Sediment transport capacity and flow-sediment relationship in different topographical units of different spatial scales in hilly loess region

Abstract: Soil erosion and sediment transport in Losses Plateau region are crucial for sustainable development of the local region and downstream of Yellow river due to the harmful sedimentation in the plain area. This study was to investigate the sediment transport capacity and flow-sediment relationship in different topographical units of different scales in loess region to explore the countermeasures for solving the sediment-related environmental issues. The Peijiamao watershed in losses region was selected as prototype for observation, including the embedded whole slope runoff plot (0.25 hm2) and 3 hydrological stations that were first branch of Qiaogou (subwatershed of Qiaogou watershed), Qiaogou watershed (subwatershed of Peijiamao watershed), and Peijiamao watershed (3930 hm2) with increasing scales, acting as 4 different topographical units. The rainfall, runoff and sediment contents data at these sites were recorded and collected from 1986 to 2008. Results showed that during the observation period the runoff depth and the sediment transport modulus at the annual timescales of the slope scale were more than that of the watershed scale, probably due to the well-known gravitational erosion occurring on the steeper slopes at slope scale in this region. When the spatial scale was up to the watershed scale, at the annual timescale the runoff depth increased with the spatial scale, but the sediment transport modulus decreased with spatial scale. However, at the inter-event timescale the maximum value of the runoff depth and the sediment transport modulus may occur at the different topographical units influenced by the rainfall process, rainfall duration, rainfall spatial distribution, the antecedent soil moisture content and other factors. Totally, 67.74% of the runoff peaks occurred at whole slope runoff plot and 29.03% at entire Peijiamao watershed. However, 38.71% of sediment transport modulus occurred at whole slope runoff plot and 48.39% occurred at entire Peijiamao watershed. With the increasing in spatial scale, the sediment transport capacity decreased dramatically. The sediment concentration carried by unit flow power of the whole slope runoff plot, the first-branch of Qiaogou and the Qiaogou was 186, 77 and 58 times that of entire Peijiamao watershed, respectively. It was further confirmed that slope scale triggered tremendous intensity of soil erosion, and the probability of hyper-concentration flow decreased when spatial scale increased. The gentler slope in the larger watersheds from first branch of Qiaogou to the whole Peijiamao watershed and more interception by the flattened terraces might lower sediment transport capacity in this region. Furthermore, the probability of hyper-concentrated flow was closely related to the ratios of width to depth of the different gully. Result showed that the smaller the ratio of width to depth the bigger the probability of hyper-concentrated flow. In this study, the ratios of width to depth of the watershed gullies were 8.15, 15.86 and 38.09, respectively at the 3 watershed scales from first branch of Qiaogou to the whole Peijiamao watershed and the gully gradient decreased with spatial scales from 4.97% at subwatershed of Qiaogou watershed to 1.51% at Peijiamao wathershed. The runoff-sediment transport modulus relationship of different topographical units was in linear function, suggesting that runoff volume was the controlling factor of sediment transport. And with the increase in spatial scale, the runoff-sediment transport modulus relationship was more significant from 0.71 at whole slope runoff plot to 0.99 (for the decisive factor) at entire Peijiamao watershed scale. It also showed that curve slope decreased with increase in watershed area, with highest slope of 1.10106 at whole slope runoff plot and 1210 at entire Peijiamao watershed indicating that the relationship between runoff and sediment was more complex in the slope scale. The results can provide valuble inforamtion for exploring the spatial scale effects and scale transformation of soil erosion in Losses Plateau region.