Abstract: [Objective] The water and soil conservation measures should be water-saving and high-efficiency for controlling soil erosion but without threatening the security of water supply in the dry Loess Plateau of Northwestern China. The effects of different water and soil conservation measures on annual runoff and sediment yield should be separated and evaluated, for guiding the integrated management of watersheds.[Method] The typical small watershed of Zhifanggou, located at the Pingliang City of Gansu Province, was selected for this study. The long-term monitoring data of runoff and sediment yield within the period of 1955-2003 were used. The multiple linear relations among the annual runoff ratio, sediment yield module and the land use area ratios were fitted through regression analysis. These relations were used to separate and evaluate the effects of different conservation measures on annual runoff and sediment yield of the small watershed.[Result] The annual precipitation showed a non-significant decreasing trend within the period of 1955-2003, while the annual runoff and sediment yield module showed a significant decreasing trend. The runoff generation capacity of the small watershed was increased after vegetation destruction, but greatly decreased after vegetation restoration. The runoff interception capacity of the small watershed was increased after the implementation of engineering measures, such as the terrace and check dams, but this effect had been declined and even disappeared with the aging of these engineering measures, caused by the silt-up of check dams and the degradation of terrace fields. The ability of unit depth of annual runoff in soil erosion and sediment yield was reduced a lot with increase of forest and greassland, but not obviously decreased with the implementation of engineering measures. The multiple and linear relation between the annual runoff, annual precipitation and the area ratio of each land use category, and the relation among the sediment yield module and the annual runoff and the area ratio of each land use category were fitted. The coefficients in these relations indicated that the 2 land use categories of forestland, terrace and dam fields will decrease the annual runoff, and the decreasing intensity of forestland was 1.48 times of that of terrace and dam fields. The runoff was mainly generated from the 2 land use categories of grassland, slope farmland and others. The runoff ratio from grassland was 1.8 times of that from slope farmland and others. The forestland and grassland showed a function of reducing the sediment yield, in which the reducing intensity of forestland was 2.5 time of that of grassland; The sediment was mainly from the 2 land use categories of terrace and dam fields, slope farmland and others, in which the contribution to sediment yield from terrace and dam fields was doubled high as that from slope farmland and others. If the slope farmland and others is converted to forestland, grassland, terrace and dam field in an intensity of 10% of the total area of watershed, the annual runoff will be changed by -9.6,+4.2, -8.1 mm, and the sediment yield module will be changed by -6 367.0, -25 32.4,+300.5 t·km^-2a^-1, respectively.[Conclusion] The functional period of vegetation measures (afforestation, grassland restoration) for soil erosion control is expected to be longer than that of engineering measures (terrace and check dams). The forestland can well reduce the sediment yield module, but consumes much more water. The grassland has lower sediment yield reduction and water use efficiency compared with forestland, but it can maintain or even increase the water yielding ability of watershed. The terrace and dam fields have also an effect of reducing runoff, which is just slightly lower than that of forestland. However, their sediment interception ability will gradually lower or even disappear with the aging of engineering measures; moreover, their water use efficiency for reducing sediment is the lowest one. In summary, to meet the requirements of water-saving and high-efficiency in soil erosion control in the dryland regions, the dominant measures should be grassland restoration. In this way, we can get a long-lasting service of sediment reduction and erosion control, an unreduced or even enhanced water yield, and higher eco-water use efficiency in erosion control.

Key words: Loess Plateau, soil erosion, land use, water yield, watershed management, Runoff, Sediment yield module