Quantification of longitudinal dispersion by upscaling Brownian motion of tracer displacement in a 3D pore-scale network model     

R.C. Acharya  M.I.J. van Dijke  K.S. Sorbie  S.E.A.T.M. Van der Zee  A. Leijnse 《Advances in water resources》2007

We present a 3D network model with particle tracking to upscale 3D Brownian motion of non-reactive tracer particles subjected to a velocity field in the network bonds, representing both local diffusion and convection. At the intersections of the bonds (nodes) various jump conditions are implemented. Within the bonds, two different velocity profiles are used. At the network scale the longitudinal dispersion of the particles is quantified through the coefficient D_L, for which we evaluate a number of methods already known in the literature. Additionally, we introduce a new method for derivation of D_L based on the first-arrival times distribution (FTD). To validate our particle tracking method, we simulate Taylor’s classical experiments in a single tube. Subsequently, we carry out network simulations for a wide range of the characteristic Péclet number Pe_? to assess the various methods for obtaining D_L. Using the new method, additional simulations have been carried out to evaluate the choice of nodal jump conditions and velocity profile, in combination with varying network heterogeneity. In general, we conclude that the presented network model with particle tracking is a robust tool to obtain the macroscopic longitudinal dispersion coefficient. The new method to determine D_L from the FTD statistics works for the full range of Pe_?, provided that for large Pe_? a sufficiently large number of particles is used. Nodal jump conditions should include molecular diffusion and allow jumps in the upstream direction, and a parabolic velocity profile in the tubes must be implemented. Then, good agreement with experimental evidence is found for the full range of Pe_?, including increased D_L for increased porous medium heterogeneity.  相似文献   

Flood hazard and risk assessment of 2014 floods in Kashmir Valley: a space-based multisensor approach     

Rajesh Kumar  Prasenjit Acharya 《Natural Hazards》2016,84(1):437-464

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大型浅水湖泊水动力模型不确定性和敏感性分析   总被引：5，自引：0，他引：5       下载免费PDF全文

李一平  唐春燕  余钟波  Acharya Kumud 《水科学进展》2012,23(2):271-277

选取国内外常用的水动力学模型(EFDC)和典型的浅水湖泊(太湖),采用拉丁超立方取样(LHS),研究湖泊水动力模块中常用的5个重要参数(风拖曳系数、床面粗糙高度、涡粘性系数、紊流扩散系数以及风遮挡系数)对湖体水位和流速的影响。结果表明:针对大型浅水湖泊,湖泊岸线形状和湖底地形、湖泊周围地形、湖泊水面风场对模拟结果产生决定性影响。尤其是在湖湾区和周边地形比较复杂的地区,风场参数对水动力模拟结果不确定性的贡献率最大。在垂向上,表层流速受到参数不确定性的影响最大,底层次之,中层最小。床面粗糙高度对水动力模拟结果不确定性贡献率较风场参数要小,水体涡粘滞系数和扩散系数影响则更小。故在选择大型浅水湖泊水动力模型参数时,要充分考虑湖泊岸线和周围地形,着重率定风场参数以及床面粗糙高度。  相似文献   

Performance of general circulation models and their ensembles for the prediction of drought indices over India during summer monsoon     

Nachiketa Acharya  Ankita Singh  U. C. Mohanty  Archana Nair  Surajit Chattopadhyay 《Natural Hazards》2013,66(2):851-871

The drought during the months of June to September (JJAS) results in significant deficiency in the annual rainfall and affects the hydrological planning, disaster management, and the agriculture sector of India. Advance information on drought characteristics over the space may help in risk assessment over the country. This issue motivated the present study which deals with the prediction of drought during JJAS through standardized precipitation index (SPI) using nine general circulation models (GCM) product. Among these GCMs, three are the atmospheric and six are atmosphere–ocean coupled models. The performance of these GCM’s predicted SPI is examined against the observed SPI for the time period of 1982–2010. After a rigorous analysis, it can be concluded that the skill of prediction by GCM is not satisfactory, whereas the ability of the coupled models is better than the atmospheric models. An attempt has been made to improve the accuracy of predicted SPI using two different multi-model ensemble (MME) schemes, viz., arithmetic mean and weighted mean using singular value decomposition-based multiple linear regressions (SVD-MLR) of GCMs. It is found that among these MME techniques, SVD-MLR-based MME has more skill as compared to simple MME as well as individual GCMs.  相似文献   

