Suspended sediment transport at the instantaneous and event time scales in semiarid watersheds of southeastern Arizona, USA



[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 (SSYe, 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 SSYe-h relationship can be generally characterized by a proportional model, SSYe = 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.