Watershed Yield

The ebb and flow of sediment movement in a watershed is spatially and temporally complex and difficult to quantify. However, attempting to quantify and understand sediment dynamics in a watershed is important for the successful management of aquatic and terrestrial ecology and [Page 3075]the associated health of the watershed. Soil erosion is a worldwide problem that has degraded small and large watersheds. Estimating the sediment yield of a watershed helps resource managers understand the amount of soil erosion that exists and how much sediment is stored in the watershed. Within the fluvial hierarchy, watersheds provide a spatial framework by which the physical and environmental metrics influencing sediment movement can be evaluated. A watershed is defined as the contributing area of land that captures and drains precipitation to an outlet point, which does not have to be a stream or lake confluence. Rather, the outlet can be a point on a stream reach, and the contributing area of the watershed would be delineated from the predefined point.

Sediment yield in a watershed is quantified using the amount of sediment passing a point in the watershed per some unit of volume per time (i.e., cubic meters per kilometer per year) or mass per time (kilograms per square meter per year). Sediment yield can be estimated by coring reservoirs and lakes, performing suspended sediment sampling at the outlet, direct site observation and calculation of erosion, and computer modeling. Reservoirs and lakes act as sediment traps and through coring or bathymetric surveys can provide an insight into temporal and spatial changes of sediment routing through the watershed. Suspended sediment is measured at the outlet and plotted against discharge. A rating curve can be established if the duration of sampling spans multiple high- and low-discharge events. The source of suspended sediment (mostly silt and clay) is mainly from surface and subsurface erosion, which includes roads, culverts, tile lines, rills, gullies, overland flow, and stream banks. One of the concerns of only measuring suspended sediment is that bed load transport is not accounted for. However, bed load normally comprises a small amount of the sediment transport in large river systems. Direct site observations are difficult and time-consuming. In turn, soil erosion models have become a popular means of establishing the erosion estimates. These include, but are not limited to, the universal soil loss equation, revised universal soil loss equation, water erosion prediction project (WEPP) model, and geographic information systems (GIS).

Sediment is mobilized and transported in a watershed by physical and chemical weathering and erosion. Only a small portion of the sediment eroded in a watershed is quantified as the sediment yield of the watershed. This is because sediment is deposited and stored throughout the watershed. Temporary and permanent storage exists on the bed of the channel, bars, floodplains, wetlands, impoundments, and hillslopes. Watersheds of contrasting size route sediment differently. Watersheds draining a small area will generally have sediment yields greater than watersheds draining a larger area. Where relief and hillslope gradient are higher, sediment has a greater opportunity to reach the stream. Watersheds draining a larger area have streams that parallel floodplains and terraces, which store flood deposits and hillslope-generated sediments.

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