The relationship between specific event sediment yields and runoff depth and its significance

Although the dynamics of suspended sediment transport are complex, they may become less complex and hence easier to quantify as the time scale increases from instantaneous to event-lumped transport. To test this hypothesis, we collected suspended sediment data from watersheds with contrasting transport behavior (supply-limited and transport-limited), together with those reported in Loess areas of China. Supply-limited watersheds (type I) included four Australia watersheds with various land use and land cover (LULC) conditions and a central New York watershed. Transport-limited watersheds (type II) contained twelve watersheds from Walnut Gulch, Arizona and an Italy watershed. Type III watersheds involved fourteen watersheds in Loess area of upstream Yellow River, China. They are different from the others in that they have hyperconcentrated flows most of time during events due to the finer grain size of loess. We calculated specific event sediment yield (SSYe, t/km2) and runoff depth (h, mm) of various rainfall events in each watershed, and statistically established a proportional relationship, SSYe = Ch where C is a constant for a given watershed, which is equivalent to the discharge-weighted event mean sediment concentration. The coefficient of determination ranges from 0.64 to 0.99, with the median of 0.95 indicating that the proportional model generally holds. Furthermore, values of C showed distinct ranges for the three types of watersheds: 0.32 - 1.93 for type I watersheds, 7.17-34.39 for type II watersheds, and 680-861 for type III watersheds. These values correctly reflect the different nature of sediment transport in three types of watersheds. We further found that C begins to decrease at different threshold values of watershed area for types I and II watersheds, whereas C remains constant for type III watersheds. These results suggest that a simple proportional model may be used to describe event-lumped behavior of suspended sediment transport.