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11.
Nutrients in an oligotrophic boundary current: Evidence of a new role for the Leeuwin Current 总被引：1，自引：0，他引：1
P.A. Thompson K. Wild-Allen M. Lourey C. Rousseaux A.M. Waite M. Feng L.E. Beckley 《Progress in Oceanography》2011,91(4):345-359
New observations along the continental shelf of Western Australia provide a novel explanation for the established ∼60 years relationship between Leeuwin Current (LC) strength and greater winter nitrate concentrations at 32°S plus the inter-annual variation in the magnitude of the annual, shelf-scale, phytoplankton bloom. The potential source of dissolved nitrogen to support the annual shelf scale phytoplankton bloom was identified as thin layers of an unprecedented areal extent, nitrate concentration and shallow nature that were observed off the northwest of Australia. We propose that the dissolved inorganic nitrogen (DIN) in these layers enters the LC at depth and then enters the euphotic zone via by three mechanisms: instability that results in a warm core eddy, cooling that deepens the surface mixed layer and shallowing of the thin layer. During the onset of the annual phytoplankton bloom along the west coast of Australia from 22°S to 34°S the poleward flowing LC was clearly evident as a surface intensified ocean boundary current transporting warmer, lower-salinity, greater-silicate waters in a shallow mixed layer rapidly southward. Between 24 and 26°S the core of the LC was present as a 50–100 m deep layer over one or more thin layers, 15–50 m thick, with high nitrate and low dissolved oxygen (DO). These layers were of lower salinity, cooler water with markedly reduced DO, high nitrate concentrations and distinct nitrate:silicate (NO3:Si(OH)4) nutrient ratios. As the LC flowed south it cooled and deepened thereby entraining the thin layers of high nitrate water into the euphotic zone. The LC also formed large (greater than 100 km diameter) warm core eddies with a deep surface mixed layer that also entrained nitrate from these thin layers. In some locations as far south as 32°S the LC was still present with the thin layer of high nitrate intact but now within the euphotic zone. Thus, the available evidence suggests the LC arises under conditions that favour rapid and shallow nitrification. This nitrification fuels a shelf-scale bloom on a downwelling favourable coast. Depending upon the rate of nitrification the source of the particular organic matter may be local or delivered from the tropics via horizontal advection in a subsurface layer of the LC. 相似文献
12.
Adrien Berchet Bruno Simon Anthony Beaudoin Pierre Lubin Germain Rousseaux Serge Huberson 《国际泥沙研究》2018,33(3):351-370
Tidal bores may appear in some estuaries when the tides quickly reach a high level. This phenomenon is rare but has a strong impact during its short duration: i.e. the river bed is significantly eroded and sediments are then transported. In this paper, the trajectories of suspended particles induced by this flow are numerically studied. Four undular bores with Froude numbers between 1.1 and 1.2 are studied. Despite similar Froude numbers, various initial flow conditions were selected to produce or not an inversion of the flow direction during the bore passage. The particle trajectories associated with each distinct flow configuration are presented and analyzed. These trajectories, estimated by solving the Maxey-Riley equation, appear to be very different even though the Froude numbers of flows are similar. These observations are important because the Froude number is often used to characterize a tidal bore as it describes well the free surface, however, it cannot describe the sediment transport. Finally, Chen's model of wave-current interactions is adapted to fit the cases studied and is applied to the four bores simulated. The results highlight that this latter model can reproduce the observed trajectories and dissociate their different components. From this model, it is shown that the inertial and Basset history effects can be neglected compared to the gravity and flow entrainment effects due to the viscous drag when one wants to determine the long-term trajectories of suspended particles. 相似文献
13.
Tree ring analysis of hydro‐climatic thresholds that trigger ice jams on the Mistassini River,Quebec 
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下载免费PDF全文 This study documents the spatiotemporal variations in the frequency and magnitude of ice jams in the Mistassini River and applies that information to the identification of the hydro‐climatic threshold conditions associated with major events. Ice jams cause severe upheavals in water flow, which result in flooding upstream of the ice jam front, and therefore represent a significant geohazard to riparian populations. To analyze the spatiotemporal variations in the magnitude–frequency of ice jams, the Mistassini River was first divided into six different sites representing different geomorphological contexts. A 50‐year ice jam chronology was constructed from 85 damaged trees from all of the study sites. This chronology was then coupled with hydro‐climatic variables to construct classification trees, which helped identify the conditions and hydro‐climatic thresholds favourable to the triggering of ice jams in a predictive model. The results indicate complex interactions between the characteristics of flow, ice cover and river morphology that affect the frequency and magnitude of ice jam events on the Mistassini River. These factors affect the frequency and magnitude of ice jam events. The triggering of extreme events seems particularly influenced by exceptional ice conditions and sites with high sinuosity and islands. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
14.
