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Trend identification is a substantial issue in hydrologic series analysis, but it is also a difficult task in practice due to the confusing concept of trend and disadvantages of methods. In this article, an improved definition of trend was given as follows: ‘a trend is the deterministic component in the analysed data and corresponds to the biggest temporal scale on the condition of giving the concerned temporal scale’. It emphasizes the intrinsic and deterministic properties of trend, can clearly distinguish trend from periodicities and points out the prerequisite of the concerned temporal scale only by giving which the trend has its specific meaning. Correspondingly, the discrete wavelet‐based method for trend identification was improved. Differing from those methods used presently, the improved method is to identify trend by comparing the energy difference between hydrologic data and noise, and it can simultaneously separate periodicities and noise. Furthermore, the improved method can quantitatively estimate the statistical significance of the identified trend by using proper confidence interval. Analyses of both synthetic and observed series indicated the identical power of the improved method as the Mann–Kendall test in assessing the statistical significance of the trend in hydrologic data, and by using the former, the identified trend can adaptively reflect the nonlinear and nonstationary variability of hydrologic data. Besides, the results also showed the influences of three key factors (wavelet choice, decomposition level choice and noise content) on discrete wavelet‐based trend identification; hence, they should be carefully considered in practice. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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为研究我国地震烈度的评定是否存在随时间变化的趋势,本文从统计的角度,选取合适的烈度衰减关系模型作标准,基于残差分析的基本原理,对1900年到2013年间392个地震的烈度评定值随年份变化的趋势进行了统计分析,并提出一种改进的衡量拟合优劣的算法,量化数据的整体偏差并直观地展现数据的离散程度.统计检验结果表明,烈度评定值无论按区域划分、按震级划分或是按烈度划分均与整体存在一致且显著的时间分布趋势:1960年之前烈度评定值较为离散,1960年到1980年左右烈度评定值逐年减小,1980年开始烈度评定值逐渐增大,2000年以后绝大多数烈度评定值高于平均水平,且该时段烈度评定值偏高程度尤为明显,平均总体偏高约0.4度,且有继续增高的趋势. 相似文献
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Temporal change of spatial heterogeneity and its effect on regional trend of annual precipitation heterogeneity indices 
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下载免费PDF全文 A statistical test on climate and hydrological series from different spatial resolution could obtain different regional trend due to spatial heterogeneity and its temporal variability. In this study, annual series of the precipitation heterogeneity indices of concentration index (CI) and the number of wet days (NW) along with annual total amount of precipitation were calculated based on at‐site daily precipitation series during 1962–2011 in the headwater basin of the Huaihe River, China. The regional trends of the indices were first detected based on at‐site series by using the aligned and intrablock methods, and field significance tests that consider spatial heterogeneity over sites. The detected trends were then compared with the trends of the regional index series derived from daily areal average precipitation (DAAP), which averages at‐site differences and thus neglects spatial heterogeneity. It was found that the at‐site‐based regional test shows increasing trends of CI and NW in the basin, which follows the test on individual sites that most of sites were characterized by increasing CI and NW. However, the DAAP‐derived regional series of CI and NW were tested to show a decreasing trend. The disparity of the regional trend test on at‐site‐based regional series and the DAAP‐derived regional series arises from a temporal change of the spatial heterogeneity, which was quantified by the generalized additive models for location, scale, and shape. This study highlights that compared with averaging indices, averaging at‐site daily precipitation could lead to an error in the regional trend inference on annual precipitation heterogeneity indices. More attention should be paid to temporal variability in spatial heterogeneity when data at large scales are used for regional trend detection on hydro‐meteorological events associated with intra‐annual heterogeneity. 相似文献
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Abstract This study presents the first comprehensive nationwide trend detection of streamflow in Nepal, a country that has been historically understudied despite its critical location as the southern pathway for most of the Himalayan snowpack melt and torrential seasonal monsoon rains. We applied Mann-Kendall and Sen's trend tests using trend-free pre-whitening and bootstrap approaches to two streamflow data sets to deal with serial and cross-correlation. The two data sets comprised 23–33 hydrometric stations with 31 years and more than 20 years of published data, respectively. The test on the 33 stations data set showed that 23% of the streamflow variables studied had statistically significant trends, evenly divided between upward and downward trends. Similarly, in the second, relatively smaller data set, 24% of variables exhibited trends, of which 41% were downward and 59% upward. The higher percentage of observed upward trends in pre-monsoon and winter seasonal average flow is noteworthy given the potential snowmelt contribution in many of the studied sites. Trends were mostly absent in stations draining the larger basins. However, some spatial patterns were seen in the observed trend directions, specifically, a downward trend in the Karnali-Mahakali River basin and an upward trend in the West Rapti River basin, as well as a nationwide absence of trend in the post-monsoon season. Editor Z.W. Kundzewicz Citation Gautam, M.R. and Acharya, K., 2011. Streamflow trends in Nepal. Hydrological Sciences Journal, 57 (2), 344–357. 相似文献
