Vegetation in an altitudinal gradient along the R?́o Loa in the Atacama Desert of northern Chile     

Julio R. Gutiérrez  Francisco López-Cortes  Pablo A. Marquet 《Journal of Arid Environments》1998,40(4):383-399

Six sites between 0 m and 4000 m were sampled for plant and soil chemical characteristics along the Río Loa, Atacama Desert, Chile. Sites located between 0 m and 1500 m showed lower species richness, higher plant cover and higher herbaceous productivity than the upper part of the altitudinal gradient. The number of species varied non-linerly with precipitation along the altitudinal gradient. Plant cover and herbaceous productivity in the lowlands is characterized by thePluchea absinthioides Distichlis spicataassociation of anthropic origin. We propose that vegetation structure along the altitudinal gradient has been affected by past and present human activities, and climatic and edaphic factors.  相似文献   

The impact of stream-groundwater exchange on seasonal nitrate loads in an urban stream     

Julio Beltran  Laura K. Lautz  John R. Slosson 《水文研究》2021,35(8):e14324

Urbanization negatively impacts water quality in streams by reducing stream-groundwater interactions, which can reduce a stream's capacity to naturally attenuate nitrate. Meadowbrook Creek, a first order urban stream in Syracuse, New York, has an inverse urbanization gradient, with heavily urbanized headwaters that are disconnected from the floodplain and downstream reaches that have intact riparian floodplains and connection to riparian aquifers. This system allows assessment of how stream-groundwater interactions in urban streams impact the net sources and sinks of nitrate at the reach scale. We used continuous (15-min) streamflow measurements and weekly grab samples at three gauging stations positioned longitudinally along the creek to develop continuous nitrate load estimates at the inlet and outlet of two contrasting reaches. Nitrate load estimates were determined using a USGS linear regression model, RLOADEST, and differences between loads at the inlet and outlet of contrasting reaches were used to quantify nitrate sink and source behaviour year-round. We observed a nitrate load of 1.4 × 10⁴ kg NO₃⁻ per water year, on average, at the outlet of the urbanized reach while the nitrate load at the outlet of the downstream, connected reach was 1.0 × 10⁴ kg NO₃⁻ per water year, on average. We found the more heavily urbanized, hydrologically-disconnected reach was a net source of nitrate regardless of season. In contrast, stream-groundwater exchange caused the hydrologically connected reach to be both a source and sink for nitrate, depending on time of year. Both reaches alter nitrate source and sink behaviour at various spatiotemporal scales. Groundwater connection in the downstream, connected reach reduces annual nitrate loads and provides more opportunities for sources and sinks of nitrate year-round than the hydrologically disconnected stream reach. Mechanisms include groundwater discharge into the stream with variable nitrate concentrations, surface-water groundwater interactions that foster denitrification, and stream load loss to surrounding near-stream aquifers. This study emphasizes how loads are important in understanding how stream-groundwater interactions impact reach scale nitrate export in urban streams.  相似文献   

On the influence of etch pits in the overall dissolution rate of apatite basal sections     

Igor Alencar  Sandro Guedes  Rosane Palissari  Julio C. Hadler 《Physics and Chemistry of Minerals》2015,42(8):629-640

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Sources and distribution of organic matter in northern Patagonia fjords,Chile (∼44–47°S): A multi-tracer approach for carbon cycling assessment     

Julio Sepúlveda  Silvio Pantoja  Konrad A. Hughen 《Continental Shelf Research》2011,31(3-4):315-329

We investigated the provenance of organic matter in the inner fjord area of northern Patagonia, Chile (～44–47°S), by studying the elemental (organic carbon, total nitrogen), isotopic (δ¹³C, δ¹⁵N), and biomarker (n-alkanoic acids from vascular plant waxes) composition of surface sediments as well as local marine and terrestrial organic matter. Average end-member values of N/C, δ¹³C, and δ¹⁵N from organic matter were 0.127±0.010, ?19.8±0.3‰, and 9.9±0.5‰ for autochthonous (marine) sources and 0.040±0.018, ?29.3±2.1‰, and 0.2±3.0‰ for allochthonous (terrestrial) sources. Using a mixing equation based on these two end-members, we calculated the relative contribution of marine and terrestrial organic carbon from the open ocean to the heads of fjords close to river outlets. The input of marine-derived organic carbon varied widely and accounted for 13–96% (average 61%) of the organic carbon pool of surface sediments. Integrated regional calculations for the inner fjord system of northern Patagonia covered in this study, which encompasses an area of ～4280 km², suggest that carbon accumulation may account for between 2.3 and 7.8×10⁴ ton C yr^?1. This represents a storage capacity of marine-derived carbon between 1.8 and 6.2×10⁴ ton yr^?1, which corresponds to an assimilation rate of CO₂ by marine photosynthesis between 0.06 and 0.23×10⁶ ton yr^?1. This rate suggests that the entire fjord system of Patagonia, which covers an area of ～240,000 km², may represent a potentially important region for the global burial of marine organic matter and the sequestration of atmospheric CO₂.  相似文献   

