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 1n = the base e logarithm
For an individual field:
D = distance (m) from the center of the land parcel along the down slope gradient to the
nearest surface water body
For a subwatershed or watershed:
D = one half the length of the watershed measure along the mainstem stream
A area-based SD (Roehl, 1961) is:
SD = 0.47A-1.25
(1.1.20)
Where:
A = watershed area in km2.
The scatterplot of the original data to derive equation 1.1.19 is from southeastern U.S. watersheds
(Figure 1.1.5). Modelers should consider the most likely SD and chose a range of high and low SDs
based on data variability.
Figure 1.1.5 Sediment Delivery Ration as a Function of
Watershed Drainage Area. (after Roehl, 1962)
Haith et al. (1985) have observed that large watershed sediment yields often do not coincide
with major erosion periods. For example, in the eastern United States, most soil erosion is caused by
1.1-18
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