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Robin T. Clarke 《水文科学杂志》2013,58(2):139-144
Abstract This paper discusses four areas where, in the author's view, statistical methods are misused in reporting results of hydro-climatological research. They are: (i) the use of the same data set both to suggest a hypothesis (commonly of trend over time) and to test it; (ii) failure to use an appropriate significance level for tests in which a number of hypotheses are tested, even when the data sets used are mutually uncorrelated; (iii) failure to account for spatial correlations between variables, whether these are explanatory or response variables; and (iv) exaggerated importance given to statistical tests of significance, in particular to the 5% and 1% significance levels. Citation Clarke, R. T. (2010) On the (mis)use of statistical methods in hydro-climatological research. Hydrol. Sci. J. 55(2), 139–144. 相似文献
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Spatiotemporal characteristics of precipitation and extreme events on the Loess Plateau of China between 1957 and 2009 总被引：1，自引：0，他引：1 下载免费PDF全文
下载免费PDF全文 Spatiotemporal trends in precipitation may influence vegetation restoration, and extreme precipitation events profoundly affect soil erosion processes on the Loess Plateau. Daily data collected at 89 meteorological stations in the area between 1957 and 2009 were used to analyze the spatiotemporal trends of precipitation on the Loess Plateau and the return periods of different types of precipitation events classified in the study. Nonparametric methods were employed for temporal analysis, and the Kriging interpolation method was employed for spatial analysis. The results indicate a small decrease in precipitation over the Loess Plateau in last 53 years (although a Mann–Kendall test did not show this decrease to be significant), a southward shift in precipitation isohyets, a slightly delayed rainy season, and prolonged return periods, especially for rainstorm and heavy rainstorm events. Regional responses to global climate change have varied greatly. A slightly increasing trend in precipitation in annual and sub‐annual series, with no obvious shift of isohyets, and an evident decreasing trend in extreme precipitation events were detected in the northwest. In the southeast, correspondingly, a more seriously decreasing trend occurred, with clear shifts of isohyets and a slightly decreasing trend in extreme precipitation events. The result suggests that a negative trend in annual precipitation may have led to decreased soil erosion but an increase in sediment yield during several extreme events. These changes in the precipitation over the Loess Plateau should be noted, and countermeasures should be taken to reduce their adverse impacts on the sustainable development of the region. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
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Long-term trend analysis using discrete wavelet components of annual precipitations measurements in Marmara region (Turkey) 总被引：4，自引：0，他引：4
The purpose of this study is to determine the possible trends in annual total precipitation series by using the non-parametric methods such as the wavelet analysis and Mann-Kendall test. The wavelet trend (W-T) analysis is for the first time presented in this study. Using discrete wavelet components of measurement series, we aimed to find which periodicities are mainly responsible for trend of the measurement series. We found that some periodic events clearly affect the trend of precipitation series. 16-yearly periodic component is the effective component on Bal?kesir annual precipitation data and is responsible for producing a real trend founded on the data. Also, global wavelet spectra and continuous wavelet transform were used for analysis to precipitation time series in order to clarify time-scale characteristics of the measured series. The effects of regional differences on W-T analysis are checked by using records of measurement stations located in different climatic areas. The data set spans from 1929 to 1993 and includes precipitation records from meteorological stations of Turkey. The trend analysis on DW components of the precipitation time series (W-T model) clearly explains the trend structure of data. 相似文献
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抚仙湖水质变化(1980-2011年)趋势与驱动力分析 总被引：17，自引：10，他引：7
水质恶化是湖泊系统生态健康的主要制约因素之一,科学地判别水质变化趋势是正确认识和解决水环境问题的首要步骤.基于抚仙湖1980-2011共32年湖体水质监测数据,采用Mann-Kendall和Daniel趋势检验法分析了高锰酸盐指数(CODMn)、总氮(TN)、总磷(TP)、透明度(SD)、叶绿素a(Chl.a)和浮游植物丰度等6项指标的年际变化趋势;从人类活动和自然环境变化2个角度,筛选出9项水质变化的驱动指标,建立了水质指标与驱动指标之间的灰色关联模型.研究结果表明:①统计的6项指标年际波动较大,其中TP、TN和浮游植物丰度的变异系数最大,达到0.6以上;②在α=0.05显著性水平上,CODMn、TN、SD、Chl.a和浮游植物丰度等5项指标均有恶化趋势,Mann-Kendall和Daniel趋势检验2种方法的检验结果一致;③驱动因子与水质指标之间有较强的相关水平,其中,人口数量和水温是主要驱动因子,与主要水质指标的灰色关联系数达到0.7以上的强相关水平. 相似文献
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T.E. McHugh L.M. Beckley C.Y. Liu C.J. Newell 《Ground Water Monitoring & Remediation》2011,31(2):92-101
