Hydrological responses to the combined influence of diverse human activities in the Pearl River delta, China

Abstract

In this paper, temporal variations in the hydrological process of the Pearl River delta (PRD) are examined based on monthly datasets for water discharge, sediment load and suspended sediment concentration (SSC) since the 1950s using the methods of power spectrum analysis and Fourier transform. In addition, the underlying causes of the variations are examined to address the influence of diverse human activities on hydrological process. The results indicate that (1) for the monthly low pass filtering series with a timescale of more than 8a, the water discharge from the Pearl River increased insignificantly with an average rate of 1.66m3/s per year, whereas the sediment load and SSC decreased significantly with an average rate of 18.54kg/s and 0.002kg/m3 per year, respectively, between 1957 and 2009; (2) decadal change in the water discharge of the Pearl River can be identified with low periods in 1950s–1972, 1984–1992 and 2003–2009 and high periods in 1973–1983 and 1993–2002, which is in good agreement with precipitation changes. Decadal change in the sediment load generally differed among three tributaries. For the West River and the North River, before the 1970s, no significant change in the sediment load can be observed because of the balanced influences of dam construction and deforestation. In the 1980s, a significant increase occurred because deforestation exceeded dam construction. Since the 1990s, large reservoirs were constructed in the West River and North River basins. And total deposition rate of all the reservoirs in two river basins was estimated to be 168.8×106m3/a in the 1990s and reached to 881.1×106m3/a in the 2000s, respectively. The influences of dam construction and soil conservation have outweighed the impact of deforestation, which has resulted in a significant decrease in the sediment load. For the East River, since the dam construction in 1960, the influence of deforestation on the sediment load did not outweigh the influence of the dam construction, which gradually decreased the sediment load; (3) for the PRD, temporal change in the water discharge and sediment load in the West River and North River deltas correspond well with the change in the water discharge and sediment load upstream of the West River and the North River before the 1990s. Due to the uneven down-cutting of the riverbed in the upper PRD since the mid-1980s, which is primarily caused by intensified sand excavation from the river channel, the divided flow ratio and divided sediment ratio at Sanshui station in the North River delta increased by 7.8% and 7.7%, respectively, in the 1990s–2000s compared with the 1960s–1980s. Therefore, the water discharge and sediment load in the North River delta increased by 57.0% and 12.2%, respectively. These changes have resulted in morphological changes in the PRD and caused many environmental problems across the PRD, which should be paid special attention.