This document discusses soil erosion, its causes, impacts, and methods for measuring and addressing it. It provides context on the development of the Universal Soil Loss Equation to quantitatively predict erosion. While useful, the USLE does not capture extreme weather events that cause most erosion. Direct measurements are most accurate but also most resource-intensive. The document outlines various on-site and off-site impacts of erosion and early efforts to address the problem through the Soil Conservation Service and projects like Coon Creek Watershed.

1. A 4 trillion cubic meter slice through ~ 2 billion years of strata!!
Erosion is a natural process !

2. Natural erosion rates
(m/m.y. = meters per million years)
http://bulletin.geoscienceworld.org/cgi/reprint/119/1-2/140

3. Cropland erosion rates
= 0.6 mm/y

4. Natural vs. accelerated erosion globally
Natural Erosion
Accelerated Erosion
acclerated erosion = accelerated
by human activities

5. Impact of climate on natural erosion rates
Brady and Weil (2002)

6. Variation in natural erosion rates

7. Intense thunderstorms
provide the desert areas
of the Southwestern US
with most of their
meager annual
precipitation.

8. What do you think this arroyo
looks like during a thunderstorm?

9. Fortunately most of land
surface not covered by
vegetation is covered by
desert pavement

10. Fortunately most of land
surface not covered by
vegetation is covered by
or cryptobiotic crusts
desert pavement

11. What created this?

13. Understanding water erosion processes
300 – 900 PSI
20- 40 feet / s
1) Detachment
2) Transport
3) Deposition
Brady and Weil (2002)

14. Detachment Transport Deposition

15. Manobolo River
Detachment estimates >> river sediment loads
Why ??
Most sediment is
deposited within the same
landscape

16. Deposition zone
Elevated SOM and
nutrient levels

17. Sheet erosion
Brady and Weil (2002)

18. Rill erosion
Rills result from concentrated flow and can
be filled by normal tillage operations
Brady and Weil (2002)

19. We had better do
something before this
rill turns into a gully.

20. Gully erosion
Dad !!!
I think the rill
has turned
into a gully!

21. From : Steve Groff sgroff@hughes.net
Sent : Thursday, June 29, 2006 5:49 AM
Hi Joel,
We ended up with 14.8" of rain over a 4 day period. These pictures
show how my neighbor’s plowed fields look. The ditch was up to
4' 4" deep and averaged about 3' deep in a 400' section- that is not
a typo! The staked tomato picture is on my side of the property
line looking from that ditch-100 feet away. I did have a little bit of
erosion but could find nothing over 1" deep…
Steve

22. Fresh Market Tomatoes planted no-till into cover crops on Steve Groff’s farm

23. Neighbor’s conventional-till corn field ~100’ away

24. When a stream is straightened or widened,
streambank erosion increases. Accelerated
streambank erosion occurs until the stream
reestablishes a stable size and pattern.
Streambank
erosion
When land use changes occur in a
watershed, such as clearing of land for
agriculture or development, runoff
increases. With this increase in runoff the
stream channel will adjust to accommodate
the additional flow, increasing streambank
erosion.

25. Shoreline erosion
As a boater, you can reduce
shoreline erosion by reducing the speed and
resulting wake from your boat, especially when
the water level is above normal.
Hydrologists estimate a wake 10” high
is five times as destructive to the shoreline
as a 5” wake and a wake that is 25” high has
a destructive potential that is 30 times
greater than a 5” wake.

26. Engineering properties of soil
Liquid limit
Brady and Weil, 2002
low Moisture content high
When moistened to its liquid limit, a soil starts to flow.

27. La Conchita
landslide
On January 14th 2005,
250,000 cubic yards of
soil flowed into the La
Conchita neighborhood
destroying 13 houses
and severely damaging
23 others.

28. Slump

29. Creep

30. Major causes of accelerated
erosion in Illinois
Arrival of tractors and the
moldboard plow
Arrival of soybean as a
major crop replacing oats
Construction and Mining
Highest rates

31. It looks like we have made
lots of progress… but how
much confidence should we
have in these #s??

32. ―It is questionable
whether there has
ever been another
perceived public
problem for which so
much time, effort and
money were spent in
light of so little
scientific evidence.‖

33. History of the Universal Soil Loss Equation (USLE)
During the 1940s and 50s, soil scientists in the Corn Belt region began to
develop quantitative methods of predicting soil loss. It was recognized that
a soil loss equation would be a valuable tool for farm planning. In 1946, a
group of erosion specialists held a workshop in Ohio to reappraise the
factors previously used to predict erosion (slope and management) and
added a rainfall intensity factor. The National Runoff and Soil Loss Data
Center was established at Purdue University in 1954 to locate, assemble,
and consolidate data from soil erosion studies throughout the United
States.
Pioneering the use of computers to analyze more than 11,000 plot-years of
basic runoff and soil loss data from studies at 47 locations in 24 states,
Walter Wischmeier (director of the NRSLDC) and collaborators developed
the Universal Soil Loss Equation (USLE) in the late 1950s. By the 1970s,
the USLE (first introduced in 1958) was widely used for conservation
planning worldwide.
Lots of good science went into developing the USLE

