Based on the data of rainfall intensity and sediment concentration measured in different periods of three typical small watersheds in the loess hilly region, the relationship between rainfall intensity and sediment concentration at different spatial scales was analyzed in this study. The results show that the sediment concentration generally increases with the increase of rainfall intensity on the slope scale. However, on the catchment scale of both slopes and gullies, regardless of the level of the watershed or the area of the valley, there is a significant threshold phenomenon in the effects of rainfall intensity on sediment concentration. When the rainfall intensity exceeds the threshold, the flood sediment concentration will no longer increase with the increase of rainfall intensity. The rainfall intensity threshold increases with the increase of forest and grass coverage. For example, the threshold of rain intensity under the condition of underlying surface in the study area from 1956 to 1969 was 10-15 mm/h, and it reached 20 mm/h from 1990 to 1997, and about 40 mm/h now. Because the rainfall with the intensity of 10-15 mm/h happened almost every year on the basin scale, the vegetation did not change much from the 1950s to the 1980s, and the sediment yield in loess hilly area mainly depended on the soil erosion caused by surface flow while the surface flow speed did not increase indefinitely with the increase of the flow discharge, so the annual maximum sediment concentration of the tributaries in loess area was basically stable before the 1990s.

In order to study the response characteristics of precipitation-runoff under the influence of human activities, this paper took Shima River, a typical urbanized catchment in the Pearl River Delta as the research area. Daily samples of precipitation and river water were collected from January to December and hourly samples were collected during three typhoon rainstorms in 2017. Based on the stable isotope data (δD, δ ¹⁸O), the characteristics of hydrogen and oxygen isotopes were analyzed. Two-component isotope-based hydrograph separation was used to study the contribution of pre-event water and event water to the runoff process during three typhoon events. The results showed that δD and δ ¹⁸O in precipitation ranged from -105.10‰ to 9.98‰ and -14.80‰ to -0.55‰, respectively, and the annual weighted mean values were -57.88‰ and -8.61‰. The Local Meteoric Water Line was δD=7.70δ ¹⁸O+8.61(R ²=0.98). δD and δ ¹⁸O in river water ranged from -91.23‰ to -15.96‰ and -12.66‰ to -4.01‰, respectively. δD-δ ¹⁸O basically fell on the LMWL indicated that precipitation was the main source of runoff in the Shima River catchment. During the three typhoons, the proportion of event water was 59.7%, 55.0% and 69.4%, respectively, which was higher than that of pre-event water. In the early stage of flood, pre-event water and event water increased synchronously. In the late stage of flood, the proportion of event water increased gradually which was more than 80% during the peak period. This indicated that the increase of impervious areas in the urban regions would significantly alter the hydrological cycle. The results of this study could provide the theoretical foundations for hydrological forecast of urbanized basins in Pearl River Delta.

Since 2000, the amount of sediment loads in the Yellow River has decreased sharply. It is of great practical significance to scientifically understand the reasons for the sediment load change in the Yellow River. This study selected the Hekou-Longmen (HL) section, which consists of main sand producing areas of the Yellow River, as the study region, analyzed the sediment discharge change trend, constructed the calculation method of the contribution rate for the sediment reduction of major soil and water conservation measures, such as terraces, check dams, and vegetation, and expounded the reasons of sediment load reduction on the HL section since 2000. The main results were as follows: (1) From 1952 to 2017, there was no significant change of annual precipitation in the HL section, and the annual runoff and annual sediment load both showed an extremely significant (p < 0.001) declining trend. (2) The two mutation years for sediment load reduction in the HL section were in 1979 and 1999, respectively. The average annual sediment loads decreased from 9.30×10 ⁸ t during 1952-1979, to 4.20×10 ⁸ t during 1980-1999, it was, and in 2000-2017, it dropped to 1.03×10 ⁸ t, a decrease of 89%. (3) Influenced by vegetation and terraces, the amount of soil erosion on the slope of the HL section decreased year by year from 2000 to 2015, changing from 1.90×10 ⁸ t to 5.13×10 ⁸ t, and the average annual sediment reduction of check dams from 2000 to 2011 was 1.38×10 ⁸ t. (4) Vegetation restoration was the main reason for the reduction of sediment loads in the HL section, with a contribution rate of 54%. Terraces and check dams, and reservoir and water diversion contributed 34%, and 12%, respectively. (5) Vegetation restoration mainly led to reduced runoff sediments, while check dam construction played a major part in reducing the sediment transport ratio of the basin. According to the changing trend of sediment load, countermeasures are put forward.