reported in continental Spain in the 20th century. 相似文献
977.
978.
Abstract An improved definition of the lacustrian thermocline is introduced, allowing a minimization of the random type fluctuations, with the aim of establishing more precise predictive models of the internal energy of lakes. The method consists first of determining the limits of the zone of thermal transition, referred to as the “thermoclinal zone”, and then of defining the plane of the thermocline by calculating its depth, either from the temperatures or from the depths to the limits of this zone. Six new definitions, each borrowing more or less from existing concepts in the literature, are compared with the Brönsted and Wesenburg‐Lund's definition. The best definitions are those for which the plane of the thermocline is defined from the mean temperatures to the limits of the thermoclinal zone. Of these, the most satisfactory is the one obtained from a mean thermal gradient limited to about twice the thermocline depth. 相似文献
979.
Abstract The measurement of radiation fluxes suffers from inaccuracies at low solar elevations and this poses a problem for determining the snow albedo at high latitudes. From the data of Resolute, NWT, three situations were observed: (1) an often‐reported situation when albedo decreases with increasing solar elevation, (2) an inverse situation when albedo increases with increasing solar elevation and (3) no obvious relationship. There were also cases when albedo exceeded 100%. The possible causes for such anomalous conditions or for erroneous albedos include instrument response deviating from the cosine law, instrument tilt, sensing of the sun by the inverted pyranometer and change in the spectral quality of incoming radiation with changing solar height. However, omission of the radiation values measured during the period of low solar elevation will not seriously affect the prediction of snowmelt. In this note, we have identified the anomalies and suggested possible causes; but further investigations are required to verify the causative mechanisms. 相似文献
980.
Abstract Evaluating horizontal gradients in three‐dimensional shallow water models that use bottom‐following sigma coordinates can lead to large errors near steep bathymetry. A technique that has been proposed to minimize this problem involves computing horizontal gradients in cartesian coordinates, while treating all other terms in a sigma coordinate framework. We study this technique through both truncation error analysis and numerical experimentation, and compare it to the direct application of sigma coordinates. While the Cartesian coordinate method has better convergence properties and generally smaller truncation errors when horizontal gradients are zero, the sigma coordinate method can be more accurate in other physically relevant situations. Also, the Cartesian coordinate method introduces significant numerical diffusion of variable sign near the bottom (where physical diffusion is particularly small), thus potentially leading to instabilities. Overall, we consider the sigma coordinates to be the best approach. 相似文献