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排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Streamflow Simulation by SWAT Using Different Precipitation Sources in Large Arid Basins with Scarce Raingauges 总被引:2,自引:1,他引:1
Meiyan Yu Xi Chen Lanhai Li Anming Bao Mupenzi Jean de la Paix 《Water Resources Management》2011,25(11):2669-2681
Streamflow from the mountains is the main source of water for the lower plain in arid regions. Accurate simulation of streamflow
is of great importance to the arid ecosystem. However, many large arid drainage basins in northwestern China have low density
of precipitation stations, which makes the streamflow modeling and prediction very difficult. Based on raingauge data and
Tropical Rainfall Measuring Mission (TRMM) data combined with raingauge data, different approaches were explored for spatializing
precipitation in large area with scarce raingauges. Spatialized precipitation was then input into Soil and Water Assessment
Tool (SWAT), a semi-distributed hydrological model, to simulate streamflow. Results from a case study in the Manas river basin
showed that simulated hydrographs using both the approaches are able to reproduce the watershed hydrological behavior. Moreover,
statistical assessment indicated that hydrological model driven by the spatialized precipitation based on radar combined with
raingauge data performed better than that based on gauge data. Radar precipitation estimator can provide a practical data
source for hydrological modeling at a basin scale where the raingauge network is sparse. 相似文献
952.
Gajanan Kisan Khadse Moromi D. Kalita S. N. Pimpalkar Pawan K. Labhasetwar 《Water Resources Management》2011,25(13):3321-3342
To ascertain the quality of drinking water being supplied and maintained, it is necessary to conduct water quality surveillance
for evolving suitable strategy for future planning. In the present investigation, water quality was monitored in treatment
plants, service reservoirs, and at consumer ends in three seasons to assess the baseline water quality status at Shillong
in Meghalaya. There are three water treatment plants at Shillong namely Umkhen, Mawlai and GSWS with design capacities of
1.5, 3.4 and 34 Million liter per day (MLD) respectively. Each treatment plant is having rapid sand filtration followed by
disinfection. The study reveals that the physico-chemical parameters of water quality at consumer end meets Indian drinking
water quality standards (BIS 1991) after conventional treatment followed by disinfection, whereas the bacteriological parameters for raw water sources exceed
the permissible limit indicating the treatment need for drinking purposes. Throughout year the average feacal coliform contamination
at service reservoir and to consumer end were found as 44 to 156 CFU/100 ml which may be attributed to the general management
practices for maintenance of service reservoirs and the possibility of en route contamination. 相似文献
953.
The effects of bed-load movement on mean flow characteristics were experimentally investigated using particle image velocimetry
(PIV). The gravel beds associated with open-channel flow are hydraulically rough, and two types of grain with different median
diameters were used. The flow field and bed load motion images were simultaneously obtained via the particular experimental
setup. The mass of sediment particles instantaneously moving on the bed, i.e., the sand-wave, was observed in the experiment.
It was found that the sand-waves have an average friction angle of 11.2° and heights ranging from 2.2 d
50 to 3.8 d
50. The mean velocity profiles on the mobile bed can also be represented with logarithmic distribution with deviations occurred
in the near wall region. Based on the Schultz–Grunow formula, a modified equation is developed in this study to better express
the flow resistance factor in terms of the Reynolds number and the ratio of flow depth to roughness height. 相似文献
954.
Conventionally drought severity is assessed based on drought indices. Recently the Reconnaissance Drought Index (RDI) was
proposed to assess drought severity based on the precipitation to potential evapotranspiration ratio (P/PET). In this paper
RDI is studied as a bivariate index under a set of assumptions and simplifications. The paper presents a simple computational
procedure for estimating the P/PET ratio for selected reference periods varying from 3 to 12 months, for any return period
of drought. Alternatively, based on this procedure, the severity of any drought episode is rationally assessed. A bivariate
probability analysis is employed based on the assumption that P and PET values are normally distributed and often negatively
correlated. Examples for the application of the proposed procedure are presented using data from several meteorological stations
in Greece. It is shown that the assumption of normality of both P and PET holds for long periods at all examined stations. 相似文献
955.
We consider the problem of approximately integrating a Lipschitz function f (with a known Lipschitz constant) over an interval. The goal is to achieve an additive error of at most ε using as few samples of f as possible. We use the adaptive framework: on all problem instances an adaptive algorithm should perform almost as well
as the best possible algorithm tuned for the particular problem instance. We distinguish between
and
, the performances of the best possible deterministic and randomized algorithms, respectively. We give a deterministic algorithm
that uses
samples and show that an asymptotically better algorithm is impossible. However, any deterministic algorithm requires
samples on some problem instance. By combining a deterministic adaptive algorithm and Monte Carlo sampling with variance reduction,
we give an algorithm that uses at most
samples. We also show that any algorithm requires
samples in expectation on some problem instance (f,ε), which proves that our algorithm is optimal. 相似文献
956.
An instance of the path hitting problem consists of two families of paths,
and ℋ, in a common undirected graph, where each path in ℋ is associated with a non-negative cost. We refer to
and ℋ as the sets of demand and hitting paths, respectively. When p∈ℋ and
share at least one mutual edge, we say that p
hits q. The objective is to find a minimum cost subset of ℋ whose members collectively hit those of
. In this paper we provide constant factor approximation algorithms for path hitting, confined to instances in which the underlying
graph is a tree, a spider, or a star. Although such restricted settings may appear to be very simple, we demonstrate that
they still capture some of the most basic covering problems in graphs. Our approach combines several novel ideas: We extend
the algorithm of Garg, Vazirani and Yannakakis (Algorithmica, 18:3–20, 1997) for approximate multicuts and multicommodity flows in trees to prove new integrality properties; we present a reduction
that involves multiple calls to this extended algorithm; and we introduce a polynomial-time solvable variant of the edge cover
problem, which may be of independent interest.
