Climate change coupled with intensified human activity could significantly affect the hydrological processes and have posed a serious threat to the sustainable management of the river system. This research aimed to investigate the impact of climate change and human activities on water discharge and, particularly, suspended sediment flux with a case study in the Longchuanjiang catchment, the Upper Yangtze River, China. Non-updating artificial neural network (ANN) was used as the modelling tool to assess the influence from human activities and to project the response of water discharge and sediment flux under hypothetical climate scenarios.
The study area had experienced a sharp increase in suspended sediment flux in the post-1990 period. The research indicated that compared with the background condition (1960-1990), the intensification of human activity had lead to an increase of 2.76 million t yr-1 in years from 1991 to 2001. Of the total change in sediment flux this period, the contribution of the intensified human activity exceeded that of the increased rainfall, with the former accounting for 66~75% and the latter for 25~34%. Among the various human activities, road construction was the dominant variable for the increase of the sediment. During the period from 1991 to 2001, road construction was estimated to have result in an increase of 30.01 million t in the total sediment flux. But meanwhile, conversion of barren land to range land in areas along the channel resulted in a reduction of 9.88 million t in sediment. Reservoir was another factor that contributed to reduce the sediment in the river. The trapping efficiency of the reservoirs in was estimated to be approximately 90%. The change of forest in the catchment was failed to be related to sediment in the river due to various reason like the immaturity of the trees, the lack of the undergrowth and the location of the reforested area.

Potential climate change will affect water and sediment in a river. The sensitivities of water discharge and sediment flux to 25 hypothetical climate changes were predicted by ANNs. Under the possible future climate change in the catchment till 2050, the change of sediment flux was estimated to be between -0.7%~13.7%. In addition, sediment under intensified human activity was found to be more sensitive to the climate change.

ANN provides a competitive alternative to the physical and conventional empirical models in hydrological, modelling, especially in sediment modelling. The current study indicated that ANN is capable of modelling the monthly water discharge and sediment flux with fairly good accuracy when proper input variables representing drivers and their lag effect are included. One significant feature of the ANN in the current study is that it relates sediment directly to the drivers that have physical influence on it. Such ANN can be used to investigate the physical relationship between the drivers and the water/sediment and it permits the assessment of hydrological responses to climate change and human activity.

The current research demonstrated a method for use in studying the impact of climate change and human activity on water discharge and sediment flux. The conclusion drawn may provide information for understanding the complicated hydrological system and its response to the changing climate and human activity. Further research on the influences from variables such as gully erosion, sediment re-transportation, location of road and reservoir retention may help to elucidates hydrological change in the catchment.