Suspended-sediment transport in an intensively cultivated watershed in southeastern California

The purpose of this study is to investigate the variation of sediment movement at multiple spatial and two temporal scales of an agricultural watershed in southeastern California where surface runoff is primarily supplied by irrigation. An extensive turbidity and stage monitoring program based on grab sampling using 500-mL plastic bottles and in situ monitoring using both turbidity sensors and pressure transducers was designed to capture the spatial and temporal heterogeneities of suspended-sediment loads. The obtained turbidity values and stages at weekly and 5-min time scales were subsequently converted to suspended-sediment concentrations and water discharges, respectively, using established calibration methods. Weekly data were analyzed using a mass-balance approach to distinguish erosion and deposition processes in drain channels at various spatial scales. Further analysis on weekly data led to an empirical model describing the process of sediment transport at various spatial scales. The model revealed that drain channels at larger spatial scales (i.e. downstream channels) tended to be dominated by weekly net deposition, while those at smaller spatial scales (i.e. upstream channels) were controlled by weekly net erosion. However, analysis of the 5-min data showed that in small-scale drain channels, large amount of sediment was indeed deposited over a short period of one week (about one day). In the rest of the week, low-intensity erosion prevailed. Therefore, sediment transport in the study watershed was generally controlled by deposition, which supports the necessity of on-going drain-channel dredging by Imperial Irrigation District.