WATERSHEDS - Water Quality Engineering in Natural Systems - page 230

• Determining how much more intensively a given
field could be cropped safely if contoured, terraced,
or strip cropped
• Determining the maximum slope length on which
given cropping and management can be tolerated
in the field
• Providing local soil loss data to agricultural techni-
cians, conservation agencies, and others to use
when discussing erosion plans with farmers and
contractors.

fields are plowed up and down the land slope. The
annual runoff volume is 30 cm and the maximum
runoff rate is 5 cm/h. (a) If the typical overland flow
distance from the beginning of overland flow to a
drainage stream is 1.4 km, estimate the average annual
soil loss. (b) If the delivery ratio is estimated as 15%,
what is the annual sediment loading on the receiving
water body?

Solution

(a) The annual soil loss can be estimated using the
RUSLE, Equation (6.23), given by

The rainfall erosivity factor, R , has a value greater
than zero for every rainfall; hence, erosion and soil loss
is anticipated by the soil loss equation for any precipita-
tion. A hydrological rainfall excess model in combina-
tion with the RUSLE would eliminate erosion by
rainfall with no excess rain, and the accuracy of such
combined models has been demonstrated in small
watersheds (Rai and Mathur, 2007). It has been found
that soil erosion models are always very sensitive to
parameters related to infiltration (Léonard et al., 2006).
All of the soil that is eroded from the land surface in
a watershed does not end up in the receiving water body.
The amount of eroded soil that ends up in the water
body is called the sediment yield , and the fraction of the
gross erosion that ends up as sediment yield is called the
delivery ratio ; hence,

A RK LS CP = ( )

According to Figure 6.19, the average annual rain-
fall erosivity factor, R r , for central Georgia is

R r = 300 tonnes/acre = 2 24 300 . × tonnes/ha

= 672 tonnes/ha

From the data given, Q = 30 cm and q = 5 cm/h;
hence, the rainfall factor, R , can be estimated by
Equation (6.27) as

1 3 / 1 3 /

R = 0 5 . R + 7 5 . Qq = 0 5 672 . ( ) + 7 5 30 5 . ( )( )

r
tonnes/ha = 721

sediment yield
gross erosion

delivery ratio =

(6.31) For clay loam soil with 2% organic matter, Table 6.8

gives K = 0.25. From the data given, the distance
from the origin of overland flow to the drainage
channel, L , is 1.4 km = 1400 m, the average slope, S ,
is 3%; Table 6.9 gives m = 0.3, and Equation (6.30)
gives the slope length factor, LS , as

The gross erosion in the denominator of Equation (6.31)
is given by the RUSLE ( A in Eq. 6.23), and delivery
ratios for agricultural lands are typically in the range of
1-30% (Novotny et al., 1986), with 10% being typical
for watersheds larger than 200 km 2 (77 mi 2 ) (Quilbé
et al., 2006). Sediment transport in streams is usually
related to the stability of the stream channel and the
characteristics of soils in the watershed. In the south-
eastern United States, median loads in stable streams
range from 2.8 tonnes/yr/km 2 (8 tons*/year/mi 2 ) for the
Southern Coastal Plain to about 79 tonnes/yr/km 2 (230
tons/year/mi 2 ) for the Mississippi Valley Loess Plains
(Simon and Klimetz, 2008).

m

L

=

[ ]

2

LS 0 065 0 04579 . + . S + 0 0065 . S

22 1 .

m

1400
22 1

=

[ ]

2

0 .065 0 04579 3 + . ( ) + 0 0065 3 . ( )

.

= 1 37 .

For cotton, Table 6.10 gives the crop management
factor, C , as 0.4. During the off-season, when the
land is plowed up and down the land slope, Table
6.10 gives C = 1; therefore, the average annual value
of C can be taken as

EXAMPLE 6.4

A 200-ha cotton farm in central Georgia consists
of predominantly clay loam soil with approximately
2% organic matter, and the ground surface has approx-
imately a 3% slope. During the off season, the cotton

1 0 0 4
2

. + .

C = = 0 7 .

Since no special erosion control measures are
implemented, P = 1, and the RUSLE, Equation
(6.23), gives * 1 ton = 2000 lbs.