Utility of scaled chrysophytes for inferring lakewater pH in northern New England lakes     

Sushil S. Dixit  John P. Smol  Dennis S. Anderson  Ronald B. Davis 《Journal of Paleolimnology》1990,3(3):269-286

Scaled chrysophytes in the surface sediments of 58 soft-water northern New England lakes were analyzed to assess their usefulness for inferring pH. The distributions of many taxa are correlated with lakewater pH and associated variables. Canonical correspondence analysis (CCA) and clustering grouped chrysophyte taxa according to their distributions along the pH gradient. For example, Chrysodidymus synuroideus, Mallomonas hindonii, and M. hamata commonly occur in acidic waters (pH<5.5), whereas M. caudata and M. pseudocoronata are common in circumneutral to alkaline waters. Of the five predictive models developed to infer pH, CCA based calibration had the lowest standard error (0.35 pH units). A CCA based predictive model was also developed to infer total alkalinity. The study provides strong evidence that, in the absence of past measured pH data, stratigraphic studies of sedimentary chrysophyte scales will provide accurate reconstructions of pH in northern New England lakes.This is the sixth of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.  相似文献   

Validating quantitative precipitation forecast for the Flood Meteorological Office,Patna region during 2011–2014     

R K Giri  Jagabandhu Panda  Sudhansu S Rath  Ravindra Kumar 《Journal of Earth System Science》2016,125(4):709-723

In order to issue an accurate warning for flood, a better or appropriate quantitative forecasting of precipitation is required. In view of this, the present study intends to validate the quantitative precipitation forecast (QPF) issued during southwest monsoon season for six river catchments (basin) under the flood meteorological office, Patna region. The forecast is analysed statistically by computing various skill scores of six different precipitation ranges during the years 2011–2014. The analysis of QPF validation indicates that the multi-model ensemble (MME) based forecasting is more reliable in the precipitation ranges of 1–10 and 11–25 mm. However, the reliability decreases for higher ranges of rainfall and also for the lowest range, i.e., below 1 mm. In order to testify synoptic analogue method based MME forecasting for QPF during an extreme weather event, a case study of tropical cyclone Phailin is performed. It is realized that in case of extreme events like cyclonic storms, the MME forecasting is qualitatively useful for issue of warning for the occurrence of floods, though it may not be reliable for the QPF. However, QPF may be improved using satellite and radar products.  相似文献   

A comprehensive assessment framework for attributing trends in streamflow and groundwater storage to climatic and anthropogenic changes: A case study in the typical semi-arid catchments of southern India     

Rajesh Nune  Biju A. George  Andrew W. Western  Kaushal K. Garg  Sreenath Dixit  Ragab Ragab 《水文研究》2021,35(8):e14305

The clearest signs of hydrologic change can be observed from the trends in streamflow and groundwater levels in a catchment. During 1980–2007, significant declines in streamflow (−3.03 mm/year) and groundwater levels (−0.22 m/year) were observed in Himayat Sagar (HS) catchment, India. We examined the degree to which hydrologic changes observed in the HS catchment can be attributed to various internal and external drivers of change (climatic and anthropogenic changes). This study used an investigative approach to attribute hydrologic changes. First, it involves to develop a model and test its ability to predict hydrologic trends in a catchment that has undergone significant changes. Second, it examines the relative importance of different causes of change on the hydrologic response. The analysis was carried out using Modified Soil and Water Assessment Tool (SWAT), a semi-distributed rainfall-runoff model coupled with a lumped groundwater model for each sub- catchment. The model results indicated that the decline in potential evapotranspiration (PET) appears to be partially offset by a significant response to changes in rainfall. Measures that enhance recharge, such as watershed hydrological structures, have had limited success in terms of reducing impacts on the catchment-scale water balance. Groundwater storage has declined at a rate of 5 mm/y due to impact of land use changes and this was replaced by a net addition of 2 mm/y by hydrological structures. The impact of land use change on streamflow is an order of magnitude larger than the impact of hydrological structures and about is 2.5 times higher in terms of groundwater impact. Model results indicate that both exogenous and endogenous changes can have large impacts on catchment hydrology and should be considered together. The proposed comprehensive framework and approach demonstrated here is valuable in attributing trends in streamflow and groundwater levels to catchment climatic and anthropogenic changes.  相似文献   

