Page 130 - Biosystems Engineering

Soil and W ater Conservation 111

mutually exclusive. Journey of a soil particle may follow many paths.
A particle detached from the soil may be picked up by flowing water
to the stream, transported to another location, and stay suspended
until it leaves the watershed outlet. Alternatively, the same particle
may be deposited somewhere along the way on an overland flow
plane or in a streambed. It may be later picked up again by flowing
water during another storm event and deposited somewhere else or
not. Therefore, the rate of soil erosion from upland areas is not neces-
sarily a surrogate for sediment yield. Sediment yield is the amount of
sediment leaving a watershed over a specific time period. The ratio of
sediment yield to gross soil erosion is smaller than unity and is called
the sediment delivery ratio.
Soil detachment could be triggered either by rainfall or by flow-
ing water. Production of eroded soil by the rainfall impact is called
rainsplash erosion. Water from raindrops, in addition to providing
energy for soil detachment, also acts as a wetting source for soils. As
soil becomes wetter, its shear strength decreases, and it becomes eas-
ier to detach particles from the soil. Maximum soil splash occurs
when the land is covered by a very thin layer of sheet flow (Piest et al.
1975). Soil erosion caused by the shear stress exerted on the soil sur-
face by the flowing water is called hydraulic erosion. Total erosion is
the combination of the two. One important note here is that if the
flowing water already contains a significant amount of sediment in it,
that is, if it has reached its sediment transport capacity (more to come on
this concept), then instead of further soil detachment it is possible to
observe sediment deposition. Therefore, although rainsplash erosion
is always positive, net hydraulic erosion could be either positive or
negative depending on the sediment supply from upland areas.
Figure 3.7 is a conceptual diagram of how sediment yield can be
computed in a watershed. The conceptualization assumes that the
watershed is divided into smaller hydrologic units or cells in a cas-
cading fashion. In this conceptualization, flow and sediment from
each unit is routed to a downstream unit. This process continues until
flow and sediment reach the watershed outlet. For each unit, theo-
retical rainsplash and hydraulic erosion is first computed. These two
theoretical erosion values are added to erosion supplied from upland
units, if there are any. Total erosion is checked against the transport
capacity of the unit. If transport capacity has not yet been reached,
the computed value represents the total sediment and is transported
to the next (downstream) unit. Conversely, if the transport capacity is
exceeded, the sediment at the amount of the transport capacity is car-
ried off to the downstream unit. Whether there is any net erosion
from the unit depends on the amount of sediment delivered from the
upland cells. If that amount exceeds the transport capacity of the cell,
there is no more erosion but a net deposition. If the amount is smaller
than the transport capacity, there will be net erosion from the unit to
supplement the sediment deficit (transport capacity minus sediment
supplied by the upstream unit).