“Random” variability in groundwater monitoring data sets reduces the ability to identify long‐term concentration trends. This, in turn, increases the time and cost required to evaluate the effectiveness of natural attenuation and other groundwater remedies. To better understand the factors influencing variability in groundwater monitoring results, we have analyzed three large groundwater monitoring data sets. For the three data sets, the long‐term trend in contaminant concentration in each well accounted for an average of 30% to 40% of the overall variation in contaminant concentration. Understanding the causes of the remaining variability would support the development of improved groundwater monitoring methods. All three data sets show large differences in the temporal monitoring records between individual wells (e.g., coefficient of variation for monitoring results from individual wells ranges from 0.08 to 4.6) indicating that well and aquifer factors are more important contributors to variability than sample collection and analysis factors. However, the depth to groundwater (R2 = 0.020) and distance between water level and screened interval (R2 = 0.049) accounted for only a portion of the differences in variability between wells and other aquifer characteristics evaluated and were not correlated with the observed variability in monitoring results. Unidentified factors were apparently much more important contributors to variability than these factors. The monitoring data sets exhibited two distinct timescales for variability: Time‐independent variability that was apparent even when wells were re‐sampled within a few days and a long‐term variability likely associated with the long‐term concentration trend. The observation of time‐independent variability suggests that frequent monitoring of contaminated monitoring wells serves primarily to characterize sources of variability unrelated to the long‐term trend of primary interest. 相似文献
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Gerd Bürger 《水文研究》2017,31(22):4039-4042
A main obstacle to trend detection in time series occurs when they are autocorrelated. By reducing the effective sample size of a series, autocorrelation leads to decreased trend significance. Numerous recipes attempt to mitigate the effect of autocorrelation, either by adjusting for the reduced effective sample size or by removing the autocorrelated components of a series. This short note deals with the latter, also called prewhitening (PW). It is known that removal of autocorrelation also removes part of the trend, which may affect the signal‐to‐noise ratio. Two popular methods have dealt with this problem, the trend‐free prewhitening (TFPW) and the iterative prewhitening. Although it is generally accepted that both methods reduce the adverse effects of PW on the trend magnitude, corresponding effects on statistical significance have not been clearly stated for TFPW. Using a Monte Carlo approach, it is demonstrated that both methods entail quite different Type‐I error rates. The iterative prewhitening produces rates that are generally close to the nominal significance level. The TFPW, however, shows very high Type‐I error rates with increasing autocorrelation. The corresponding rate of false trend detections is unacceptable for applications, so that published trends based on TFPW need to be reassessed. 相似文献
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A sequence of occurrence times of floods may be considered to be part of a realization of a binary-valued time series or of a stochastic point process. In this paper a criterion for detecting the presence of a monotonic trend in the rate of the process is considered. The criterion is based on linear functions of the data with the coefficients chosen to emphasize a monotonic rate. In the case that the process is stationary and mixing, the null distribution of the test statistic is approximately standard normal. 相似文献
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Estimating Land Surface Evaporation: A Review of Methods Using Remotely Sensed Surface Temperature Data 总被引：10，自引：0，他引：10
This paper reviews methods for estimating evaporation from landscapes, regions and larger geographic extents, with remotely sensed surface temperatures, and highlights uncertainties and limitations associated with those estimation methods. Particular attention is given to the validation of such approaches against ground based flux measurements. An assessment of some 30 published validations shows an average root mean squared error value of about 50 W m?2 and relative errors of 15–30%. The comparison also shows that more complex physical and analytical methods are not necessarily more accurate than empirical and statistical approaches. While some of the methods were developed for specific land covers (e.g. irrigation areas only) we also review methods developed for other disciplines, such as hydrology and meteorology, where continuous estimates in space and in time are needed, thereby focusing on physical and analytical methods as empirical methods are usually limited by in situ training data. This review also provides a discussion of temporal and spatial scaling issues associated with the use of thermal remote sensing for estimating evaporation. Improved temporal scaling procedures are required to extrapolate instantaneous estimates to daily and longer time periods and gap-filling procedures are needed when temporal scaling is affected by intermittent satellite coverage. It is also noted that analysis of multi-resolution data from different satellite/sensor systems (i.e. data fusion) will assist in the development of spatial scaling and aggregation approaches, and that several biological processes need to be better characterized in many current land surface models. 相似文献
