Spatial and Temporal Variabilities of Sediment Delivery Ratio

Sediment contribution of specific areas of a watershed is an important consideration, but one that is often overlooked, when developing watershed management plans. Understanding sediment delivery to the watershed outlet is one method for identifying high contribution areas. In this study, a new methodology for calculating individual subbasin sediment delivery ratio (SDR) was developed within the Soil and Water Assessment Tool (SWAT) for two Michigan watersheds. Subbasins with the greatest SDR had two defining characteristics: high erosion rates and close proximity to the watershed outlet. The impact of best management practice (BMP) implementation (no-tillage and Conservation Reserve Program) on sediment yield reduction at the watershed outlet was compared for two targeting methods (SDR and erosion rate). Identifying SDR of individual subbasins for implementation of BMPs is more effective method for addressing water quality concerns at the watershed outlet than the widely used targeting high risk erosion areas. Watershed SDR was found to be highly variable on a monthly basis, due to changes in sediment yield, which is in turn affected by factors such as precipitation and surface runoff. Finally, watershed SDR calculated using the physically-based SWAT model was compared to four empirically-based areal relationships with SDR. While some area methods reproduced the SWAT-SDR, the variations between areal methods indicate that more detailed physical characteristics should be considered in watershed SDR calculation.