Sediment delivery ratios of typical watersheds on different spatial scales in Songhua River Basin and its affecting factors
Gao Yan, Zhang Yanling, Jiao Jian, Xie Yun
1. Songliao Water Resources Commission, Ministry of Water Resources, 130021,Changchun, China;
2. China Institute of Water Resources and Hydropower Research, 100048, Beijing, China;
3. School of Geography, Beijing Normal University, 100875, Beijing, China
[Background] There is widespread concern about the severe soil loss in the black soil region of Northeastern China. To date, however, few studies have examined the relationships between soil loss and sediment yields of different watersheds in this area. In this contribution, we calculated sediment delivery ratios of the suspended load (SDR) for selected watersheds of different sizes to obtain an overview of sedimentation and erosion in this region. [Methods] We chose a total of 36 watersheds of different sizes, small, medium, and large, in the Songhua River Basin for our study. Nineteen of the 36 watersheds were in hilly region, where the altitudes ranged from 200 to 500 m; while the other 17 watersheds were in mountainous region, where the altitudes exceeded 500 m. The SDR of each watershed was the ratio of the sediment yields to the amount of soil loss. We used rainfall and land use data, and remote sensing images to calculate amount of soil loss for the different watersheds, with the adoption of the Chinese Soil Loss Equation (CSLE). We calculated sediment yields for the different watersheds from runoff and sediment yields data from 36 corresponding hydrological stations. [Results] The results indicated that the annual average SDR for the small watershed was 0.33. The SDR values for the medium and large-scale watersheds ranged from 0.005 to 0.365, and the average was only 0.051. The drainage area (A) and slope of the main channel (SLP ) were the two main controls on the SDR values in the medium and large-scale watersheds: SDR decreased by a power function as A increased, and the relationship for the tested watersheds in the Songhua River Basin was SDR =1.25A-0.41. In the watersheds of the hilly region, this relationship was more obvious, the formula was SDR = 4.01A -0.52. In the mountainous region, SDR increased as SLP increased and the relationship was expressed as: SDR =0.007SLP +0.004; however, this relationship did not apply to the hilly region. The slopes in the hilly region generally had gradients that were less than 5 and the slope lengths varied between 500 and 2 000 m, meaning that the eroded particles were easily deposited at the foot of the slope. The SDR values in the Songhua River Basin were clearly lower than those of the Yellow (0.5 -1.0) and the Yangtze (0.3 -0.7) River Basins. There were sizeable seasonal differences in the values of the SDR. The SDR for the rainy season in the small watersheds was 0.38 but was only 0.17 in the snowmelt season, probably because the eroded particles transported in the snowmelt season were larger than those transported in the rainy season. [Conclusions] The results of this study will support the establishment of statistical relationships between soil loss and sediment yields at the watershed and basin scale, and will hopefully provide a robust scientific basis for soil conservation in this region.