Aylin CastilloJuan Valdes Jose SibajaIlena Vega Rosa AlfaroJosé Morales Germain EsquivelElisa Barrantes Paleah BlackDavid Lean 《Applied Geochemistry》2011,26(2):242-248
Monitoring of Hg in the atmosphere near volcanoes is limited with no previous data for Costa Rica. Seasonal and daily patterns of total gaseous mercury (TGM) were observed at the main sampling location at the Universidad Nacional, Heredia, Costa Rica. The area (lat. 10.000230 long. −84.109499) is located in the Central Valley of Costa Rica and is 27 km SE of the Poas volcano (lat. 10.199486 long. −84.231388). Measurements were made from May 2008 to May 2009 at this location with some additional values obtained at other sites near the Poas volcano including San Luis and Grecia as well as near, Turrialba and Irazu volcanoes. Total gaseous Hg was determined in samples collected at a height of 2 m using the Tekran 2537A (Tekran Inc.) gas-phase Hg vapor analyzer. Meteorological data (temperature, relative humidity, wind speed, wind direction, radiation and precipitation) were obtained from the airport weather station located at Alajuela. Monthly precipitation is typically 85 mm during the dry season (December to April) with winds from the west. The wet season begins in late April and continues to December with monthly rainfall of 328 mm and winds from the NE. The annual mean temperature is 20 °C. With the onset of the wet season TGM increased from typical values near 10 to 905 ng m−3. Measurements made within 5 km of the Poas volcano were higher than at Heredia at that time. Diel values measured at the university site increased until midday along with temperature and radiation. Relative humidity showed a reciprocal pattern. It was found that high values of TGM were not related to wind velocity or direction. The strong diel pattern increased with sunrise, peaked at midday and was lowest during the night time. It would seem that Hg0 from the volcano is oxidized and is deposited to the soils during the dry season when winds are blowing from the volcano. With the onset of heavy rains in April, Hg in the soil is reduced and re-volatilized resulting in the high levels in the atmosphere. Values at other volcano sites were provided. The role of atmospheric pollutants such as H2O2 and O3 should be included in future studies as they may result in oxidation of reduced Hg. The instability in the air masses may also be a factor and local pollution sources may result in high levels that is emitted from the volcanoes of Hg being circulated to ground level as radiation intensity increases. 相似文献
15.
Abstract Various inversion algorithms have been developed to obtain estimates of soil moisture and surface roughness parameters from multifrequency, multiangle, and multipolarization radar reflectances. Since the penetration depth for radar signals increases with wavelength, an inversion algorithm using widely separated frequencies does not yield comparable probing depths. Furthermore, existing algorithms assume a linear relationship between the radar backscatter coefficient (in dB) and soil parameters, such as the volumetric soil moisture, soil surface roughness and surface slope. This assumption is valid only over a narrow range of soil parameters, thereby restricting its operational use under realistic conditions. Our research specifically explored the use of inversion algorithms based on L‐Band radar reflectances at 1 GHz and 2 GHz frequencies in order to retain relatively consistent probing depths. In order to extend the range of applicability, a non‐linear exponential‐type relationship was developed between radar reflectance at a specified frequency, polarization and incidence angle combination, and soil parameters of interest, viz., soil moisture, surface roughness, and surface slope. An over‐constrained inversion algorithm using a six‐parameter combination was found to yield relatively accurate estimates of soil parameters over a wide range of soil conditions even in the presence of system error. 相似文献
16.
Germain Esquivel‐Hernndez Giovanny M. Mosquera Ricardo Snchez‐Murillo Adolfo Quesada‐Romn Christian Birkel Patricio Crespo Rolando Clleri David Windhorst Lutz Breuer Jan Boll 《水文研究》2019,33(13):1802-1817
High‐elevation tropical grassland systems, called Páramo, provide essential ecosystem services such as water storage and supply for surrounding and lowland areas. Páramo systems are threatened by climate and land use changes. Rainfall generation processes and moisture transport pathways influencing precipitation in the Páramo are poorly understood but needed to estimate the impact of these changes, particularly during El Niño conditions, which largely affect hydrometeorological conditions in tropical regions. To fill this knowledge gap, we present a stable isotope analysis of rainfall samples collected on a daily to weekly basis between January 2015 and May 2016 during the strongest El Niño event recorded in history (2014–2016) in two Páramo regions of Central America (Chirripó, Costa Rica) and the northern Andes (Cajas, south Ecuador). Isotopic compositions were used to identify how rainfall generation processes (convective and orographic) change seasonally at each study site. Hybrid Single Particle Lagrangian Integrated Trajectory model (HYSPLIT) air mass back trajectory analysis was used to identify preferential moisture transport pathways to each Páramo site. Our results show the strong influence of north‐east trade winds to transport moisture from the Caribbean Sea to Chirripó and the South American low‐level jet to transport moisture from the Amazon forest to Cajas. These moisture contributions were also related to the formation of convective rainfall associated with the passage of the Intertropical Convergence Zone over Costa Rica and Ecuador during the wetter seasons and to orographic precipitation during the transition and drier seasons. Our findings provide essential baseline information for further research applications of water stable isotopes as tracers of rainfall generation processes and transport in the Páramo and other montane ecosystems in the tropics. 相似文献
17.
