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Water level, as an intuitive factor of hydrologic conditions, is of great importance for lake management. In this study, periodic structures of water level and its fluctuations in Lake Baiyangdian are analyzed based on wavelet analysis and seasonal-trend decomposition using local error sum of squares (STL). Data of monthly time series are divided into three types with emphasis on anthropogenic influence from water allocation. It is found that intra-annual characteristics of water level fluctuations are the common periodic structures. Water allocation alters the periodic structures by decreasing and weakening the oscillations of water level, compared with the slight effects of natural hydrologic water supplies and short-term climate changes. An irregular water level decline and short-term oscillation with irregular periodicity are deduced from seasonal-trend decomposition analysis using STL. With seasonality depicted monthly, the influence of water allocation implies irregular oscillations with high-frequency components, especially for monthly changes. The water level fluctuations are influenced by seasonal changes, as demonstrated by three types of time series. The impacts of water allocation on seasonality show the differences with continuous single-peak oscillations representing no influences and continuous double-peak oscillations representing frequent influences. Furthermore, the accumulation of water allocation shows a slight rising trend in average monthly level fluctuations over the last several years. The study helps understand periodic structures and long-term trend changes of water level fluctuations, which will facilitate lake management of Lake Baiyangdian. [ABSTRACT FROM AUTHOR]

Abstract: In this study, periodic structures of water level and chlorophyll a (Chl a) concentration in Lake Baiyangdian and their correlations were analyzed using Morlet wavelet transform, wavelet power spectrum and cross wavelet transform analyses. Monthly data of water level (from 1950 to 2009) and Chl a concentration (from 2000 to 2009) were obtained from the Anxin county water conservancy and environmental bureaus, respectively. Results from the Morlet wavelet transform analysis show that water level fluctuations follow intra-annual and 60-month periodicity. In contrast, the wavelet transform analysis shows a water level variation structure of 6 and 16 month periodicity at a high confidence level (>95%). The periodic structure of Chl a concentrations shows 6 and >12 month patterns in continuous wavelet transform analysis whereas the periodic structures generated using wavelet power spectrum analysis exhibit 4 and >12 month variance, with the 4 month periodic change above the 95% confidence level. Comparative cross wavelet and coherence and phase analysis of water levels and Chl a concentrations in the lake results in a 16-month periodic structure that displays a complicated nonlinear relationship between the two parameters. Several short-term periodic nonlinear relationships also appear at other time intervals. In conclusion, we show for the first time that a 16-month periodic structure demonstrates the most robust influence of water level on Chl a concentrations in Lake Baiyangdian. Our results can help in understanding the interactions between specific hydrological changes and their corresponding ecological effects. [Copyright &y& Elsevier]