Hydrodynamic Study of Weathered Zone of Hardrock Aquifers Using Dugwell Data     

Tapas Acharya  Rajesh Prasad 《Natural Resources Research》2014,23(3):331-340

The present study discusses a technique of locating high-discharging dugwells in Precambrian metamorphic rocks in Balarampur, Purulia district, West Bengal, India and describes the hydrodynamic properties of hardrock aquifers using bedrock joints/fractures, lineaments and weathered zone. Dugwells with higher discharges are mainly concentrated in metapelites, rich in biotite. In groups of dugwells, high-discharging wells exhibit linear spatial distribution, denoted as “well-line,” and consistently show high well discharge along each well-line in every dugwell group. The spatial distributions of high-discharging wells are associated with thick weathered-zones. Well-lines, comprising abruptly high-discharging wells, are spatially correlated to the lithostratigraphic contacts and lower elevation zones.  相似文献   

Development of an artificial neural network based multi-model ensemble to estimate the northeast monsoon rainfall over south peninsular India: an application of extreme learning machine   总被引：2，自引：0，他引：2  

Nachiketa Acharya  Nitin Anand Shrivastava  B. K. Panigrahi  U. C. Mohanty 《Climate Dynamics》2014,43(5-6):1303-1310

The south peninsular part of India gets maximum amount of rainfall during the northeast monsoon (NEM) season [October to November (OND)] which is the primary source of water for the agricultural activities in this region. A nonlinear method viz., Extreme learning machine (ELM) has been employed on general circulation model (GCM) products to make the multi-model ensemble (MME) based estimation of NEM rainfall (NEMR). The ELM is basically is an improved learning algorithm for the single feed-forward neural network (SLFN) architecture. The 27 year (1982–2008) lead-1 (using initial conditions of September for forecasting the mean rainfall of OND) hindcast runs (1982–2008) from seven GCM has been used to make MME. The improvement of the proposed method with respect to other regular MME (simple arithmetic mean of GCMs (EM) and singular value decomposition based multiple linear regressions based MME) has been assessed through several skill metrics like Spread distribution, multiplicative bias, prediction errors, the yield of prediction, Pearson’s and Kendal’s correlation coefficient and Wilmort’s index of agreement. The efficiency of ELM estimated rainfall is established by all the stated skill scores. The performance of ELM in extreme NEMR years, out of which 4 years are characterized by deficit rainfall and 5 years are identified as excess, is also examined. It is found that the ELM could expeditiously capture these extremes reasonably well as compared to the other MME approaches.  相似文献   

Evaluation of drought and wetness episodes in a cold region (Northeast China) since 1898 with different drought indices     

Binquan Li  Zhongmin Liang  Zhongbo Yu  Kumud Acharya 《Natural Hazards》2014,71(3):2063-2085

Drought identification and drought severity characterization are crucial to understand water scarcity processes. Evolution of drought and wetness episodes in the upper Nen River (UNR) basin have been analyzed for the period of 1951–2012 using meteorological drought indices and for the period of 1898–2010 using hydrological drought indices. There were three meteorological indices: one based on precipitation [the Standardized Precipitation Index (SPI)] and the other two based on water balance with different formulations of potential evapotranspiration (PET) in the Standardized Precipitation Evapotranspiration Index (SPEI). Moreover, two hydrological indices, the Standardized Runoff Index and Standardized Streamflow Index, were also applied in the UNR basin. Based on the meteorological indices, the results showed that the main dry period of 1965–1980 and wet periods of 1951–1964 and 1981–2002 affected this cold region. It was also found that most areas of the UNR basin experienced near normal condition during the period of 1951–2012. As a whole, the UNR basin mainly had the drought episodes in the decades of 1910, 1920, 1970 and 2000 based on hydrological indices. Also, the severity of droughts decreased from the periods of 1898–1950 to 1951–2010, while the severity of floods increased oppositely during the same periods. A correlation analysis showed that hydrological system needs a time lag of one or more months to respond to meteorological conditions in this cold region. It was also found that although precipitation had a major role in explaining temporal variability of drought, the influence of PET was not negligible. However, the sole temperature driver of PET had an opposite effect in the UNR basin (i.e., misestimating the drought detection) and was inferior to the SPI, which suggests that the PET in the SPEI should be determined by using underlying physical principles. This finding is an important implication for the drought research in future.  相似文献   