Ricardo Sánchez-Murillo Germain Esquivel-Hernández José L. Corrales-Salazar Laura Castro-Chacón Ana M. Durán-Quesada Manuel Guerrero-Hernández Valeria Delgado Javier Barberena Katia Montenegro-Rayo Heyddy Calderón Carlos Chevez Tania Peña-Paz Saúl García-Santos Pedro Ortiz-Roque Yaneth Alvarado-Callejas Laura Benegas Antonio Hernández-Antonio Marcela Matamoros-Ortega Lucia Ortega Stefan Terzer-Wassmuth 《水文研究》2020,34(11):2660-2675
Numerous socio-economic activities depend on the seasonal rainfall and groundwater recharge cycle across the Central American Isthmus. Population growth and unregulated land use changes resulted in extensive surface water pollution and a large dependency on groundwater resources. This work combines stable isotope variations in rainfall, surface water, and groundwater of Costa Rica, Nicaragua, El Salvador, and Honduras to develop a regionalized rainfall isoscape, isotopic lapse rates, spatial–temporal isotopic variations, and air mass back trajectories determining potential mean recharge elevations, moisture circulation patterns, and surface water–groundwater interactions. Intra-seasonal rainfall modes resulted in two isotopically depleted incursions (W-shaped isotopic pattern) during the wet season and two enriched pulses during the mid-summer drought and the months of the strongest trade winds. Notable isotopic sub-cloud fractionation and near-surface secondary evaporation were identified as common denominators within the Central American Dry Corridor. Groundwater and surface water isotope ratios depicted the strong orographic separation into the Caribbean and Pacific domains, mainly induced by the governing moisture transport from the Caribbean Sea, complex rainfall producing systems across the N-S mountain range, and the subsequent mixing with local evapotranspiration, and, to a lesser degree, the eastern Pacific Ocean fluxes. Groundwater recharge was characterized by (a) depleted recharge in highland areas (72.3%), (b) rapid recharge via preferential flow paths (13.1%), and enriched recharge due to near-surface secondary fractionation (14.6%). Median recharge elevation ranged from 1,104 to 1,979 m a.s.l. These results are intended to enhance forest conservation practices, inform water protection regulations, and facilitate water security and sustainability planning in the Central American Isthmus. 相似文献
18.
Snow avalanche regime and climatic conditions in the Chic-Choc Range, eastern Canada 总被引：4，自引：1，他引：3
Because snow avalanches occur at altitudes close to the 0°C isotherm in mountain environments, they should respond quickly to climatic variations. This study provides tree-ring-based high-magnitude avalanche chronologies for 12 subalpine avalanche paths in the Chic-Choc Range of Québec (eastern Canada). For the period covered by the chronologies, i.e., between 1895 and 1999, high-magnitude avalanches occurred with an average return interval of 5.3 years, which represents an average annual probability of 21% for all paths. A regional avalanche activity index (RAAI) was developed to help differentiating widespread regional avalanche activity from avalanche events resulting from local factors. Nineteen years of high-magnitude avalanche occurrence were identified (1898, 1936, 1939, 1941, 1965, 1969, 1970, 1971, 1977, 1981, 1984, 1987, 1988, 1989, 1990, 1991, 1992, 1994 and 1995). Of these, 15 occurred after 1950 and 10 after 1980. Propitious climatic conditions that account for widespread avalanche activity in the study region were identified as follows: (1) above-average total snowfall, (2) high-frequency of snowstorms, (3) major rain events and facet–crust development, (4) sequences of freezing rain and strong winds, and (5) early-season weak layers of faceted crystals and depth hoar. The number of days with air temperature above 0°C has strong implications on the internal structure and stratigraphy of the snowpack, and consequently on avalanche release. Land managers should thus consider more closely the impact of climatic conditions and warming on avalanche activity. 相似文献
19.