34. History of the Universal Soil Loss Equation (USLE)
During the 1940s and 50s, soil scientists in the Corn Belt region began to
develop quantitative methods of predicting soil loss. It was recognized that
a soil loss equation would be a valuable tool for farm planning. In 1946, a
A = R * K * LS * C * P
group of erosion specialists held a workshop in Ohio to reappraise the
factors previously used to predict erosion (slope and management) and
added a rainfall intensity factor. The National Runoff and Soil Loss Data
Rainfall erosivity factor
Soil erodibility factor
Topgraphy factor
Cover factor
Erosion control practices factor
Predicted soil loss (tons/acre)
Center was established at Purdue University in 1954 to locate, assemble,
and consolidate data from soil erosion studies throughout the United
States.
Pioneering the use of computers to analyze more than 11,000 plot-years of
basic runoff and soil loss data from studies at 47 locations in 24 states,
Walter Wischmeier (director of the NRSLDC) and collaborators developed
the Universal Soil Loss Equation (USLE) in the late 1950s. By the 1970s,
the USLE (first introduced in 1958) was widely used for conservation
planning worldwide.

35. Newer versions of the USLE, (RUSLE (Revised Universal Soil Loss Equation)
and RUSLE 2) have been developed and are now in use by the National
Resources Conservation Service (NRCS) for program planning and
implementation.
Major changes to the USLE incorporated into RUSLE(2) include:
 new and improved isoerodent maps and erodibility index (EI) distributions
for some areas
 new soil erodibility factors which reflect freeze-thaw in some geographic
areas
 new equations to account for slope length and steepness
 additional sub-factors for evaluating the cover and management factor for
cropland and rangeland
 includes new conservation practice values for cropland and rangeland.
A new Agriculture Handbook (No. 703) which describes RUSLE in great detail
was published in 1997 and is now accessible on-line.
http://www.ott.wrcc.osmre.gov/library/hbmanual/rusle703.htm

36. R values in IL
Isoerodent
lines

37. K values vary with texture and OM

38. LS factors

39. Impact of management (C and P factors)

40. The main reason why RUSLE #s do not
match up very well with real measurements
is because most erosion occurs during
extreme weather events (that are not
included in the RUSLE model).

41. Extreme soil erosion occurred in IA during the first 2 weeks of June 2008

42. Direct measurements of erosion are the most accurate method
of quantifying soil erosion, but are also the most laborious, time
consuming, and expensive. They involve collecting deposited
materials and taking volumetric and weight measurements.

43. Indirect measurements of erosion use natural benchmarks
and established benchmarks to evaluate long term changes in
soil depth/elevation.
1) A-horizon reconstruction is the comparison of A-horizon thicknesses between
lands suspected of being eroded and surrounding tracks of otherwise similar soil but
that are known not to have been affected by human action.
2) Natural benchmarks such as trees or boulders might have soil marks, not unlike
the high water marks on buildings in recently flooded areas. Volumetic
remeasurements can be estimated on the basis of the distance between the surface
and the mark.
3) Erosion pins are metal rods set into the ground, typically with a portion sticking up
above the surface some known and recorded amount (10 cm). Flagging is tied to the
stake to warn possible disturbers. The distance between the top of the pin and the
surface are recorded over time. A variation on this theme is to use a very long spike
driven through a washer to ground level. Over time, the distance the washer drops
from the top of the spike to the eroded ground surface can be recorded.
4) Erosion pipes are similar to pins except that soil remains undisturbed within the
pipe while it erodes away on the outside. Differences between soil height inside and
outside of the pipe can be compared over time.

44. The economics of off-site erosion
Karl L. Guntermann, Ming T. Lee and Earl R. Swanson - 1976
Erosion and sedimentation in agriculture has traditionally
been thought to result in substantial costs to the
producer implying that voluntary measures at soil
conservation would be in the individual's and society's
interest. The research reported here indicates that off-
site sediment damages are far greater than the on-
site productivity effects of erosion and that there is
considerable justification for stronger public policies
in this area. The development of the efficient production
frontier reveals that conventional production techniques
are quite inefficient compared to procedures that could
be adopted.

