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 late spring and summer intense rainstorms, but most sediment discharge occurs during late winter and
early spring runoff. They reason that runoff during erosive periods is often insufficient to transport
eroded soil far from a source area. Subsequent large events are assumed to flush portions of the
accumulated sediment from the watershed drainage network to the receiving water body. Haith et al.
(1985) propose a "sediment year" for the Eastern United States running fro April through March.
Although general procedures are not available for estimating seasonal sediment yields, Haith et
al. (1984) achieved "satisfactory" results in an 850 km2 New York watershed using monthly
approximations. They assumed sediment yield during a given month m, Ym , to be proportional to
Qm1.2 where Qm is the watershed runoff during month m. Ym is then derived from historical values for
average annual sediment yield Y as obtained from equation 4-18. A seasonal yield is the sum of the Ym
for the months in a given season.
Sediment-Attached Nutrient Loading. The loading function for a sediment-attached nutrient
runoff (Haith and Tubbs, 1981) from a source area k, at time t, is:
LSkt = 0.001 Cskt Xkt SDk and Ak
(1.1.22)
Where:
= solid-phase nutrient load (kg)
= concentration of the nutrient in sediment (mg/kg)
= soil loss (t/ha) (from the USLE)
= sediment delivery ratio (dimensionless)
= area (ha)
0.001 is a units conversion constant
The concentration Cs may be determined by direct analysis of eroded soil (sediment) or in situ
soil samples. Since erosion is selective for smaller soil particles, in situ concentrations must be adjusted
by an enrichment ration, thus:
Cs = en Ci
(1.1.21)
Where:
cs = nutrient concentration in sediment (mg/1)
en = enrichment ration (dimensionless)
ci = nutrient concentration in situ soil (mg/1)
Mills et al. (1985) discuss methods for arriving at values for en. However, Haith and Tubbs (1981)
maintain that enrichment ratios for nutrients typically vary from 1 to 4 and are essentially unpredictable.
1.1-19
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