Snow avalanches are a major natural hazard for road users and infrastructure in northern Gaspésie. Over the past 11 years, the occurrence of nearly 500 snow avalanches on the two major roads servicing the area was reported. No management program is currently operational. In this study, we analyze the weather patterns promoting snow avalanche initiation and use logistic regression (LR) to calculate the probability of avalanche occurrence on a daily basis. We then test the best LR models over the 2012–2013 season in an operational forecasting perspective: Each day, the probability of occurrence (0–100%) determined by the model was classified into five classes avalanche danger scale. Our results show that avalanche occurrence along the coast is best predicted by 2 days of accrued snowfall [in water equivalent (WE)], daily rainfall, and wind speed. In the valley, the most significant predictive variables are 3 days of accrued snowfall (WE), daily rainfall, and the preceding 2 days of thermal amplitude. The large scree slopes located along the coast and exposed to strong winds tend to be more reactive to direct snow accumulation than the inner-valley slopes. Therefore, the probability of avalanche occurrence increases rapidly during a snowfall. The slopes located in the valley are less responsive to snow loading. The LR models developed prove to be an efficient tool to forecast days with high levels of snow avalanche activity. Finally, we discuss how road maintenance managers can use this forecasting tool to improve decision making and risk rendering on a daily basis. 相似文献
20.
DyeLIF™: A New Direct‐Push Laser‐Induced Fluorescence Sensor System for Chlorinated Solvent DNAPL and Other Non‐Naturally Fluorescing NAPLs 下载免费PDF全文
下载免费PDF全文 Murray Einarson Adrian Fure Randy St. Germain Steven Chapman Beth Parker 《Ground Water Monitoring & Remediation》2018,38(3):28-42
DyeLIF? is a new version of laser‐induced fluorescence (LIF) for high‐resolution three‐dimensional subsurface mapping of nonaqueous phase liquids (NAPLs) in the subsurface. DyeLIF eliminates the requirement that the NAPL contains native fluorophores (such as those that occur in compounds like polynuclear aromatic hydrocarbons [PAHs]) and can therefore be used to detect chlorinated solvents and other nonfluorescing NAPLs that had previously been undetectable with conventional LIF tools. With DyeLIF, an aqueous solution of water and nontoxic hydrophobic dye is continuously injected ahead of the sapphire detection window while the LIF probe is being advanced in the subsurface. If soil containing NAPL is penetrated, the injected dye solvates into the NAPL within a few milliseconds, creating strong fluorescence that is transmitted via fiber‐optic filaments to aboveground optical sensors. A detailed field evaluation of the novel DyeLIF technology was performed at a contaminated industrial site in Lowell, Massachusetts, USA where chlorinated solvent dense nonaqueous phase liquid (DNAPL) persists below the water table in sandy sediments. Continuously cored boreholes were drilled adjacent to 5 of 30 DyeLIF probes that were advanced at that site. The cores were subsampled in high resolution to generate discrete‐depth soil samples as splits at the same depths where DNAPL was detected in the colocated DyeLIF probes. The cores were analyzed above ground using (1) colorimetric screening using hydrophobic dye tests, (2) laboratory extraction and quantitative chemical analysis, (3) “Benchtop” DyeLIF, and (4) volumetric moisture content. Correlation between DyeLIF and aboveground analyses of the soil cores was excellent: 98% agreement with positive DNAPL detections in samples where DNAPL pore saturations were >0.7% (based on quantitative soil analyses) and the ex situ tests. DyeLIF produced the equivalent of one aboveground colorimetric dye test every 0.2 inch (0.5 cm) of probing. With average daily probing of 395 linear feet (120.4 m), this was the equivalent of 12,039 discrete‐depth colorimetric dye tests/day. Because DyeLIF is an in situ measurement, there are no issues with soil core recovery like there would be for conventional ex situ colorimetric dye tests and 100% characterization of the probed intervals is achieved. Tracking the injection rate and pressure of the dye solution provides simultaneous data regarding relative soil permeability, similar to other direct push (DP) hydraulic profiling tools. Conventional LIF is considered the premier DP tool to identify and map NAPL containing PAHs in the subsurface or confirm its absence. While chlorinated solvent DNAPLs at some field sites contain impurities (e.g., solvated greases or oils) that make them detectable with conventional LIF techniques, at other sites, the DNAPL cannot be detected with conventional LIF. At such sites, the injection of a hydrophobic dye ahead of the sapphire window with the DyeLIF system now makes the LIF technology applicable to the many types of NAPLs that were previously invisible using conventional LIF techniques. 相似文献