Quantifying time lag of epikarst-spring hydrograph response to rainfall using correlation and spectral analyses

Understanding spring-flow characteristics in karst areas is very important for efficient utilization of water resources. The time lag of a spring-hydrograph response to rainfall is related to karst hydrogeological properties such as thickness, porosity and hydraulic conductivity. The length of the time lag can be determined based on results of the time-series analysis. However, some approaches, with different identifying indicators, give different lengths of the time lag. In this study, the flow-discharge series of two hillslope springs located in a karst area of southwest China were used to compute lengths of the time lag. The thickness and porosity of the epikarst-zone fractures on the two hillslopes were estimated based on a ground-penetrating radar investigation and field measurement. Based on comparison of lengths of the time lag computed by auto- and cross-correlation analyses, the identifying indicators of the time lag were classified into three types for measuring short, intermediate and long-term responses of the spring hydrograph to rainfall. The study also reveals that the time lag of spring-hydrograph response to rainfall in the thick epikarst zone is much longer than that in the thin epikarst zone.