Figure 11 shows linear regression between total suspended sediment discharge and total runoff using 17 typical observed rainstorm events. In the flood rising section of flow hydrographs, the runoff-sediment data points often fall below the regression equation line, while in the flood falling section, the data points are above the regression line, indicating that sediment transport processes lag behind flow processes (Fig. 11). Such behaviour is consistent with land cover since, although sparse, the secondary forest and shrub land promotes shading and inhibits development of an herbaceous layer, favouring the sediment detachment process and decreasing surface roughness66. Among the three relationships of rainstorm, runoff and sediment yield, runoff amount has a strong correlation with sediment yield, and they can be statistically regressed as simple linear or approximate exponential associations. The results are consistent with Guo et al. (2017) that the relationship between runoff and sediment yield under the different land disturbances could be described by an exponential function67. Therefore, the corresponding association between sediment transport rate and flow processes can be quantitatively expressed by a mathematical model. Total sediment yield may be quantitatively estimated by the runoff-sediment relationship under different rainstorm events in the Zhifanggou Watershed. However, statistics show that the particularly large sediment concentration is often caused by the collapse of steep loess slopes, due to the flow scouring effects on the gully slope control line. But there is some uncertainty regarding the collapse of steep loess slopes, so the occurrence of large high sediment content also has some randomness. Besides, Poesen et al. (2003) found that collapse of stream banks and gully formation increased specific sediment yield, which occurs more intensively when soil is unprotected upon land use changes68. Both these phenomena will cause the scattered correlation between runoff and sediment concentration.Figure 11Regression analysis curve between total suspended sediment discharge and total runoff using 17 typical observed rainstorm events.