Trend and persistence of precipitation under climate change scenarios for Kansabati basin,India

With increasing uncertainties associated with climate change, precipitation characteristics pattern are receiving much attention these days. This paper investigated the impact of climate change on precipitation in the Kansabati basin, India. Trend and persistence of projected precipitation based on annual, wet and dry periods were studied using global climate model (GCM) and scenario uncertainty. A downscaling method based on Bayesian neural network was applied to project precipitation generated from six GCMs using two scenarios (A2 and B2). The precipitation values for any of three time periods (dry, wet and annual) do not show significant increasing or decreasing trends during 2001–2050 time period. There is likely an increasing trend in precipitation for annual and wet periods during 2051–2100 based on A2 scenario and a decreasing trend in dry period precipitation based on B2 scenario. Persistence during dry period precipitation among stations varies drastically based on historical data with the highest persistence towards north‐west part of the basin. Copyright © 2009 John Wiley & Sons, Ltd.     

辽宁省夏季降水的日变化特征

文章利用辽宁省25个气象观测站1961—2008年夏季6—8月的逐时降水自记资料,分析了辽宁省夏季降水日变化的基本特征。发现：降水日变化特征地域性较强,沿海站与内陆站存在差异。总体来说,沿海站降水量的最大值基本出现在午前04-08时,内陆站点则呈双峰值的形式,峰值分别出现在午前和午后,午后1 4—20时为降水量最大值出现的时间;降水频次的日变化特征和降水量基本相同;持续时间长的降水多在午前达到峰值,持续时间短的降水多在午后达到峰值;沿海站点午前的降水峰值区基本是由持续时间在6 h以上的长时间降水造成,内陆站点午后最大降水峰值则为持续时间6 h以内的短时降水,这与内陆下午对流能量强相适应。研究表明,降水日变化的存在能够影响到降水预报的评分。     

青藏高原积雪异常的持续性研究

选取青海省和西藏自治区境内的72个气象站，自建站至1999年12月逐日观观测的积雪深度资料，规定观测到地面有0．5cm以上积雪深度的日子作为积雪日，分别统计了月、冬春、年的积雪日数，通过对积雪日数的分析，确定出高原冬春积雪的异常年份。分析了这些异常年份高原积雪的持续特征，结果表明，高原冬春总体积雪的异常主要是由11月至翌年3月的积雪异常决定的，高原冬春的积雪异常和春末夏初的积雪异常是不相一致的；一般来讲，高原积雪负异常至少持续4个月，大多数般到3月份就结束了；高原冬春总体积雪的正负异常都很难持续到春末夏初，高原冬春异常多积雪的大规模消融期在2月末到3月初。     

月平均位势高度场异常持续性初步分析

使用NCEP/NCAR的500hPa月平均位势高度场球函数展开系统资料，分析了北、南半球1、7月具有较大年际异常方差贡献的5种低阶球函数（全波数n=0-4)分量振幅异常的持续性，以及它们与热带太平洋海表温度年际异常的关系。结果表明：振幅异常的持续性随n增大迅速减小，明显的持续性只存在于少数低n值超长波分量；均匀球函数分量振幅异常的持续性显著，且存在季节、半球际差异；北、南半球均匀球函数分量（n=0)振幅异常呈准同步变化，它们与热带太平洋SSTA在ENSO尺度上准同步；南半球n=2的带型球函数分量振幅异常也具有较强的持续性，且其变化与热带太平洋SSTA在年代际尺度上准同步。     

Long-term rainfall trends and change points in hot and cold arid regions of India

This study examined trends and change points in 100-year annual and seasonal rainfall over hot and cold arid regions of India. Using k-means clustering, 32 stations were classified into two clusters: the coefficient of variation for annual and seasonal rainfall was relatively high for Cluster-II compared to Cluster-I. Short-term and long-term persistence was more dominant in Cluster-II (entirely arid) and Cluster-I (partly arid), respectively. Trend tests revealed prominent increasing trends in annual and wet season rainfall of Cluster-II. Dry season rainfall increased by 1.09 mm year^?1 in the cold arid region. The significant change points in annual and wet season rainfall mostly occurred in the period 1941–1955 (hot and cold), and in the dry season in the period 1973–1975 (hot arid) and in 1949 (cold arid). The findings are useful for managing a surplus or deficiency of rainwater in the Indian arid region.

EDITOR A. Castellarin; ASSOCIATE EDITOR S. Kanae     

The distribution of Kendall's tau for testing the significance of cross-correlation in persistent data

Abstract

Kendall's tau (τ) has been widely used as a distribution-free measure of cross-correlation between two variables. It has been previously shown that persistence in the two involved variables results in the inflation of the variance of τ. In this paper, the full null distribution of Kendall's τ for persistent data with multivariate Gaussian dependence is derived, and an approximation to the full distribution is proposed. The effect of the deviation from the multivariate Gaussian dependence model on the distribution of τ is also investigated. As a demonstration, the temporal consistency and field significance of the cross-correlation between the North Hemisphere (NH) temperature time series in the period 1850–1995 and a set of 784 NH tree-ring width (TRW) proxies in addition to 105 NH tree-ring maximum latewood density (MXD) proxies are studied. When persistence is ignored, the original Mann-Kendall test gives temporally inconsistent results between the early half (1850–1922) and the late half (1923–1995) of the record. These temporal inconsistencies are largely eliminated when persistence is accounted for, indicating the spuriousness of a large portion of the identified cross-correlations. Furthermore, the use of the modified test in combination with a field significance test that is robust to spatial correlation indicates the absence of field significant cross-correlation in both halves of the record. These results have serious implications for the use of tree-ring data as temperature proxies, and emphasize the importance of utilizing the correct distribution of Kendall's τ in order to avoid the overestimation of the significance of cross-correlation between data that exhibit significant persistence.

