The effects of climate change and human actions on runoff and its seasonal variation

The impacts of climate change and human activities on basin runoff are crucial for effective basin management. This study employs the M-K mutation test to determine the baseline and impact periods of runoff in the Hulu River basin. Based on simulation results from the SWAT model, the contributions of climate change, land use change, and other human activities to runoff changes in the basin are quantified. The findings indicate that: (1) The annual runoff in the Hulu River basin exhibited an initial increase followed by a decrease from 1961 to 2014, displaying an overall declining trend, with a notable change occurring in 1986. (2) The SWAT model effectively reproduces the natural runoff process in the Hulu River basin, meeting the criteria of Nash Sutcliffe efficient coefficient (N_S) > 0.74 and coefficient of determination (R²) > 0.75 for both the calibration and validation periods. (3) The average annual runoff in the Hulu River basin during 1986-2014 decreased by 295 million m³ compared to the base period (1961-1985). Climate change, land use change, and other human activities contributed 67%, 2%, and 31%, respectively. Within impact periods I (1986-1999) and II (2000-2014), climate change accounted for 96% and 44% of the runoff reduction, while other human activities contributed 3% and 52%, and land use changes contributed 1% and 4%, respectively. (4) During 2000-2014, both land use change and other human activities led to a decrease in monthly runoff. Climate change influenced runoff with an increase in January and December and a decrease in the remaining months. Overall, climate change was the primary factor influencing runoff in the Hulu River basin before 2000, whereas human activities took precedence after 2000. These findings provide valuable insights for optimizing land use patterns and promoting the sustainable utilization of water resources.