Spatial and temporal characteristics of water level change and its causes in the Zhujiang Delta in recent decades

Based on the measured tide gauge records at 25 stations in the Zhujiang Delta, the periodic oscillations and secular trends of water level series were studied with Power spectra analysis and Fourier transformation analysis. The Power spectra analysis of monthly mean time series reveals that there were several predominant periods of water level change, varying from 0.3 a to 11.7 a. Results from Linear regression of low-passed time series which were the outcome of Fourier transform process suggest that the water level of river network of Zhujiang Delta was rising in general, and the rate of water level change generally ascended as shifting from the top of the delta to the outlets of the Zhujiang River. The rates of water level change of 25 tide gauge stations differ from -3.8 mm/a to 3.7 mm/a, and the secular trend of seven stations among them was descending, which was largely due to large-scale disordered sand excavation in the upper reaches of the Xijiang and Beijiang River network. Analysis on the change in the rates of water level change of different periods indicates that large-scale sand excavation in the river network which led to bed scour and deepening resulted in the significant fluctuation of water level, and even reversed the secular trend of water level change.