Hydrological responses to precipitation variation and diverse human activities in a mountainous tributary of the lower Xijiang,China

Hydrological regimes of river systems have been changing under the impacts of both climate variation and human activities in the global context. The Luodingjiang River, a mountainous tributary of the lower Xijiang in South China, was chosen to investigate the hydrological responses to the precipitation variation and diverse human activities (land use change, water diversion, reservoir construction, and in-channel damming) in this study. Two non-parametric statistical methods, Mann–Kendall and Pettitt test, were employed to detect the long-term changes in the time series of water discharge and sediment load (1959–2002) at the annual, monthly and seasonal scales. Significant increasing changes were detected in the water discharge time series in the dry season. To the contrary, significant decreasing changes were detected in the annual sediment load and sediment load time series in the wet-season. The impacts of precipitation variation and human activities on water discharge and sediment load were discerned and quantified using double mass curve and linear regression methods. By taking the period 1959–1968 as the reference period, the contribution of human activities to the increasing trend of dry-season water discharge in the period 1969–2002 was estimated to be 80%. Because the change of sediment load during the period 1969–2002 was not monotonic with an abrupt change around 1986, the contribution estimations were made for the two periods before and after 1986 respectively. For the period 1969–1985, human activities, particularly deforestation during the period 1981–1985, contributed 43% to the increasing change of sediment load and for the period 1986–2002, the impact of human activities dominated the decreasing change of sediment load with the contribution slightly higher than 100% because of the opposite role of precipitation variation. The operation of reservoirs and hydropower stations is considered to be responsible for the observed increasing trends of water discharge in the dry-season and decreasing trends of sediment load after 1986, and for the latter, reforestation program in the catchment is another contributing factor. The distinct seasonal changing patterns of both water discharge and sediment load in this study highlight the importance of involving monthly or seasonal time series in the change detection in hydrological data.