Upwelling-triggered near-geostrophic recirculation in an equatorward facing embayment     

Julio Moraga-Opazo  Arnoldo Valle-Levinson  Marcel Ramos  Matías Pizarro-Koch 《Continental Shelf Research》2011,31(19-20):1991-1999

Underway current velocity profiles were combined with hydrographic profiles at the entrance to Tongoy Bay, an equatorward facing bay in north-central Chile, with the objective of determining its exchange hydrodynamics. To the west, Tongoy Bay is bounded by Lengua de Vaca Point, a ～6 km-long northward protruding peninsula. Observations were obtained during three surveys (April 2005, December 2005, May 2009) along cross-bay transects for at least one full day. During the surveys, winds were upwelling-favorable and displayed diurnal variations. Non-tidal (tidally averaged) flows showed a consistent clockwise or southern hemisphere cyclonic, recirculation during the three surveys. This recirculation was likely part of a cyclonic gyre (10–20 km in diameter), not entirely resolved by the surveys, and formed by flow separation off Lengua de Vaca Point. Estimates of the baroclinic pressure gradient, combined with analytical solutions of density-driven and wind-driven flows, indicated that the recirculation in Tongoy Bay was nearly in geostrophic balance. An ageostrophic contribution to the dynamics was related to frictional effects derived from local upwelling-favorable winds. A linear superposition of the analytically derived density-driven and wind-driven exchange resulted in a flow pattern that resembled the observed net exchange flows at the bay mouth.  相似文献   

Groundwater chemical characterization of a Rio de Janeiro coastal aquifer,SE – Brazil     

Emmanoel V. Silva-Filho  Renato G. Sobral Barcellos  Christophe Emblanch  Bernard Blavoux  Silvia Maria Sella  Michel Daniel  Roland Simler  Julio Cesar Wasserman 《Journal of South American Earth Sciences》2009,27(1):100-108

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Spatio-temporal climate regionalization using a self-organized clustering approach     

Mihaela I. Chidean  Antonio J. Caama&#;o  Carlos Casanova-Mateo  Julio Ramiro-Bargue&#;o  Sancho Salcedo-Sanz 《Theoretical and Applied Climatology》2020,140(3):927-949

The authors present a novel self-organized climate regionalization (CR) method that obtains a spatial clustering of regions, based on the explained variance of physical measurements in their coverage. This method enables a microscopic characterization of the probabilistic spatial extent of climate regions, using the statistics of the obtained clusters. It also allows for the study of the macroscopic behaviour of climate regions through time by using the dissimilarity among different cluster size probability histograms. The main advantages of the presented method, based on the Second-Order Data-Coupled Clustering (SODCC) algorithm, are that SODCC is robust to the selection of tunable parameters and that it does not require a regular or homogeneous grid to be applied. Moreover, the SODCC method has higher spatial resolution, lower computational complexity, and allows for a more direct physical interpretation of the outputs than other existing CR methods, such as Empirical Orthogonal Function (EOF) or Rotated Empirical Orthogonal Function (REOF). These facts are illustrated with an example of winter wind speed regionalization in the Iberian Peninsula through the period (1979 − 2014). This study also reveals that the North Atlantic Oscillation (NAO) has a high influence over the wind distribution in the Iberian Peninsula in a subset of years in the considered period.  相似文献   

The effects of damming on the materials flux in the Colorado River delta   总被引：1，自引：0，他引：1  

José D. Carriquiry  Julio A. Villaescusa  Víctor Camacho-Ibar  L. Walter Daesslé  Pedro G. Castro-Castro 《Environmental Earth Sciences》2011,62(7):1407-1418

