Driving forces of nature and human activities on water and sediment changes in the middle reaches of the Yellow River in the past 100 years

Purpose The runoff and sediment load of the Yellow River have changed greatly in recent decades and therefore it is important to understand the drivers of such changes. In the assessment of water and soil erosion, multiple questions can only be addressed given a better understanding of the past. Materials and methods In this study, taking the He-Long section of the Middle Yellow River as the research site, the vegetation coverage in the He-Long district since 1888 was reconstructed based on tree rings, and the natural and social driving factors of water and sediment change were analyzed with hydrometeorological data, reconstructed vegetation coverage, and data on population; cultivated land; policy systems; and other factors over the last 100 years. Results and discussion The results show that there was no significant change in the precipitation-sediment loads and runoff relationship during the period from 1919 to 1970, whereas the vegetation coverage fluctuated significantly (from 42 to 49%). The runoff and sediment in the He-Long area decreased significantly from 2000. The annual average vegetation coverage in the He-Long section was 51.6%, reaching the highest level in the past 100 years. But the effect of vegetation on runoff and sediment reduction may have been overestimated, considering that the average vegetation coverage at this stage was only slightly higher than the vegetation coverage (49.8%) from 1946 to 1957. The reduction of sediment may be the result of the combined impact of vegetation, terrace farming, and check dams. Conclusions Human activities play a leading role in the reduction of runoff and sediment in the He-Long section of the Yellow River owing to vegetation coverage, terrace farming, and check dams, but the single human activity factor on runoff and sediment reduction is not significant. Reasonable spatial distribution and optimal allocation of multiple soil and water conservation factors are dominant in the runoff and sediment load reduction.