Agriculture Reference
In-Depth Information
In 1940, Austin W. Zingg published the first equation relating soil erosion to land
slope and length:
A
=
CS
m
L
n
-1
(17.1)
where
A
is the average soil loss per unit area from a land slope of unit width,
S
is the
land slope in percent,
L
is the horizontal length of the land slope, and
C
is a constant
of variation. Data evaluated by Zingg were from work reported by Duley and Hays
on two Kansas soils: by Diseker and Yoder on a Cecil clay soil in Alabama and from
soil erosion plots located at Tyler, TX; Guthrie, OK; Clarinda, IA; Bethany, MO; and
LaCrosse, WI. The work of Duley and Hays was conducted using a sprinkling-can
rainfall simulator. At Bethany, MO, a rainfall simulator was used in addition to the
soil erosion plots. Zingg recommended values of 1.4 for
m
and 1.6 for
n
.
Zingg's work ushered in a long period of research that ultimately led to the devel-
opment of the USLE and provided direction in soil erosion research that soon led to
the concept of an allowable, tolerable, or limiting soil loss. Further, the equations,
methods, and factors developed were used to select and design practices that limited
soil loss to levels supporting sustainable land use.
In 1941, Dwight D. Smith, building on the work of Zingg, extended Equation 17.1
by the introduction of a factor
P
to provide for the effect of practices on soil loss. The
factor
P
is the ratio of
A
1
/
A
, where
A
1
is the soil loss with a given practice and
A
is
given by Equation 17.1. He further developed the concept of designing soil conserva-
tion systems by substituting
A
1
/
P
for
A
in Equation 17.2:
A
1
/
P
=
CS
7/5
L
3/5
(17.2)
Equation 17.2 was rewritten to develop an equation to determine a maximum
acceptable slope length based on the allowable soil loss:
L
= (
A
1
/
PC
)
5/3
S
-7/3
(17.3)
where
L
would be the maximum length of slope allowable for a specific site with
values of
C
,
S
,
A
1
, and
P
.
Smith applied this approach to conditions for the Shelby soil at the soil conserva-
tion experiment station at Bethany, MO. Based on a 3-year rotation of corn, wheat,
clover, and timothy, he computed a
C
value for the Shelby surface soil.
The value of
A
1
(allowable average annual soil loss) Smith recommended for the
Shelby soil was 9 Mg/ha. Smith expressed the view that the allowable soil loss limit
should be based upon that rate of soil loss that permits a constant, or preferably an
increasing, time gradient of soil fertility.
Smith indicated that the 9-Mg/ha soil loss per year may be too great a loss for
maintenance of soil fertility when erosion has progressed to the point that plowing
is diluting the surface soil with thin layers of subsoil. He also stated that the rate of
allowable loss may be even greater than 9 Mg/ha/year, with fertility being main-
tained because certain nutrients had larger concentrations in the Shelby subsoil than
in the surface soil.
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