During the last century, the Colorado River delta (CRD) has been dramatically altered by the diversion of river water for use in human activities. This alteration has eliminated the delivery of fresh water to the Gulf of California radically transforming the former estuary into an inverse-estuary. Under the new conditions, the new materials budget was estimated at the mouth of the Colorado River in terms of salts, total suspended sediments, organic suspended matter and nutrients. The results of this study show that, because of the asymmetry of the tidal wave, the variability of seston concentration follows a sedimentation pattern of three successive stages: re-suspension (erosion at ebb flow) > dilution (during flood flow) > sedimentation (at the end of the flood stage). The tidal asymmetry during neap tides was characterized by longer ebb (at least, 30 min longer) than flood and more intense ebb currents (as much as 43% higher), hence characterizing an ebb-dominated system. The CRD is characterized by high nutrients concentrations. Maximal levels are: nitrates (41 μM), phosphates (2.6 μM) and silicates (68 μM), nitrite (15 μM). The mass balance indicates that the system acts as a net exporter of suspended sediment with rates as high as 7 tons per tidal cycle. This behavior indicates that the CRD is in a destructive stage as a result of the lack of freshwater inflow and supply of sediment into the system.  相似文献   

Sociological Profile of Astronomers in Spain     

Julio Iglesias de Ussel  Antonio Trinidad  Diego RuIz  Eduardo Battaner  Antonio J. Delgado  José M. Rodriguez-Espinosa  Eduard Salvador-Solé  José M. Torrelles 《Astrophysics and Space Science》1997,257(2):237-248

In this paper the main findings are presented of a recent study made by a team of sociologists from the University of Granada on the professional astronomers currently working in Spain. Despite the peculiarities of this group – its youth, twentyfold increase in size over the last 20 years, and extremely high rate of specialization abroad – in comparison with other Spanish professionals, this is the first time that the sociological characteristics of the group have been studied discretely. The most significant results of the study are presented in the following sections. Section 1 gives a brief historical background of the development of Astronomy in Spain. Section 2 analyzes the socio-demographic profile of Spanish Astronomy professionals (sex, age, marital status, etc.). Sections 3–5 are devoted to the college education and study programs followed by Spanish astronomers, focusing on the features and evaluations of the training received, and pre- and postdoctoral study trips made to research centers abroad. The results for the latter clearly show the importance that Spanish astronomers place on having experience abroad. Special attention is paid to scientific papers published as a result of joint research projects carried out with colleagues from centers abroad as a result of these study trips. Section 6 describes the situation of Astronomy professionals within the Spanish job market, the different positions available and the time taken to find a job after graduation. Section 7 examines Astronomy as a discipline in Spain, including the astronomers' own opinions of the social status of the discipline within Spanish society. Particular attention is paid to how Spanish astronomers view the status of Astronomy in Spain in comparison with that of other European countries. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Inferences about winter temperatures and summer rains from the late Quaternary record of C4 perennial grasses and C3 desert shrubs in the northern Chihuahuan Desert     

Camille A. Holmgren  Jodi Norris  Julio L. Betancourt 《第四纪科学杂志》2007,22(2):141-161

Late Quaternary histories of two North American desert biomes—C₄ grasslands and C₃ shrublands—are poorly known despite their sensitivity and potential value in reconstructing summer rains and winter temperatures. Plant macrofossil assemblages from packrat midden series in the northern Chihuahuan Desert show that C₄ grasses and annuals typical of desert grassland persisted near their present northern limits throughout the last glacial–interglacial cycle. By contrast, key C₃ desert shrubs appeared somewhat abruptly after 5000 cal. yr BP. Bioclimatic envelopes for select C₄ and C₃ species are mapped to interpret the glacial–interglacial persistence of desert grassland and the mid‐to‐late Holocene expansion of desert shrublands. The envelopes suggest relatively warm Pleistocene temperatures with moist summers allowed for persistence of C₄ grasses, whereas winters were probably too cold (or too wet) for C₃ desert shrubs. Contrary to climate model results, core processes associated with the North American Monsoon and moisture transport to the northern Chihuahuan Desert remained intact throughout the last glacial–interglacial cycle. Mid‐latitude effects, however, truncated midsummer (July–August) moisture transport north of 35° N. The sudden expansion of desert shrublands after 5000 cal. yr BP may be a threshold response to warmer winters associated with increasing boreal winter insolation, and enhanced El Niño–Southern Oscillation variability. Published in 2006 by John Wiley & Sons, Ltd.  相似文献