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采用形态法和小波分析法,分析2020年3月20日蒙古MS 5.9地震震中距400 km范围内巴里坤、富蕴和芨芨台地震台倾斜观测资料的震前异常特征。结果表明：3套资料在震前均具有明显的中短期异常,分别表现为反向趋势变化、破年变形态和倾斜速率减缓的特征;使用db4小波对3套资料进行分析,其6—9阶在震前出现周期为几天至几十天的低频短期异常,同时巴里坤水平摆倾斜仪EW分量8—9阶趋势异常较显著。 相似文献
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在医用X-CT中,散射现象对重建图像的质量有重要的影响。多源锥束CT是医用CT的一种发展趋势,可成倍提高时间分辨率。但与单源CT相比,多源CT额外的光源会导致散射现象的加重,因此在实际应用中需要着重考虑这一问题。本文在开源工具EGSnrc的基础上,开发了一套基于蒙特卡罗法的CT仿真软件SimProj,用该软件获得了贴近... 相似文献
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Daily temperature and precipitation data from 136 stations of Southwest China (SWC) during the last five decades, from 1960 to 2007, were analysed to determine the spatial and temporal trends by using the Mann–Kendall trend test. Results show that SWC has become warmer over the last five decades, especially in the recent 20–25 years. The increasing trends in winter months are more significant than those in the months of other seasons, and spatially Tibet, Hengduan mountains area and west Sichuan Plateau have larger temperature trend in magnitude than the other regions have. A downward trend was detected in Sichuan Basin also, but the region with cooler temperature was shrinking due to the statistically significant increasing trend of temperature after 1990s. Both annual and seasonal means of daily maximum and minimum temperatures show an increasing trend, but trend magnitude of minimum temperature was larger than that of maximum temperature, resulting in the decrease of diurnal temperature range for SWC in the last 50 years. Annual precipitation showed slightly and statistically insignificant increasing trend, but statistically significant increasing trend has been detected in winter season while autumn witnessed a statistically significant decreasing trend. The results could be a reference for the planning and management of water resources under climate change. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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Qiang Zhang Chong-Yu Xu Zengxin Zhang Yongqin David Chen Chun-ling Liu Hui Lin 《Journal of Hydrology》2008,353(3-4):215-227
This study investigated spatial and temporal patterns of trends of the precipitation maxima (defined as the annual/seasonal maximum precipitation) in the Yangtze River basin for 1960–2005 using Mann–Kendall trend test, and explored association of changing patterns of the precipitation maxima with large-scale circulation using NCEP/NCAR reanalysis data. The research results indicate changes of precipitation maxima from relative stable patterns to the significant increasing/decreasing trend in the middle 1970s. With respect to annual variability, the rainy days are decreasing and precipitation intensity is increasing, and significant increasing trend of precipitation intensity was detected in the middle and lower Yangtze River basin. Number of rain days with daily precipitation exceeding 95th and 99th percentiles and related precipitation intensities are in increasing tendency in summer. Large-scale atmospheric circulation analysis indicates decreasing strength of East Asian summer monsoon during 1975–2005 as compared to that during 1961–1974 and increasing geopotential height in the north China, South China Sea and west Pacific regions, all of which combine to negatively impact the northward propagation of the vapor flux. This circulation pattern will be beneficial for the longer stay of the Meiyu front in the Yangtze River basin, leading to more precipitation in the middle and lower Yangtze River basin in summer months. The significant increasing summer precipitation intensity and changing frequency in the rain/no-rain days in the middle and lower Yangtze River basin have potential to result in higher occurrence probability of flood and drought hazards in the region. 相似文献
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Stefan Becker Marco Gemmer Tong Jiang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2006,20(6):435-444
Precipitation trends in the Yangtze River catchment (PR China) have been analyzed for the past 50 years by applying the Mann-Kendall trend test and geospatial analyses. Monthly precipitation trends of 36 stations have been calculated. Significant positive trends at many stations can be observed for the summer months, which naturally show precipitation maxima. They were preceded and/or followed by negative trends. This observation points towards a concentration of summer precipitation within a shorter period of time. The analysis of a second data set on a gridded basis with 0.5° resolution reveals trends with distinct spatial patterns. The combination of classic trend tests and spatially interpolated precipitation data sets allows the spatiotemporal visualization of detected trends. Months with positive trends emphasize the aggravation of severe situation in a region, which is particularly prone to flood disasters during summer. Reasons for the observed trends were found in variations in the meridional wind pattern at the 850 hPa level, which account for an increased transport of warm moist air to the Yangtze River catchment during the summer months. 相似文献