An extended abstract of this paper appeared in Proceedings of the 14th Annual European Symposium on Algorithms, 2006.
This work is part of D. Segev’s Ph.D. thesis prepared at Tel-Aviv University under the supervision of Prof. Refael Hassin. 相似文献
957.
We introduce the notion of ranking robustness, which refers to a property of a ranked list of documents that indicates how
stable the ranking is in the presence of uncertainty in the ranked documents. We propose a statistical measure called the
robustness score to quantify this notion. Our initial motivation for measuring ranking robustness is to predict topic difficulty
for content-based queries in the ad-hoc retrieval task. Our results demonstrate that the robustness score is positively and
consistently correlation with average precision of content-based queries across a variety of TREC test collections. Though
our focus is on prediction under the ad-hoc retrieval task, we observe an interesting negative correlation with query performance
when our technique is applied to named-page finding queries, which are a fundamentally different kind of queries. A side effect
of this different behavior of the robustness score between the two types of queries is that the robustness score is also found
to be a good feature for query classification.
相似文献
958.
In this paper, we propose an efficient scalable algorithm for mining Maximal Sequential Patterns using Sampling (MSPS). The MSPS algorithm reduces much more search space than other algorithms because both the subsequence infrequency-based
pruning and the supersequence frequency-based pruning are applied. In MSPS, a sampling technique is used to identify long
frequent sequences earlier, instead of enumerating all their subsequences. We propose how to adjust the user-specified minimum
support level for mining a sample of the database to achieve better overall performance. This method makes sampling more efficient
when the minimum support is small. A signature-based method and a hash-based method are developed for the subsequence infrequency-based
pruning when the seed set of frequent sequences for the candidate generation is too big to be loaded into memory. A prefix
tree structure is developed to count the candidate sequences of different sizes during the database scanning, and it also
facilitates the customer sequence trimming. Our experiments showed MSPS has very good performance and better scalability than
other algorithms.
Congnan Luo received the B.E. degree in Computer Science from Tsinghua University, Beijing, P.R. China, in 1997, the M.S. degree in Computer
Science from the Institute of Software, Chinese Academy of Sciences, Beijing, P.R. China, in 2000, and the Ph.D. degree in
Computer Science and Engineering from Wright State University, Dayton, OH, in 2006. Currently he is a technical staff at the
Teradata division of NCR in San Diego, CA, and his research interests include data mining, machine learning, and databases.
Soon M. Chung received the B.S. degree in Electronic Engineering from Seoul National University, Korea, in 1979, the M.S. degree in Electrical
Engineering from Korea Advanced Institute of Science and Technology, Korea, in 1981, and the Ph.D. degree in Computer Engineering
from Syracuse University, Syracuse, New York, in 1990. He is currently a Professor in the Department of Computer Science and
Engineering at Wright State University, Dayton, OH. His research interests include database, data mining, Grid computing,
text mining, XML, and parallel and distributed processing. 相似文献
959.
The increasing prominence of data streams arising in a wide range of advanced applications such as fraud detection and trend
learning has led to the study of online mining of frequent itemsets (FIs). Unlike mining static databases, mining data streams
poses many new challenges. In addition to the one-scan nature, the unbounded memory requirement and the high data arrival
rate of data streams, the combinatorial explosion of itemsets exacerbates the mining task. The high complexity of the FI mining
problem hinders the application of the stream mining techniques. We recognize that a critical review of existing techniques
is needed in order to design and develop efficient mining algorithms and data structures that are able to match the processing
rate of the mining with the high arrival rate of data streams. Within a unifying set of notations and terminologies, we describe
in this paper the efforts and main techniques for mining data streams and present a comprehensive survey of a number of the
state-of-the-art algorithms on mining frequent itemsets over data streams. We classify the stream-mining techniques into two
categories based on the window model that they adopt in order to provide insights into how and why the techniques are useful.
Then, we further analyze the algorithms according to whether they are exact or approximate and, for approximate approaches, whether they are false-positive or false-negative. We also discuss various interesting issues, including the merits and limitations in existing research and substantive areas
for future research. 相似文献
960.
Traditional clustering algorithms are inapplicable to many real-world problems where limited knowledge from domain experts
is available. Incorporating the domain knowledge can guide a clustering algorithm, consequently improving the quality of clustering.
In this paper, we propose SS-NMF: a semi-supervised non-negative matrix factorization framework for data clustering. In SS-NMF,
users are able to provide supervision for clustering in terms of pairwise constraints on a few data objects specifying whether
they “must” or “cannot” be clustered together. Through an iterative algorithm, we perform symmetric tri-factorization of the
data similarity matrix to infer the clusters. Theoretically, we show the correctness and convergence of SS-NMF. Moveover,
we show that SS-NMF provides a general framework for semi-supervised clustering. Existing approaches can be considered as
special cases of it. Through extensive experiments conducted on publicly available datasets, we demonstrate the superior performance
of SS-NMF for clustering.
相似文献
Ming DongEmail: |