Effect of topography on farm-scale spatial variation in extreme temperatures in the Southern Mallee of Victoria, Australia     

Prakash N. Dixit  Deli Chen 《Theoretical and Applied Climatology》2011,103(3-4):533-542

Extreme temperatures around flowering of wheat have the potential to reduce grain yield and at farm scale their impact can be spatially variable depending on topography. Twenty-five data loggers were installed at 0.8-m height across a 164-ha farm in the southern Mallee of Victoria, Australia to spatially record the daily course of temperatures around the average date of flowering of wheat in the region. The experiment was conducted during 2-years period. In 1 year, the farm had no crop cover and in another year the farm had a wheat crop. Multiple linear regression analysis techniques were used to fit models relating daily extreme temperatures to the farm topographic features of elevation, aspect and slope, and the average maximum and minimum temperatures of each day at the farm in order to identify areas of high risk of extreme temperatures around the time of the flowering of wheat. The fitted regression models explained 90% and 97% of the variability in maximum and minimum temperatures, respectively, when the farm had no crop cover and 80% and 94% of the variability in maximum and minimum temperatures, respectively, when the farm had a wheat crop cover. When the farm had no crop, only minimum temperature was partially explained by the topography however, both maximum and minimum temperatures were partially explained by the topography when the farm had a wheat crop. From this study it was concluded that, (1) high temperature variations were found across the farm (2) temperature variations were only partially explained from the developed model presumably due to the flatter topography of the farm and (3) the relationships obtained from this study could be used in a crop model which can explain variation in grain yield based on the topography of a field.  相似文献   

Acceleration covariance in hydromagnetic dusty turbulence in the presence of a very strong uniform magnetic field     

T. Dixit 《Astrophysics and Space Science》1989,159(1):57-66

We have found the Electric Dipole Radiation Fields and an estimative of the power radiated by an electric dipole in Einstein and de Sitter's cosmological backgrounds. A possible connection with the quasar problem is suggested.  相似文献   

Diatom and chrysophyte functions and inferences of post-industrial acidification and recent recovery trends in Killarney lakes (Ontario, Canada)   总被引：1，自引：0，他引：1  

Sushil S. Dixit  Aruna S. Dixit  John P. Smol 《Journal of Paleolimnology》2002,27(1):79-96

The surface sediment diatom and chrysophyte assemblages from 33 Sudbury lakes were added to our published 72 lake data set to expand and refine the diatom and chrysophyte-based inference models that we had earlier developed for this region. Our calibration data set now includes 105 lakes, representing gradients for multiple environmental variables (e.g., lakewater pH, metals, and transparency). The revised models are based on the weighted averaging calibration and regression approach and include bootstrap error estimates. The pH model was the strongest (r² _boot = 0.75, RMSE _boot = 0.50). The chrysophyte-inferred pH model (r² _boot = 0.79, RMSE _boot = 0.48) that we developed was as robust as the diatom pH model. Diatom and chrysophyte inferred pH models were then applied to top (surface sediments representing current conditions) and bottom (generally from > 30 cm deep representing pre-industrial conditions) sediment diatom and chrysophyte assemblages of 19 Killarney area lakes near Sudbury. The top and bottom inferred pH results were compared to early-1970s measured pH data. These data suggest that, although many of the poorly buffered Killarney lakes had experienced acidification, marked pH recovery has occurred in many lakes within the last 25 years. Despite the stunning pH recovery, the present-day diatom and chrysophyte assemblages are significantly different from assemblages present during pre-industrial times. Our results suggest that biological recovery may require more time than chemical recovery. It is also likely that these lakes may never recover biologically because other anthropogenic stressors (e.g., climate warming and increased exposure to UV-B radiation) may now have greater influence on biological communities in Killarney/Sudbury area lakes than acidification.  相似文献   

