Time filter

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Yang X.-H.,Beijing Normal University | Mei Y.,Beijing Normal University | She D.-X.,Beijing Normal University | Li J.-Q.,Beijing Institute of Water
Computers and Mathematics with Applications | Year: 2011

The embedding dimension and the number of nearest neighbors are very important parameters in the prediction of chaotic time series. To reduce the prediction errors and the uncertainties in the determination of the above parameters, a new chaos Bayesian optimal prediction method (CBOPM) is proposed by choosing optimal parameters in the local linear prediction method (LLPM) and improving the prediction accuracy with Bayesian theory. In the new method, the embedding dimension and the number of nearest neighbors are combined as a parameter set. The optimal parameters are selected by mean relative error (MRE) and correlation coefficient (CC) indices according to optimization criteria. Real hydrological time series are taken to examine the new method. The prediction results indicate that CBOPM can choose the optimal parameters adaptively in the prediction process. Compared with several LLPM models, the CBOPM has higher prediction accuracy in predicting hydrological time series. © 2010 Published by Elsevier Ltd. All rights reserved.

Yang X.H.,Beijing Normal University | Zhang X.J.,Beijing Normal University | Hu X.X.,Beijing Normal University | Yang Z.F.,Beijing Normal University | Li J.Q.,Beijing Institute of Water
Nonlinear Processes in Geophysics | Year: 2011

There is much uncertain information which is very difficult to quantify in the water resource renewability assessment (WRRA). The index weights are the key parameters in the assessment model. To assess the water resource renewability rationally, a novel nonlinear optimization set pair analysis model (NOSPAM) is proposed, in which a nonlinear optimization model based on gray-encoded hybrid accelerating genetic algorithm is given to determine the weights by optimizing subjective and objective information, as well as an improved set pair analysis model based on the connection degree is established to deal with certain-uncertain information. In addition, a new calculating formula is established for determining certain-uncertain information quantity in NOSPAM. NOSPAM is used to assess the water resource renewability of the nine administrative divisions in the Yellow River Basin. Results show that NOSPAM can deal with the uncertain information, subjective and objective information. Compared with other nonlinear assessment methods (such as the gray associate analysis method and fuzzy assessment method), the advantage of NOSPAM is that it can not only rationally determine the index weights, but also measure the uncertain information quantity in the WRRA. This NOSPAM model is an extension to nonlinear assessment models. © Author(s) 2011.

Zhao T.,Tsinghua University | Zhao T.,University of Illinois at Urbana - Champaign | Yang D.,Tsinghua University | Cai X.,University of Illinois at Urbana - Champaign | And 3 more authors.
Water Resources Research | Year: 2012

The use of a streamflow forecast for real-time reservoir operation is constrained by forecast uncertainty (FU) and limited forecast horizon (FH). The effects of the two factors are complicating since increasing the FH usually provides more information for decision making in a longer time framework but with increasing uncertainty, which offsets the information gain from a longer FH. This paper illustrates the existence of an effective FH (EFH) with a given forecast, which balances the effects of the FH and FU and provides the maximum information for reservoir operation decision making. With the assumption of a concave objective function, a monotonic relationship between current operation decision and ending storage is derived. Metrics representing the error resulting from a limited forecast relative to a perfect forecast are defined to evaluate reservoir performance. Procedures to analyze the complicating effect of FU and FH and to identify EFH are proposed. Results show that: (1) when FH is short, FH is the dominating factor for determining reservoir operation, and reservoir performance exhibits a quick improvement as FH increases; (2) when FH is long, the inflow information may be too uncertain to guide reservoir operation decisions and FU becomes the dominating factor; and (3) at a medium FH, reservoir performance depends on the complicating effects of FU and FH and EFH locates with a certain balanced level of FU and FH. The statistical characteristics of EFH are illustrated with case studies with deterministic forecast and ensemble forecast. Moreover, the impacts of temporal correlation of FU, inflow variability, evaporation loss, and reservoir capacity on EFH are explored. Copyright 2012 by the American Geophysical Union.

