Nat Hazards DOI 10.1007/s11069-014-1507-7 ORIGINAL PAPER Spatial and temporal characteristics of droughts in the Northeast China Transect Xiaoxue Wang • Huitao Shen • Wanjun Zhang • Jiansheng Cao • Yongqing Qi • Guopeng Chen • Xuyong Li Received: 15 May 2014 / Accepted: 4 November 2014 Ó Springer Science+Business Media Dordrecht 2014 Abstract In this study, drought trends and change magnitudes of the Northeast China Transect (NECT) were analyzed using the Mann–Kendall test and Theil–Sen’s slope esti- mator. Meteorological data from 20 meteorological stations of NECT region from 1957 to 2012 were used. Results demonstrated that five stations had significant negative trends in precipitation. The magnitudes of the significant negative trends at the 95 % confidence level varied from -2.41 ± 1.05 mm year-1 at Tonghe station to -1.11 ± 0.51 mm year-1 at Qianguoerluosi station. Analysis of the seasonal precipitation series showed a mix of neg- ative and positive trends. Many stations also exhibited strong contrasting seasonal trends that counterbalanced one other at the yearly level. In addition, cluster analysis based on discrete wavelet transform (DWT) was applied to the standard precipitation index (SPI) series. Results revealed three different and spatially well-defined subregions (east, center and west regions of NECT). Due to the decrease in precipitation from the east to the west, land use varies from forest regions in the east, to agriculture in the center, to pastoral areas in the west. Characteristics of drought events for each representative station of different subregions are explored using temporal evolution of the SPI values. Results showed that severe or extreme droughts occurred in 2001, 2003 and 2008 in Tonghe, 1980 and 2007 in Tongliao and 2005–2007 in East Ujimqin Banner. Results indicate that clustering analysis based on DWT X. Wang Á X. Li State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China X. Wang University of Chinese Academy of Sciences, Beijing 100049, China H. Shen (&) Á W. Zhang Á J. Cao Á Y. Qi Key Laboratory of Agricultural Water Resources, Hebei Key Laboratory of Agricultural Water-Saving, Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, No. 286, Huaizhong Road, Shijiazhuang 050021, China e-mail: [email protected] G. Chen Institute of Forest Sciences, Bailongjiang Forestry Management Bureau, Lanzhou 730070, Gansu, China 123 Nat Hazards has great potential for examining spatial coherence of regional drought, which was consistent with not only the precipitation spatial distribution but also the characteristics of land use in the study area. This study not only provides important information on drought variability in the NECT, but also provides useful information for improving water management strategies and planning agricultural practices. Keywords Precipitation Á Trend magnitude Á Spatial–temporal characteristics Á Cluster analysis Á Standardized precipitation index 1 Introduction Drought is one of the most poorly understood natural phenomena and is perceived as one of the most expensive. The occurrence of drought varies in frequency, severity and duration (Kao and Govindaraju 2010; Gocic and Trajkovic 2013). Trends in drought frequency and duration can be explained through changes in precipitation (Gocic and Trajkovic 2013). The spatial–temporal patterns of precipitation influence eco-hydrological processes and, consequently, regional economies that are strongly tied to agriculture and raising livestock (Guo et al. 2006; Liang et al. 2011; Zhang and Zhou 2010). A better understanding of precipitation and drought variability on a regional scale will assist in improving water management strategies, planning agricultural practices and protecting the environment (Liang et al. 2011). Cluster analysis is a statistical technique in which structure in an unlabeled data set is identified and data are objectively organized into homogeneous groups, where the within- group-object dissimilarity is minimized and the between-group-object dissimilarity is maximized (Nourani et al. 2012). The use of clustering techniques in spatial and temporal patterns in climatic and meteorological research increased in recent decades. For instance, Santos et al. (2010) and Dinpashoh et al. (2004) used an algorithm that combined clusters analysis and PCA for dividing the research region into climate subregions. Since time series of precipitation are dynamic, certain non-stationary or chaotic properties may be embedded in the data, requiring appropriate methods to extract the features of interest (Hsu and Li 2010). Discrete wavelet transform (DWT) is well suited to the study of multi-scale and non-stationary precipitation properties over finite spatial and