Dynamics of suspended sediment transport at field-scale drain channels of irrigation-dominated watersheds in the Sonoran desert, southeastern California

Suspended sediment is a major source of pollution in irrigation-dominated watersheds. However, little is known about the process and mechanisms of suspended sediment transport in drain channels directly connected to agricultural fields. This paper explains sediment dynamics using averaged 5 min flow discharge Q (m3 s^-1) and suspended sediment concentration C (mg 1^-1) collected during one crop season in a small catchment containing a first-order drain channel and its connected six agricultural fields within the Salton Sea watershed. The statistical properties and average trends of Q and C were investigated for both early (i.e. November) and late (i.e. January) stages of a crop season. Further in-depth analysis on sediment dynamics was performed by selecting two typical single-field irrigation events and two multiple-field irrigation events. For each set of irrigation events, the process of suspended sediment transport was revealed by examining hydrograph and sediment graph responses. The mechanisms underlying suspended sediment transport were investigated by analysing the types of corresponding hysteresis loop. Finally, sediment rating curves for both hourly and daily data at early and late stages and for the entire crop season were established to seek possible sediment-transport predictive model(s). The study suggests that the complicated processes of suspended sediment transport in irrigation-dominated watersheds require stochastic rather than deterministic forecasting.