Gao J.,Beijing Institute of Water | Gao J.,Tsinghua University | Gao J.,University of Colorado at Boulder | Williams M.W.,University of Colorado at Boulder | And 3 more authors.
Remote Sensing of Environment | Year: 2012

The spatial and temporal distribution of snow and its response to changes in climate were investigated from 1979 to 2005 in eastern Tibet. The Lhasa River basin, Niyang River basin and Changdu region cover an area of approximately 15×10 4km 2 and ranges in elevation from 2000 to more than 7000m. This large area necessitates innovative procedures for estimating potential spatial and temporal changes in snow cover. For this analysis we used the microwave long-term snow cover dataset of China with a spatial resolution of 25km and temporal resolution of 1day. After data validation between the microwave dataset and MODIS snow product, we defined two parameters for each pixel: (1) median date of the snow-free period (T m); and (2) duration of the snow-free period (δT). After removing transient-snow dominated areas, we find that the duration of the snow-free period was inversely correlated with elevation (R=-0.651, p<0.001). TFPW-MK (Trend-free pre-whitening Mann-Kendall) was then used to examine and highlight the trend of δT with time. At lower-elevation sites, the length of the snow-free season increased. In contrast, at higher-elevations, it decreased. Mann-Kendall tests on monthly air temperature and annual precipitation for the period 1979 to 2005 from seven climate stations in the region operated by the China Meteorological Data Sharing Service System show a significant increase in annual precipitation and an increase in monthly air temperatures for the fall through spring months. Thus, the length of the snow covered season appears to be decreasing at lower elevation because of the increase in air temperatures. However, at higher elevations the increase in precipitation appears to compensate for the increase in air temperature such that the snow-free period has decreased. © 2012 Elsevier Inc.

Huang J.,CAS Nanjing Institute of Geography and Limnology | Gao J.,CAS Nanjing Institute of Geography and Limnology | Liu J.,Beijing Institute of Water | Zhang Y.,China National Environmental Monitoring Center
Ecological Modelling | Year: 2013

Kalman filter has been successfully used in assimilating observations into the existing models, and has been continually adjusted for its wider use. In this study, one of the Kalman filter techniques (ensemble Kalman filter) was used to assimilate measured data into a spatial hydrodynamic-phytoplankton model for predicting dynamics of phytoplankton biomass in Lake Taihu. In order to investigate the effects of the initial conditions (chlorophyll a) and the model parameter on the model fit, we carried out three simulations with different update strategies of parameter and variable using ensemble Kalman filter. Two simulations updated both of model parameter and state variable once and twice a week, respectively. Another simulation updated the state variable once a week, respectively. The simulation results show that the model fit was improved when the state variable (chlorophyll a) was updated by measured data in a shorter term, and was slightly improved with time-varying parameter. In this respect, good estimates of initial chlorophyll a conditions are critical to achieve good predictions of chlorophyll a dynamics in Lake Taihu. This study demonstrates the success of the ensemble Kalman filter technique in improving models' predictive skill, and implementing time-varying parameters for ecological models. © 2013 Elsevier B.V.

Xu S.,Dalian University of Technology | Ma T.,Dalian University of Technology | Liu Y.,Beijing Institute of Water
Aquatic Botany | Year: 2011

This paper introduces a multi-cylinder evapotranspirometer method, which can directly measure evapotranspiration (ET) from emergent plants in different species and states as well as simultaneously measure evaporation (EW) from an open water surface. Values of daily ET from three contrasting reed (Phragmites australis) stands, with different leaf area indexes (LAI), were obtained through in situ measurements of the Baiyangdian wetland using this method during the growing seasons in 2008 and 2009. The results showed that the ET rate of the reed belt was very high, even exceeding 20mmd-1 under extreme weather conditions. Depending on the LAI change, the annual ET from the different reed canopies ranged from 970 to 2035mm, whereas the ET/EW ratios ranged from 2.05 to 3.98. Accuracy analysis results showed that the errors of the measurement from this method were no more than 2mm. The relative errors of the measurement were correspondingly from 0.04% to 0.33%. It is indicated that the accuracy of our measurement is good enough for the requirements of the ET measurement. © 2011 Elsevier B.V.

Hu C.,Beijing Institute of Water
Shuili Xuebao/Journal of Hydraulic Engineering | Year: 2016

Sediment deposition in a reservoir constructed in a sediment-laden river has a direct impact on the reservoir's lifespan and its comprehensive benefits of flood control, power generation, navigation and water supply. In 1970s the reservoir operation mode of 'Storing Clear Water and Discharging Muddy Flow' was put forward by Chinese scientists for coping with serious sedimentation problem in the Sanmenxia Reservoir, and further applied and developed in several key water control projects such as the Xiaolangdi Reservoir on the Yellow River and the Three Gorges Reservoir on the Yangtze River. This operation mode successfully solves the problem of the conflict between sedimentation and benefits of a reservoir on a sediment-laden river by reducing its sediment deposition and maintaining its effective storage capacity for long-term use. It gives full play to the comprehensive benefits of a reservoir and presents an effective new way to cope with reservoir sediment problem for a sediment-laden river. The operation mode of 'Storing Clear Water and Discharging Muddy Flow' will be constantly optimized and improved in practice along with the knowledge deepening on reservoir sediment problems. Base on the actual situation of changes on runoff and sediment loads of rivers, the paper puts forward suggestions on further optimizing operation mode for the Three Gorges Reservoir and Xiaolangdi Reservoir to ensure the projects'safety and comprehensive benefits and to provide scientific support for promoting development on the theory and technology of reservoir sedimentation. © 2016, China Water Power Press. All right reserved.

