Qiannan Yang

Research on flood events is one of the most important parts to study runoff and sediment transport in the typical watershed on the Loess Plateau. Based on 101 floods in Period I (PD-I, 1970 to 1990) and Period II (PD-II, 2006 to 2018), and combined with rainfall data, the study indicated the evolution of runoff and sediment transport characteristics during flood events in the Chabagou watershed, and reveal its influencing factors in both periods. Results showed: (1) Sediment yield (SY) increased linearly with runoff amount (RA), and the increasing rate of SY in PD-II was around 20% of PD-I, the relationship between peak flow (PF) and sediment concentration (SC) was the power function, and the SC in PD-II was lower than that in PD-I under the same PF. (2) SY was more sensitive to P (precipitation) of the flood event than rainfall intensity (RI), and the sensitivity of RA and SY to P in PD-II was greater than that in PD-I. The sediment delivery modulus (SDM) with rainfall erosivity (RE) was also linear, and the increasing rate of SDM in PD-II was 27% of PD-I. (3) The better improvement of the underlying surface not only raised the threshold of RA and corresponded with peak SC, but also shortened the duration of high sediment concentration and lowered the sediment transportation capacity by contrasting the flood processes.

Baseflow is a critical component of streamflow in arid areas. Determining variations in baseflow and the factors affecting it have positive roles for water resource utilization in arid watersheds. Two watersheds, the Hailiutu River watershed (HLTR) and the Dali River watershed (DLR), located in two different geomorphological regions of the middle Yellow River, were selected for this study. By using the Eckhardt segment method (fourth digital filtering method, DF4), baseflow was separated from streamflow based on its daily data. Mann-Kendall trend test analysis (M-K trend test) was used to test the trends in baseflow change at different times. Complex Morlet wavelet analysis was used to judge baseflow periodicity. Heuristic segmentation and sequential cluster analysis were used to identify the potential change points in the baseflow series for the two regions together with Pearson correlation coefficient. The results showed: (1) the annual baseflow of the HLTR and the DLR showed significantly decreasing trends (P < 0.01), more significantly for the HLTR (0.33 × 10⁸ m³) than the DLR (0.20 × 10⁸ m³). The annual base flow index (BFI, baseflow/total streamflow) in the wind-sand region (0.75) was larger than for the loess region (0.55), and the BFI in the wind-sand region was more stable in different periods. (2) The annual baseflow of the HLTR and the DLR both exhibited a complete main cycle of 42 years and 38 years, respectively. The change points of the annual baseflow in the HLTR were 1967 and 1986, and 1971 and 1996 in the DLR. (3) There was no significant change in annual precipitation in the two watersheds, while annual reference evapotranspiration (ET₀) in the DLR showed a significant increasing trend (P < 0.01). The normalized difference vegetation index (NDVI) on the DLR (0.40) was higher than on the HLTR (0.26). (4) Baseflow in the wind-sand region, where vegetation improvement was the only ecological activity, decreased faster than in the loess region where there had been numerous ecological measures such as vegetation improvement, check dams and terraces. This implied that comprehensive measures such as these were helpful in slowing the rate at which the baseflow decreases. Therefore, the effect of ecological construction should be considered in future baseflow studies in other geomorphological types within the Middle Yellow River Basin.