Processes controlling sediment yield from watersheds as functions of spatial scale

The need for estimates of sediment yield are ubiquitous throughout water resources analyses, modelling, and engineering as sediment is a major pollutant, a transporter of pollutants, and sedimentation rates and amounts determine the performance and life of reservoirs, canals, drainage channels, harbors, and other downstream structures and improvements. Moreover, as a ‘watershed wide’ measure of soil erosion, transport, and deposition, sediment yield reflects the characteristics of a watershed, its history, development, use, and management. The major factors and processes controlling sediment yield from watersheds are described and discussed in the context of spatial scale. Historical sediment yield data from selected watersheds across a range of scales are used to illustrate variations of sediment yield with watershed scale. Generalized relationships between sediment yield and drainage area from the USA and Australia are used to show the statistical variations of sediment yield with watershed area. Area is shown to be an important predictor variable which usually, but not always, is correlated with sediment yield. Experimental data from a small experimental watershed are used in a case study to illustrate processes controlling sediment yield. The case study summarizes and interprets simulation model studies using experimental field data from measurements distributed across a range of scales. Information presented here should help guide the conceptual development of sediment yield models and their mathematical formulation. It should also be useful in design and implementation of spatially distributed verification and validation studies.