Investigation of karstic hydrological processes of Niangziguan Springs (North China) using wavelet analysis Yonghong Hao, 1 * Guoliang Liu, 2 Huamin Li, 3 Zhongtang Li, 4 Jiaojuan Zhao 3 and Tian-Chyi J. Yeh 5 1 Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, Tianjin, China 2 College of Environment and Resources, Shanxi University, Taiyuan, Shanxi Province, China 3 College of Urban and Environmental Science, Tianjin Normal University, Tianjin, China 4 Department of Environmental Engineering, College of Science and Engineering, Jinan University, Guangzhou, China 5 Department of Hydrology and Water Resources, The University of Arizona, Tucson, AZ, USA Abstract: Niangziguan Spring complex is the largest karst spring in North China. We investigate the karst hydrological processes by using Morlet wavelet transform analysis and cross wavelet analysis based on monthly precipitation from 1958 to 2010 and spring discharge from 1958 to 2009. From Morlet wavelet transform coefficients of precipitation and the spring discharge in Niangziguan Springs Basin, we find that the precipitation and discharge are characterized by the multi-scale features in the time domain, and the energy distribution of the signal is highly irregular across scales. Although precipitation eventually becomes spring discharge by infiltrating and propagating through karst formations, the signals are attenuated. The results also show that the precipitation of Niangziguan Springs Basin has the main periodic components of 1-, 5-, 12-, and 17-year periods with alternating wet–drought cycle. Similarly, the spring discharge of Niangziguan Springs has the main components of 17-year periods, but the 1-, 5-, and 12-year periodicity of precipitation are not reflected in spring discharge, which is filtered by the aquifers. The results of cross wavelet analysis reveal that the precipitation and spring discharge share the common periodicity of 17 years. This means that those signals with high energy and long timescales can penetrate through the aquifer and be reflected in spring discharge, whereas other signals are filtered and modified. Copyright © 2011 John Wiley & Sons, Ltd. KEY WORDS karst aquifer; groundwater; wavelet analysis; Niangziguan Springs Received 29 August 2010; Accepted 10 August 2011 INTRODUCTION Karst aquifers are highly heterogeneous. They are dominated by secondary or tertiary porosity (i.e. fractures or conduits, respectively) and exhibit hierarchical perme- ability structures or flow paths (Atkinson, 1977). In karst hydrological systems, precipitation and runoff reach the groundwater via infiltration through heterogeneous karst aquifers and, subsequently, propagate and emerge as springs. Accordingly, variations in precipitation and heterogeneity of karst formation strongly affect spring flow and cause fluctuations in discharge volume. Labat et al. (2000a) described rainfall–runoff relations using a linear stochastic model and Fourier analysis applied to three karstic systems (i.e. the Aliou, Baget, and Fontestorbes springs) located in the Pyrenees Mountains, France. They found that linear input–output models were not very successful at characterizing hydraulic behaviour of karst systems, and their results showed that karst groundwater was likely a non-linear and non-stationary system. Then, they applied wavelet transform to the karstic systems (Labat et al., 2000b). The results demonstrated that the wavelet analysis can detect the runoff response to both natural recharge processes and human stimuli to the groundwater system and can possibly give an accurate explanation of the temporal structure of rainfall and runoff records in different timescales. Many scientists have applied of wavelet analysis to karst hydrology (Andreo et al., 2005; Massei et al., 2006; Herman et al., 2009; Salerno and Tartari, 2009). Labat et al. (2005a,b) reviewed and summarized the works of wavelet application in the field of earth science. They applied these methods to the monthly discharge of four large rivers (i.e. Amazon, Parana, Orinoco, and Congo) and two long-term climato- logical indices. The results indicate that wavelet analysis can help to identify the relation between discharge and climatological indices. Labat et al. (2005a, 2005b) also emphasized that there was great potential in applying the wavelet analysis technique to hydrological systems. China has some of the largest karst terrain in the world. One quarter of the world’s carbonate rock occurs in China (Hua, 1981; Sweeting, 1995). There is about 470 000 km 2 of karst terrains in North China. Because North China is a semiarid area where annual precipitation averages less than 800 mm, the karstification is less developed compared with the more humid areas in South China. Yet in northern China, there are large karst basins, which exceed 1000 km 2 . The aquifers in these basins are mainly recharged by infiltration of precipitation and, subsequently, provide an *Correspondence to: Yonghong Hao, Tianjin Key Laboratory of Water Resources and Environment, Tianjin Normal University, Tianjin 300387, China. E-mail: haoyh@sxu.edu.cn; haoyhong@yahoo.com Contract/grant sponsor: The National Natural Science Foundation of China; contract/grant numbers: 40972165, and 40572150, Tianjin Science and Technology Developing Strategy Foundation; contract/grant numbers: 09JCYBJC27500, and Opening Fund of Tianjin Key Laboratory of Water Resources and Environment; contract/grant numbers: 52XS1015. HYDROLOGICAL PROCESSES Hydrol. Process. 26, 3062–3069 (2012) Published online 9 January 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.8265 Copyright © 2011 John Wiley & Sons, Ltd.