Ocean and atmospheric characteristics associated with the cyclogenesis and rapid intensification of NIO super cyclonic storms during 1981–2020     

Paul  Debashis  Panda  Jagabandhu  Routray  Ashish 《Natural Hazards》2022,111(1):261-282

Natural Hazards - Flooding events in the Lower Benue valley of Nigeria are often associated with huge damage to properties and loss of life in the adjoining communities. Specific objectives of this...  相似文献   

Rayleigh wave group velocity tomography of Gujarat region,Western India and its implications to mantle dynamics     

Mayank Dixit  A. P. Singh  O. P. Mishra 《Journal of Seismology》2017,21(4):809-823

In the present study, fundamental Rayleigh waves with varying period from 10 to 80 s are used to obtain group velocity maps in the northwest Deccan Volcanic Province of India. About 350 paths are obtained using 53 earthquakes (4.8 ≤ M ≥ 7.9) recorded by the SeisNetG (Seismic Network of Gujarat). Individual dispersion curves of group velocity of Rayleigh wave for each source-station path are estimated using multiple filter technique. These curves are used to determine lateral distribution of Rayleigh wave group velocity by tomographic inversion method. Our estimated Rayleigh group velocity at varying depths showed conspicuous corroboration with three tectonic blocks [Kachchh Rift Basin (KRB), Saurashtra Horst (SH), and Mainland Gujarat (MG)] in the region. The seismically active KRB with a thicker crust is characterized as a low velocity zone at a period varying from 10 to 30 s as indicative of mantle downwarping or sagging of the mantle beneath the KRB, while the SH and MG are found to be associated with higher group velocities, indicating the existence of the reduced crustal thickness. The trend of higher group velocity was found prevailed adjacent to the Narmada and Cambay rift basins that also correspond to the reduced crust, suggesting the processes of mantle upwarping or uplifting due to mantle upwelling. The low velocities at periods longer than 40 s beneath the KRB indicate thicker lithosphere. The known Moho depth correlates well with the observed velocities at a period of about 30 s in the Gujarat region. Our estimates of relatively lower group velocities at periods varying from 70 to 80 s may correspond to the asthenospheric flow beneath the region. It is interesting to image higher group velocity for the thinner crust beneath the Arabian Sea adjacent to the west coast of Gujarat at the period of 40 s that may correspond to the upwarped or upwelled mantle beneath the Arabian Sea. Our results have better resolution estimated by a radius of equivalent circular averaging area for each period.  相似文献   

Responses of Two New Hampshire (USA) Lakes to Human Impacts in Recent Centuries   总被引：1，自引：0，他引：1  

Ronald B. Davis  Dennis S. Anderson  Sushil S. Dixit  Peter G. Appleby  Molly Schauffler 《Journal of Paleolimnology》2006,35(4):669-697

