Suspended sediment at the watershed scale has played a critical role in sediment pollution, water-quality degradation, and the impairment of riparian ecosystems, and thus has been widely studied in many disciplines. This paper synthesizes a variety of methods adopted in suspended sediment monitoring, estimation and modelling for understanding sediment transport processes and determining the suspended sediment load. Methods for sediment monitoring are described in terms of direct and indirect approaches. Estimation of suspended sediment load is commonly achieved by establishing a sediment rating curve. Different approaches toward the establishment of a sediment rating curve are examined thoroughly. Techniques of sediment modelling are summarized via depiction of various hydrological and sediment models at the watershed scale. The paper ends with the discussion of future developments in suspended sediment studies at the watershed scale.

[1] We investigated the high variability of suspended sediment transport in 16 watersheds of Walnut Gulch, southeastern Arizona, USA that were distinguished at three spatial scales: the plot (ca. 0.001–0.01 km²), unit-source (ca. 0.01–0.1 km²), and large (ca. 1–150 km²) scales. Event-based data of water discharge and suspended sediment concentration were compiled in variable periods between the 1960s and 2010s. By subjectively distinguishing five different intraevent transport patterns that may be ascribed to a combination of various hydrological and sediment-transport processes, we showed that no single sediment rating curve could be developed for these data. However, at the event temporal scale, event specific sediment yield (SSY_e, t/km²) was significantly correlated to event runoff depth (h, mm) for all transport patterns of the watersheds, suggesting that the complexity of suspended sediment transport at the intraevent scale is effectively reduced at the event scale regardless of watershed sizes. Further regression analysis indicated that the SSY_e-h relationship can be generally characterized by a proportional model, SSY_e = nh where n, is conceptually equivalent to the volume-weighted event mean sediment concentration and is mainly determined by large events. For watersheds dominated by shrub cover, the change of the n value with watershed area was limited and thus may be reasonably regarded as a constant, implying that despite the highly variable suspended sediment concentrations during individual storm events in variable-sized watersheds, the synoptic effect of suspended sediment transport was similar and may be determined by a single value.

In the saltation regime where bed-shear stress is low, bed load moves by sliding, rolling, and saltating along the bed, while in the sheet-flow regime where bed-shear stress is high, it travels by a combination of saltation and sheet flow. In this paper a theoretical model is developed for predicting the onset of the sheet-flow regime as shear stress increases. This model is based on a new variable Pb representing the proportion of grains on the bed that are entrained as bed load. The model yields the equation Pb =2.56θ G3 in which G=1− θc ∕θ , θ=dimensionless bed-shear stress; and θc =critical value of θ at which grains begin to move. The equation shows that θt , which is the value of θ at the onset of the sheet-flow regime and is assumed to occur when Pb =1 , is around 0.5 with the exact value controlled by θc . For example, when θc =0.045 , θt =0.52 . The theoretical model is verified by performing a nonlinear regression analysis on data from 285 flume experiments. Additional flume experiments with a h...