Quantitative Ascription of Sediment Discharge Changes of Two Highly Sediment-Loaded Tributaries of the Yellow River in China: The Importance of Selecting Rainfall Index and Timescale

River sediment load has been changing conspicuously worldwide. The famous sediment-laden Yellow River in China has also had a declining sediment load in the past decades. This study made a quantitative ascription of the sediment discharge changes by non-parametric Pettitt test, rainfall and sediment discharge characteristic index calculation, correlation test, double cumulative curve regression and solving differential equation in the Huangfu and the Kuye basins in the main sediment sources of the Yellow River. The results indicated that: (1) The sediment discharge decreased significantly from 1956 to 2016, while the rainfall and rainfall erosivity only had a visible but insignificant decreasing trend. (2) Rainfall erosivity is better than other rainfall characteristic indexes in relation with sediment discharge. (3) The anthropogenic factor was the main driver for the reduction of sediment discharge with a percentage of 70.2–90.5% in different periods. The contribution of rainfall changes in flood season to sediment discharge reduction surpassed that in the whole year. (4) A percentage of 88% and 93% of suspended sediment yield reduction was attributed to streamflow reduction, and 12% and 7% to changes in C-Q relationship in the Huangfu and the Kuye basins, respectively.