Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 221

the surface of the soil, and adds to its own sediment load. The further the runoff runs
uncontrolled, the greater its erosive force and the greater the resulting damage.
The fate of eroded material within a watershed is influenced by hydrologic,
topographic, vegetative, and groundcover characteristics.

6.5.5 Sediment Yield and Its Estimation

6.5.5.1 Concept

Sediment yield is defined as the amount of sediment per unit area removed from a
watershed by flowing water during a specified period of time. Sediment yield affects
rates of soil development and influences the recovery of disturbed surfaces downs-
lope from source areas in desert landscapes. Sediment yield is strongly affected by
surficial materials, topography, rainfall seasonality, and vegetation cover and can be
increased by soil disturbance, which often occurs as the result of land use.

6.5.5.2 Mechanism

Sediment yield from upland areas is simply the final and net result of detachment,
transport, and deposition processes occurring from the watershed down to the point
of interest where sediment yield information is needed. Depending upon the scale of
investigation and definition of the problem, this point of interest can be the edge of
a farm field, delivery point to a stream channel, a position on a hillslope, a property
boundary at a construction site, watershed outlet or some other location dependent
upon topography. In any event, sediment yield at the point of interest is determined
by the occurrence of physical processes of sediment detachment, transport, and
deposition at all positions in the contributing watershed area above the point of
interest. When the energy in the stream (containing sediment) dissipates to a level
that can no longer support the transport of the sediment, the sediment falls out of
the water column and deposits.

6.5.5.3 Estimation

(A) Delivery Ratio Method

Sediment yield is often computed based on the use of a delivery ratio defined as
the change per unit area from the source to the point of interest. The delivery ratio
( D in percent) is often expressed as

D = 100 Y / T (6.13)

where Y is the total sediment yield at the downstream point of interest, and T is
the total material eroded (gross erosion) on the watershed area above the point of
interest. Values of Y and T are given in units of mass per unit area per unit time (e.g.,
T / A /yr).