Quantitative assessment of the impacts of climate change and human activities on runoff change in a typical karst watershed, SW China
Graphical abstract
Introduction
In recent decades, global climate change and human activities have accelerated the global hydrological cycle (Allen and Ingram, 2002, Stocker and Raible, 2005, Xu et al., 2013, Xu et al., 2013a, Xu et al., 2013b, Millan, 2014, Amin et al., 2017) and have changed the spatiotemporal patterns of rainfall and evaporation (Beniston, 2002, Coulibaly, 2006, Chu et al., 2008, Hsu and Li, 2010, Liang et al., 2011, Liu et al., 2014). Thus, the runoffs of many rivers around the world exhibit a significantly decreasing trend (Fu et al., 2004, Xu et al., 2010, Wang et al., 2012a, Li and Zhou, 2016, Zhang et al., 2016) that greatly threatens global water security. In the karst watershed in Southwest China, prominent runoff attenuation has triggered a serious water crisis that threatens water resources and ecological security (Qin et al., 2015), as well as influences the survival and development of the 0.22 billion people in this region. Therefore, investigating the changing characteristics of runoff and quantitatively assessing the effects of climate change and human activities on runoff change in the karst watershed is crucial. Solving the water crisis in the karst watershed is conducive to water resources planning and management and has vast strategic significance in global water security.
Quantifying the impacts of climate change and human activities on runoff change has become a hot topic in climatic and hydrologic researches (Li et al., 2016, Yuan et al., 2016, Zhang et al., 2016). Previous reports have indicated that that human activities have altered the water cycle and have increased runoff loss. Researchers have utilized many methods to evaluate the contribution rates of the main influencing factors to runoff changes and have revealed that runoff changes involve the superposition of the effects of climate change and human activities, with human activities being the dominant factor (Beniston, 2012, Zhan et al., 2013, Gao et al., 2017). The effects of climate are mainly reflected in the inter-annual variability, multi-timescale, and future sustainability of runoff. The hydrological model and quantitative evaluation method are the main methods for the quantitative separation of the impacts of climate change and human activities at the watershed scale (Wang, 2014). In the former method, hydrological models are used to assess the influence of climate and human activities on runoff change by runoff simulation. The latter method is based on the methods of climatic elastic coefficient (Hu et al., 2012), multivariate regressive (Xu, 2011), sensitivity analysis (Zuo et al., 2013), water balance (Wang et al., 2009), and double mass curve of runoff and precipitation (Guo et al., 2014, Zhao et al., 2014, Niu et al., 2016). Many researchers have conducted a wide range of studies on the Yangtze River basin (Zhao et al., 2015), Yellow River basin (Wang et al., 2014, Kong et al., 2016), Wei River basin (Huang et al., 2016a), Hei River basin of China (K.S. Luo et al., 2016), as well as on several typical watersheds in other countries (Buendia et al., 2016, Zare et al., 2016, Farenhorst et al., 2017, Griffioen, 2017, Marcos et al., 2017). Based on their findings, runoff changes involved the superposition of the effects of climate change and human activities, and human activities was the dominant factor. Although these studies have quantitatively evaluated the comprehensive effects of climate change and human activities on runoff change, some shortcomings and deficiencies in research methods and research areas have been observed. For example, although the hydrological model has a good physical foundation, its structure and parameter sensitivity possess several uncertainties (Leavesley, 1994). Unverified simulation results likely overestimate the effect of climate change on runoff change (Legesse et al., 2003). Furthermore, although the quantitative evaluation method requires less data, a longer data series is needed to reach the effect of quantitative assessment, and the noise in the long-time data series interferes with the evaluation results (Sankarasubramanian et al., 2001). Eliminating the influence of inter-annual fluctuations is also difficult. The factors that affect runoff change and their specific contributions to runoff change in a karst watershed are significantly different from those in a non-karst watershed because of special hydro geographical structures and complex humanistic background (Kong et al., 2007, Bai et al., 2010, Patterson et al., 2013, Liu et al., 2016). Thus, the identification of main driving factors of water resource change and their contributions to runoff change in a karst watershed by water resource researchers and managers remains difficult. Quantifying the contributions of the major influencing factors in climate and human activities to runoff change in a karst watershed is now a major problem.
Runoff in a karst watershed has received little attention so far, with most studies focusing only on the research and development of water resources utilization technology (Hartmann et al., 2014, Qin et al., 2015, Luo et al., 2016), runoff simulation (Meng et al., 2015, Tian et al., 2016), and soil and water loss (Wang et al., 2014, Chen and Lian, 2016). Given the lack of theoretical basis, good technical methods, and data support, researchers have seldom considered the trend, periodicity, and future change of the hydrological and climatic factors in a karst watershed and have ignored the impacts of climate change and human activities on runoff change. International research on karst watersheds is not only short of relevant data but also lack experience and contribution. Therefore, the main contribution of the present study is the identification of the main contributing factors to runoff change in a typical karst watershed. Furthermore, the present study establishes a basic theoretical understanding of the influential mechanisms and contribution values of runoff change is established in a karst watershed with the support of hydrological and meteorological data from a long period of 32 years.