Guan C.M.,Beijing Institute of Water
Applied Mechanics and Materials | Year: 2014

The project of substituting small hydropower for fuel (SHPF project in short) in China has been implemented for ten years and has achieved significant economic, social and ecological benefits. The construction, completion situations, results and major experience of the SHPF project are summarized. The existing problems are analyzed. Some suggestions are given for the construction and management of similar projects in future. © (2014) Trans Tech Publications, Switzerland.

Liu J.,Beijing Institute of Water | Liu J.,University of Bristol | Han D.,University of Bristol
Hydrology and Earth System Sciences | Year: 2013

With the advancement in modern telemetry and communication technologies, hydrological data can be collected with an increasingly higher sampling rate. An important issue deserving attention from the hydrological community is which suitable time interval of the model input data should be chosen in hydrological forecasting. Such a problem has long been recognised in the control engineering community but is a largely ignored topic in operational applications of hydrological forecasting. In this study, the intrinsic properties of rainfall-runoff data with different time intervals are first investigated from the perspectives of the sampling theorem and the information loss using the discrete wavelet transform tool. It is found that rainfall signals with very high sampling rates may not always improve the accuracy of rainfall-runoff modelling due to the catchment low-pass-filtering effect. To further investigate the impact of a data time interval in real-time forecasting, a real-time forecasting system is constructed by incorporating the probability distributed model (PDM) with a real-time updating scheme, the autoregressive moving-average (ARMA) model. Case studies are then carried out on four UK catchments with different concentration times for real-time flow forecasting using data with different time intervals of 15, 30, 45, 60, 90 and 120 min. A positive relation is found between the forecast lead time and the optimal choice of the data time interval, which is also highly dependent on the catchment concentration time. Finally, based on the conclusions from the case studies, a hypothetical pattern is proposed in three-dimensional coordinates to describe the general impact of the data time interval and to provide implications of the selection of the optimal time interval in real-time hydrological forecasting. Although nowadays most operational hydrological systems still have low data sampling rates (daily or hourly), the future is that higher sampling rates will become more widespread, and there is an urgent need for hydrologists both in academia and in the field to realise the significance of the data time interval issue. It is important that more case studies in different catchments with various hydrological forecasting systems are explored in the future to further verify and improve the proposed hypothetical pattern. © 2013 Author(s).

Xinshan S.,Donghua University | Qin L.,Donghua University | Denghua Y.,Beijing Institute of Water
Procedia Environmental Sciences | Year: 2010

This paper carried on a series of experiments with coupled vertical subsurface flow constructed wetlands(VSSFCWs) and horizontal subsurface flow constructed wetlands(HSSFCWs) for the nitrogen removal of the high concentration nitrogenous domestic sewage. According to the transformation results of inorganic nitrogen in VSSFCWs and HSSFCWs, the paper analyzed the key factors to influence inorganic nitrogen, and discussed the nitrogen removal effects under the conditions of external carbon source addition. The results show that: First point, the VSSFCWs has more powerful nitrification ability, and HSSFCWs has more powerful denitrification ability. Under the condition of excessive high concentration nitrogen in inlet water, not enough carbon source become the restriction of denitrification in HSSFCWs. Second point, in VSSFCWs, when DO is greater than 1.5 mg/L, hydraulic retention time is about 2 days, and ammonia nitrogen concentration in inlet water is less than 80 mg/L, the ammonia nitrogen concentration in inlet water can be transformed sufficiently into nitrate nitrogen. And so on, in HSSFCWs, adding external carbon source can cause a lower DO level system, and that is helpful to denitrificate successfully. When TOC(Total Organic Carbon)/TN(Total Nitrogen) in inlet water of HSSFCWs is greater than 2.5, the carbon source for denitrification is sufficiently, and the excessive TOC/TN is not constantly advantageous to increase nitrogen removal efficiency. © 2010 Published by Elsevier Ltd.

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