Probabilistic multi-model ensemble prediction of Indian summer monsoon rainfall using general circulation models: A non-parametric approach     

Nachiketa Acharya  Uma Charan Mohanty  Lokanath Sahoo 《Comptes Rendus Geoscience》2013,345(3):126-135

Probabilistic prediction has the ability to convey the intrinsic uncertainty of forecast that helps the decision makers to manage the climate risk more efficiently than deterministic forecasts. In recent times, probabilistic predictions obtained from the products from General Circulation Models (GCMs) have gained considerable attention. The probabilistic forecast can be generated in parametric (assuming Gaussian distribution) as well as non-parametric (counting method) ways. The present study deals with the non-parametric approach that requires no assumption about the form of the forecast distribution for the prediction of Indian summer monsoon rainfall (ISMR) based on the hindcast run of seven general circulation models from 1982 to 2008. Probabilistic prediction from each of the GCM products has been generated by non-parametric methods for tercile categories (viz. below normal (BN), near-normal (NN), and above normal (AN)) and evaluation of their skill is assessed against observed data. Five different types of PMME schemes have been used for combining probabilities from each GCM to improve the forecast skill as compared to the individual GCMs. These schemes are different in nature of assigning the weights for combining probabilities. After a rigorous analysis through Rank Probability Skill Score (RPSS) and relative operating characteristic (ROC) curve, the superiority of PMME has been established over climatological probability. It is also found that, the performances of PMME1 and PMME3 are better than all the other methods whereas PMME3 has showed more improvement over PMME1.  相似文献   

Observations of AGNs using PACT     

D. Bose  V. R. Chitnis  P. R. Vishwanath  P. Majumdar  M. A. Rahman  B. B. Singh  A. C. Gupta  B. S. Acharya 《Astrophysics and Space Science》2007,309(1-4):111-117

We report our observations on 4 AGNs viz, Mkn 421, Mkn 501, 1ES1426+428 and ON231 belonging to a sub-class called blazars. The observations were carried out using the Pachmarhi Array of Cherenkov Telescopes and span about 6 years period from 2000 to 2005. We discuss our methods of analysis adopted to extract the gamma ray signal from cosmic ray background. We present our results on the emission of TeV gamma rays from these objects. Also, we report on the status of the new high altitude experiment, a 7 telescope array at Hanle in the Himalayas at an altitude of about 4200 m above mean sea level for detecting celestial gamma-rays.  相似文献   

Search for TeV gamma-rays from Geminga pulsar     

B.B. Singh  V.R. Chitnis  D. Bose  M.A. Rahman  S.S. Upadhya  K.S. Gothe  B.K. Nagesh  P.N. Purohit  Shobha K. Rao  Kamesh K. Rao  S.K. Sharma  P.V. Sudersan  B.L. Venkateshmurthy  P.R. Vishwanath  B.S. Acharya 《Astroparticle Physics》2009,32(2):120-128

We present the results of observation of the Geminga pulsar carried out in the TeV energy band during the 6 year period spanning 2000–2006 using the Pachmarhi Array of Cherenkov Telescopes (PACT). A long stretch of data, new computer codes and the “Tempo” package have been used in the present analysis. We have searched for evidence of pulsed emission of γ-rays from the Geminga pulsar using the post-glitch pulsar elements obtained by Jackson and Halpern from X-ray/γ-ray satellite data. We do not see any significant evidence for pulsed emission from the Geminga pulsar at a threshold energy of 825 GeV. In this paper we present our results on the light curve in the TeV energy band, set an upper limit on the time averaged flux of γ-rays, and compare our results with other ground based observations.  相似文献