In this study, a new separation method of slope changing ratio of cumulative quantity (SCRCQ) (Wang et al., 2012b) was used to conveniently eliminate the noise and separate the influence values of climatic factors and human activities on runoff change. Accumulative amount was introduced to decrease the influence of inter-annual fluctuations in the measured data. Linear fitting between year and accumulation considerably improved the correlation coefficient of the fitting relationship. Cumulative anomaly (Wang et al., 2012a, Wang et al., 2012b) was used to accurately detect the abrupt change features of hydrological and climatic time series, thus avoiding the right avertence defect from double mass curve of runoff and precipitation (Ran et al., 2010) and the multi-point abrupt change from the non-parameter Mann-Kendall detection. The periodical influence and multi-timescale evolution features of hydrological and climatic time series were accurately reflected by continuous wavelet analysis, which was used for signal time-frequency analysis (Labat, 2008, Mustapha et al., 2013, Nourani et al., 2015, Rashid et al., 2015). The current changing trends of hydrological and climatic time series were accurately analyzed by the combined application of the Theil-Sen median trend analysis and Mann-Kendall rank correlation trend test method (Dong et al., 2009, Guo et al., 2011, Nalley et al., 2012, Araghi et al., 2016).
The Yinjiang River watershed, which is located in a typical karst valley area in Southwest China, was selected as the research site of the present study. This study combined Mann-Kendall rank correlation trend test, cumulative anomaly, continuous wavelet analysis, Hurst exponent, and SCRCQ to quantitatively evaluate the impacts of climate change and human activities on runoff change. The study has the following objectives: (1) to statistically detect the trends and the abrupt change points of hydrological and climatic time series of the watershed and to analyze its multi-timescale characteristics and future sustainability features, (2) to effectively separate the influential values of climate change and human activities on the runoff change and to quantitatively calculate the contributions of its main influential factors, and (3) to explore the influential mechanisms of the impacts of climate change and human activities on runoff change. This study provides a methodologies and data references for international counterparts for studying the impacts of climate change and human activities on runoff change in karst watershed. This study also provides new understanding for balancing the security pattern of global water resources.
Section snippets
Study site
The Yinjiang River watershed (108°18′–108°47′ E, 27°53′–28°15′ N), which is located northeast of Guizhou Province (Fig. 1a), is a typical karst valley watershed in the Yinjiang County's karst test site. The watershed, which belongs to the Yangzi River system, is an important branch of the Wujiang River. It covers an area of 830 km2, with an elevation range of 399–2465 m above sea level, and the mean elevation is 1033 m (Fig. 1b). The watershed's topography is mountainous. Elevation in the study
Methodology
Five approaches were used in this study: Mann-Kendall rank correlation trend test, cumulative anomaly, continuous wavelet analysis, Hurst exponent, and SCRCQ. The specific steps of these approaches are shown in Fig. 3. The Mann-Kendall rank correlation trend test was applied to detect changes in trends. Cumulative anomaly was used to detect the abrupt change times of runoff, precipitation, and evaporation. As a useful tool for the analysis of the time-frequency properties of various time
Inter-annual change characteristics
As shown in Fig. 4, the highest values of annual runoff was 7.64 × 108 m3 in 2014, and the lowest was 2.91 × 108 m3 in 2015. The maximum precipitation and evaporation values were 1435 mm in 2014 and 920.20 mm in 1985, respectively. The minimum precipitation and evaporation values were 718.90 mm in 2013 and 568.10 mm in 2014, respectively. Runoff and precipitation have similar inter-annual fluctuation patterns, and that evaporation has inverse characteristics with runoff and precipitation. The change in
Comparison of the SCRCQ results with that in non-karst watershed
SCRCQ was first proposed in 2012 and had been applied in the investigation of Huangfuchuan River basin of the Yellow River basin in China (Wang et al., 2012b). In recent years, SCRCQ has been widely used in the basins of Songhua River (Wang et al., 2015), Liao River (Ma et al., 2015), and Hei River (K.S. Luo et al., 2016). Given regional differences, most of the results of previous studies were slightly different from those of the present study. For example, a study on the whole Yellow River
Conclusions
This study takes the Yinjiang River watershed as the research site, analyzes the change trend, abrupt change characteristic, multi-timescale characteristics between 1984 and 2015 based on Mann–Kendall rank correlation trend test method, cumulative anomaly, and continuous wavelet transform, calculates the contributions of climate change and human activities to runoff change by SCRCQ, and analyzes the long-term memory characteristics and future trends based on Hurst exponent. The main conclusions
Acknowledgement
This research work was supported jointly by National Key Research Program of China (No. 2016YFC0502300, 2016YFC0502102, 2013CB956700 & 2014BAB03B02), United Fund of Karst Science Research Center (No. U1612441), International cooperation research projects of the National Natural Science Fund Committee (No. 41571130074 & 41571130042), Science and Technology Plan of Guizhou Province of China (No. 2012-6015 & 2013-3190 & 201742920512120000), Science and Technology Cooperation projects (No. 2014-3).
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