Citation Hamed, K. H. (2011) The distribution of Kendall's tau for testing the significance of cross-correlation in persistent data. Hydrol. Sci. J. 56(5), 841–853.     

A new measure for assessing the efficiency of hydrological data-driven forecasting models

Abstract

There is a lack of consistency and generality in assessing the performance of hydrological data-driven forecasting models, and this paper presents a new measure for evaluating that performance. Despite the fact that the objectives of hydrological data-driven forecasting models differ from those of the conventional hydrological simulation models, criteria designed to evaluate the latter models have been used until now to assess the performance of the former. Thus, the objectives of this paper are, firstly, to examine the limitations in applying conventional methods for evaluating the data-driven forecasting model performance, and, secondly, to present new performance evaluation methods that can be used to evaluate hydrological data-driven forecasting models with consistency and objectivity. The relative correlation coefficient (RCC) is used to estimate the forecasting efficiency relative to the naïve model (unchanged situation) in data-driven forecasting. A case study with 12 artificial data sets was performed to assess the evaluation measures of Persistence Index (PI), Nash-Sutcliffe coefficient of efficiency (NSC) and RCC. In particular, for six of the data sets with strong persistence and autocorrelation coefficients of 0.966–0.713 at correlation coefficients of 0.977–0.989, the PIs varied markedly from 0.368 to 0.930 and the NSCs were almost constant in the range 0.943–0.972, irrespective of the autocorrelation coefficients and correlation coefficients. However, the RCCs represented an increase of forecasting efficiency from 2.1% to 37.8% according to the persistence. The study results show that RCC is more useful than conventional evaluation methods as the latter do not provide a metric rating of model improvement relative to naïve models in data-driven forecasting.

Editor D. Koutsoyiannis, Associate editor D. Yang

Citation Hwang, S.H., Ham, D.H., and Kim, J.H., 2012. A new measure for assessing the efficiency of hydrological data-driven forecasting models. Hydrological Sciences Journal, 57 (7), 1257–1274.     

气候变暖背景下降水持续性与相态变化的研究综述

持续性降水和固态降水（或近地面气温为0℃左右的降水）都能导致洪涝和低温雨雪冰冻等灾害性的极端事件,对人民群众生命和财产安全以及社会经济发展也会造成严重危害。目前中外围绕降水量、极端降水事件变化等已开展了大量研究,但在降水持续性和相态变化的特征及其影响机理方面的研究仍显不足。因此,围绕降水持续性和相态变化的相关研究,对近20余年来取得的一些重要研究进展进行回顾。研究指出,在气候变暖背景下降水持续性和相态变化的特征在全球范围内表现出了区域上的不一致性。有关降水持续性变化方面,中国南方地区持续性降水过程及其产生的降水量呈现增多趋势,但北方地区呈现减少的趋势,而西南地区长持续性降水呈下降趋势。至于降水相态变化方面,中国南方地区持续性雨雪冰冻事件在气候变暖背景下总体呈减少趋势。这些变化除了与气候变暖有关外,可能还与大气遥相关模态、低频振荡及ENSO事件等引起的大气环流异常有关。今后应该更多开展气候变暖背景下降水持续性和相态变化的特征、可能机理以及其与气候变暖的可能联系方面的研究,以期通过相关研究深入理解中国降水持续性与相态变化的规律、成因及其与旱涝、低温雨雪灾害等的联系,进一步加深对气候变暖背景下中国天气、气候的影响及其机理的认识。      

Comparative evaluation of statistical tests for time series analysis: application to hydrological time series / Evaluation comparative de tests statistiques pour l'analyse de séries temporelles: application à des séries temporelles hydrologiques

Abstract

Statistical analyses of hydrological time series play a vital role in water resources studies. Twenty-nine statistical tests for detecting time series characteristics were evaluated by applying them to analyse 46 years of annual rainfall, 47 years of 1-day maximum rainfall and consecutive 2-, 3-, 4-, 5- and 6-day maximum rainfalls at Kharagpur, West Bengal, India. The performance of all the tests was evaluated. No severe outliers were found, and both the annual and maximum rainfall series were found to be normally distributed. Based on the known physical parameters affecting the homogeneity, the cumulative deviations and the Bayesian tests were found to be superior to the classical von Neumann test. Similarly, the Tukey test proved excellent among all the multiple comparison tests. These tests indicated that all the seven rainfall series are homogeneous. Two parametric t tests and the non-parametric Mann-Whitney test indicated stationarity in all the rainfall series. Of 12 trend detection tests, nine tests indicated no trends in the rainfall series. The Kendall's Rank Correlation test and the Mann-Kendall test were found equally powerful. Moreover, the Fourier series analysis revealed no apparent periodicities in all the seven rainfall series. The annual rainfall series was found persistent with a time lag of nine years. All the rainfall series were subjected to stochastic analysis by fitting 35 autoregressive moving-average (ARMA) models of different orders. The best-fit models for the original annual rainfall and 1-, 2- and 3-day maximum rainfall series were found to be ARMA(0,4), ARMA(0,2), ARMA(0,2) and ARMA(3,0), respectively. The best-fit model for the logarithmically transformed 4-day maximum rainfall was found to be ARMA(0,2). However, for the inversely transformed 4-, 5- and 6-day maximum rainfall series, ARMA(0,1) was obtained as the best-fit model. It is concluded that proper selection of time series tests and use of several tests is indispensable for making useful and reliable decisions.     

环境污染中三维捕食_被捕食系统生存和绝灭的阈值

研究了一类受环境污染的3种群系统,得到了三维捕食_被捕食系统持续生存与绝灭的阈值,并由此对系统的正平衡点的稳定性进行了分析。