This study contrasts the effects of human disturbances on two very different lake/catchment systems: Hatch Pond and Beaver Lake, New Hampshire. Hatch is in a steep mountain catchment remote from urban/industrial centers, and is a small, relatively deep, primarily seepage lake with moderate flushing rate. The catchment has been continuously forested, but disturbed by partial loggings and a forest fire. Beaver’s more extensive and gently sloping catchment is much closer to urban/industrial centers, and has been agricultural and recently suburban. Beaver is a larger but shallower drainage lake with much more rapid flushing than Hatch. We compared the sedimentary records and inferred limnological responses to catchment land uses and air pollution inputs of these mesotrophic lakes, and interpreted differing responses as a function of basic differences between the lakes and their catchments. Some chemical and biological variables in the sediment of both lakes respond sensitively to the first disturbances of the catchments by Euro-American settlers. Diatom-inferred Cl at both lakes indicates a modest increase of salinity at this time. Hatch sediment contains a record of soil erosion starting 1810 with first logging and fire. For ~125 years, erosion was largely of upper eluviated soil, but by 1935 it reached deeper into the illuviated (enriched) horizon. Maximal lake trophic state based on diatom-inferred limnological variables occurred in the mid-1900’s period of maximum sedimentation of illuviated soil. The sediment record since 1964 reflects forest maturation, and soil stabilization and acidification, but not lake acidification. At Beaver, the limnological effects of forest clearance and agriculture starting ~1700 were relatively mild due to gentleness of slopes and soil characteristics favoring stability. A near doubling of earth elements, and major increases in trace metal pollutants and coal combustion particles (CCP) occurred in sediment dating around 1900. Landscape analysis as well as historical and sedimentary records preclude the catchment as the major source of these increases. The most likely source is atmospheric fallout of CCP with its metal load. Despite controls on CCP and other emissions starting 1960, concentrations of earth elements and trace metals in Beaver sediment remain high due to soil mobilization by residential development of the catchment. The trophic state of Beaver Lake has increased, but the relative increase has been smaller than at Hatch, despite more intensive land uses and greater aerial inputs at Beaver. We propose that this lesser eutrophication is due to rapid flushing of Beaver Lake with stream water from relatively undeveloped parts of the extensive catchment, and lower sensitivity to nutrient inputs of this naturally more eutrophic lake. A major shift from unicellular to colonial chrysophytes at Hatch starts in the late-1800’s, and at Beaver in the early 1900’s. Colonial taxa in lesser quantities appear at the time of first settlement of the Beaver catchment. At both lakes the shifts are correlated with increasing metal fluxes from the opening of catchment biogeochemical cycles, as with the aforementioned erosional sequence at Hatch. But at Hatch aerial inputs of trace metals, and at Beaver aerial inputs major and trace metals associated with CCP are also correlated with the major chrysophyte shift. It appears that the chrysophytes have responded to stressors associated with both catchment disturbance and regional atmospheric inputs.  相似文献   

Mid-Holocene vegetation and climatic variability in tropical deciduous sal (<Emphasis Type="Italic">Shorea robusta</Emphasis>) forest of Lower Brahmaputra valley,Assam     

Swati Dixit  S. K. Bera 《Journal of the Geological Society of India》2011,77(5):419-432

One 2.5 m lacustrine sedimentary profile dated back to 6340 years BP from Deosila swamp under Rangjuli Reserve forest of Assam, Northeast India has been pollen analyzed for tracing past vegetation vis à vis climate variability since mid-Holocene. The pollen diagram has deciphered that during 6340 to 2970 years BP, tropical tree savannah type of vegetation grew in the region chiefly constituted of grasses interspersed with scattered trees of Salmalia, Dillenia, Emblica, Meliaceae along with sporadic presence of Artocarpus chaplasha, Symplocos, Ilex, Schima and Shorea robusta under relatively less cool and dry climatic condition with a little ameliorating trend at the upper column. The vegetation scenario implies poor growth of arboreals due to harsh abiotic dynamics causing changes in drainage system. Subsequently during 2970 to 1510 years BP, tropical mixed deciduous forest succeeded tree savannah with invasion of Shorea robusta, Lagerstroemia, Lannea, Semecarpus and Acacia under warm and humid climate. The gradual enrichment of organic soil might have been conducive for better growth of both deciduous and semi evergreen arboreals as found today. However, during 1510 to 540 years BP, the forest groves became more strengthened resulting establishment of tropical deciduous Sal forest. Steady increment of Shorea robusta along with Lannea, Lagerstroemia, Terminalia, Sapotaceae, Albizia and Adina was observed probably due to influence of active SW monsoon under increased warm and humid climatic regime. This is well substantiated by the consistent occurrence of marshy and aquatic taxa along with ferns and fungal remains. Finally during 540 years BP onward the reduced forest floristics have obviously been envisaged the change in climate which turned to warm and relatively dry probably attributable to the weak monsoon rainfall. The acceleration in human settlement during this phase is evidenced by Cerealia and by the increase of Melastoma, Ziziphus and Areca catechu implying forest clearance.  相似文献