temporal domains (Aldrich 2012; Kumar and Foufoula-Georgiou 1997; Nalley et al. 2012). Partal and Ku¨c¸u¨k (2006) suggested that DWT plays an important role in analyzing precipitations data as its local analysis and multi-resolution decomposition make the analysis process more efficient and accurate. Considering the dynamic characteristics and non-uniform distribution of precipitation data and the need for homogeneous regions in water resource management, cluster analysis based on DWT is employed for exploring the temporal–spatial charac- teristics of drought time series. Terrestrial transect has become an important and effective for the study of global change (Zhu et al. 2006). The Northeast China Transect (NECT) is identified as a mid-latitude semiarid terrestrial transect by the International Geosphere–Biosphere Programme (IGBP) (Ni and Zhang 2000), which is mainly driven by precipitation, and has become an effective platform for the global change research in China (Nie et al. 2012; Zhou et al. 2002; Zhu et al. 2006). Meanwhile, the NECT also represents an important base of forestry, agricultural and pastoral productions in China. Land across the NECT produces wood, hay, grain crops 123 Nat Hazards (maize, soybean, wheat and rice) and cattle (leather, wool and milk) (Ni and Zhang 2000). Consequently, it is worthwhile to study the long-term time series of precipitation regarding the non-homogeneous climatic and hydrological conditions for water supply management and for crop and cattle productivity in this transect. However, a comprehensive analysis of trends and variability in drought series over the NECT is still lacking. In this study, the analysis was based on 56 years (1957–2012) of precipitation data from 20 meteorological stations in the NECT region. One of the main objectives of the present study was to investigate the spatial distribution of temporal trends of the annual and seasonal precipitation time series using the Mann–Kendall and Theil–Sen’s nonparametric methods. This study also aimed to identify the temporal patterns of droughts within each subregion identified by the spatial classification using standard precipitation index (SPI) values. A clustering analysis was used to explore the characteristics of SPI values for multiple time scales (1–12 months) for each cluster. Analyzing the variation of precipi- tation may lead to a better understanding of climate variability in the NECT and could be extended to similar semiarid/arid regions. 2 Data and methods 2.1 Precipitation data Examination of climate changes requires long and high-quality records of climatic vari- ables (Shifteh Some’e et al. 2012). In the present study, daily precipitation measurements from 20 meteorological stations in NECT were obtained from the China Meteorological Data Sharing Service System (http://cdc.cma.gov.cn/home.do). The China Meteorological Data Sharing Service System is the national authority responsible for collection and val- idation of hydrological data. Measurements at all of the meteorological stations were made using the same standards and instrumentation, ensuring homogeneity of data quality (Liu 2005). Spatial distribution of the 20 stations is shown in Fig. 1, and their characteristics and data availability are presented in Table 1. 2.2 Precipitation trend detection and change magnitude 2.2.1 Mann–Kendall test The nonparametric Mann–Kendall (MK) test is routinely used to assess the increasing or decreasing trends in climate variables (Hanif et al. 2013). This test has the advantage of no assuming any form for the distribution function of the data, while having predictive power nearly as high as comparable parametric tests (Hanif et al. 2013). In this study, MK test was applied to all considered series to examine the annual and seasonal trends of pre- cipitation in NECT of China. The MK test is given as: XnÀ1 Xn S ¼ signðxj À xkÞð1Þ k¼1 j¼kþ1 8 < þ1ifðxj À xkÞ [ 0 signðxj À xkÞ¼: 0ifðxj À xkÞ¼0 ð2Þ 1ifðxj À xkÞ\0 123 Nat Hazards Fig. 1 Land use/cover of the study area and spatial distribution of meteorological stations P ½nðn À 1Þð2n þ 5Þ À m t ðt À 1Þð2t þ 5Þ VarðSÞ¼ i¼1 i i i ð3Þ 18 where n is the number of data points, ti is the number of ties for the i value and m is the number of tied values. Equations (1) and (3) were used to compute the test statistic Z from the following equation: 8 > S À 1 > pffiffiffiffiffiffiffiffiffiffiffiffiffiffi if S [ 0 < VarðSÞ Z ¼ > 0ifS ¼ 0 > S À 1 :> pffiffiffiffiffiffiffiffiffiffiffiffiffiffi if S\0 VarðSÞ A positive/negative value of Z indicates an upward/downward trend (Gocic and Trajkovic 2013). The null hypothesis H0 that there is no trend in the records is either accepted or rejected depending if the computed Z statistics