Location: Southwest Watershed ResearchTitle: Suspended sediment transport at the instantaneous and event time scales in semiarid watersheds of southeastern Arizona, USA Author
Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2013
Publication Date: 10/23/2013
Citation: Gao, P., Nearing, M.A., Commons, M. 2013. Suspended sediment transport at the instantaneous and event time scales in semiarid watersheds of southeastern Arizona, USA. Water Resources Research. 49:1-14. DOI: 10.1002/wrcr.20549. Interpretive Summary: This study looks at the sediment that is transported from watersheds ranging in size from approximately one acre up to 50 square miles and located in southern Arizona. We looked at both the rate that sediment was exported from the watersheds during a rainfall event, as well as the total amount of sediment exported during entire events. The changes in sediment export rates during events did not follow any common pattern, which indicates that a multiplicity of processes occur at different times and with different storms, resulting in complex sediment responses. However, we did find that the total amount of sediment exported during entire events was more or less linearly related to the total amount of surface water runoff volumes. Finding this simple relationship between runoff volumes and sediment exports might be an important tool for general characterization of differences across geographic regions as well as in easily identifying general trends over time, such as might potential changes associated with climate change.
Technical Abstract: We investigated the high variability of suspended sediment transport in 16 watersheds of Walnut Gulch, southwestern United States that may be distinguished at three spatial scales: the plot (ca. 0.001 – 0.01 km2), unit-source (ca. 0.01 – 1 km2), and large (ca. 1 – 150 km2) scales. Event-based data of water discharge and suspended sediment concentration were compiled in the period varying 1960s to 2010s. By subjectively distinguishing six different intra-event transport patterns that may be ascribed to a combination of various hydrological and sediment-transport processes, we showed that no single sediment rating curve may be identified at this time scale. However, at the event scale, event specific sediment yield (SSYe, t/km2) was very well correlated to event runoff depth (h, mm) for all transport patterns of selected watersheds, suggesting that the complexity of suspended sediment transport at the intra-event scale is effectively reduced at the event scale regardless of the sizes of watersheds. Further regression analysis indicated that the SSYe-h relationship may be generally characterized by a proportional model, SSYe = nh where n, is conceptually equivalent to the discharge-weighted event mean sediment concentration and is mainly determined by large events. For watersheds dominated by shrub cover, the value of n changed with watershed area in a limited way and thus may be reasonably regarded as a constant, implying that despite the highly variable suspended-sediment concentrations during individual storm events in individual watersheds with different sizes, the synoptic effect of suspended sediment transport, which is influenced by rainfall amount and duration, topography, vegetation cover, and soil properties in Walnut Gulch, is similar regardless of spatial variations and may be quantified by a single value.