45. (Pimental et al., 1995)

46. The U. S. Army Corps of Engineers spends ~ $ 100 million a year
dredging the main channel of the Mississippi.

47. The U. S. Army Corps of Engineers spends ~ $ 100 million a year
dredging the main channel of the Mississippi.
Dustpan
dredge
Cutterhead
dredge

49. On-site effects of erosion
The main on-site impacts of accelerated erosion are
loss of soil fertility and water-holding capacity.
Eroded sediment normally contains elevated levels of
nutrients and SOM relative to the soil left behind. Also,
because the finest constituents of sediment tend to be
transported furthest, eroded soils become preferentially
depleted of their finer constituents over time; which
often reduces their water-holding capacity. In other
words, ―Erosion removes the cream of the soil‖.
In affluent areas of the world, accelerated erosion’s on-
site effects can often be mitigated by increased use of
fertilizer and irrigation; however this is not an option for
much of the earth’s population.

50. What are the on-site costs of this erosion?

51. Hill-top erosion on Midwest farms
http://outdoors.webshots.com/photo/1236540189056376852HRFRXH

52. Tillage erosion
Tillage erosion has only recently been recognized as a form of soil erosion.
Studies across North America and Europe have concluded that tillage
erosion is the primary cause of the severe soil loss and crop yield loss
observed on hilltops.
Tillage erosion is the progressive downslope movement of soil by tillage
causing soil loss on hilltops (knolls) and soil accumulation at the base of
slopes (depressions). Large, aggressive tillage implements, operated at
excessive depths and speeds are more erosive, with more passes resulting
in more erosion. Landscapes that are very topographically complex (with
many short, steep, diverging slopes) are more susceptible to tillage erosion.

53. Visual evidence of tillage erosion includes loss of topsoil and
exposure of subsoil at the summit of ridges and knolls; and
undercutting of field boundaries, such as fence lines, on the
downslope side and burial on the upslope side.
The soil loss on hilltops resulting from tillage erosion
reduces crop productivity and increases field variability.
Rates of soil loss on hilltops are often more than 10 times
what is considered to be tolerable for sustainable
production. Consequently, yield losses associated with
these areas are often as high as 30 to 50%.

55. http://assets.knowledge.allianz.com/img/soil_conservation_q1_14282.jpg

56. Moving topsoil from
deposition areas to
degraded areas on
the Mitchell Farm in
Waterloo, IA
Light green zones have
excess topsoil. Darker green
zones have successively less
topsoil and red zones have
the smallest amount of
topsoil. Black lines represent
optimal routes for a tractor
to follow when redistributing
top soil. The map on the
right is the expected
outcome.

57. Hugh Hammond Bennett
From “Soil Erosion:
A National Menace (1928)
“What would be the feeling of this
Nation should a foreign nation
suddenly enter the United States
and destroy 90,000 acres of land,
as erosion has been allowed to
do in a single county?”
“To visualize the full enormity of
land impairment and devastation
brought about by this ruthless
agent is beyond the possibility of
the mind. An era of land wreckage
destined to weigh heavily upon
the welfare of the next generation
is at hand.”
Soil scientist and showman

58. Dust from the High Plains
blotted out the sun in
Washington DC and helped
HH Bennett convince
Congress to fund the SCS.
http://www.nrcs.usda.gov/Internet/FSE_MEDIA/nrcs143_020944.jpg

59. On September 13, 1933, the Soil Erosion Service was
formed in the Department of the Interior, with Bennett as
chief. The service was transferred to the Department of
Agriculture on March 23, 1935, and was shortly
thereafter combined with other USDA units to form the
Soil Conservation Service (SCS) by the Soil
Conservation and Domestic Allotment Act of 1935.
Hugh Bennett continued as chief, a position he held
until his retirement in 1951.
On October 20, 1994, the agency was renamed the
Natural Resources Conservation Service (NRCS) to
reflect its broader mission.

60. Hugh Hammond Bennett inspecting strip cropping
in the Coon Creek Watershed – the nation’s first
watershed demonstration project

61. The ~ 500 Coon Creek farmers who signed five-year contracts
received $0.50/acre payments, foreshadowing future financial
assistance programs designed to promote wise stewardship of
natural resources.
The Soil Conservation Service supplied Coon Creek farmers
with seed, fertilizer and fencing, and the Civilian Conservation
Corps (known as the ―CCC Boys) provided a huge amount of
labor. They quarried and crushed millions of tons of limestone,
installed nearly 29,000 miles of terracing, cultivated and
planted many millions of trees and cleared channels and
reservoirs of nearly 400,000 square yards of sediment and
debris, in addition to other back-breaking tasks.

62. Coon Creek Watershed today
More than 95% of the 92,000-acre watershed is currently covered by a
conservation plan. Some current landowners still adhere to agreements
their parents worked out with Bennett’s team more than 70 years ago.

63. So what has been the long term impact of the Coon Creek
project that Aldo Leopold once called an ―adventure in
cooperative conservation?‖
Going by the numbers, a satisfying portrait emerges:
•Erosion within the watershed has decreased 75%
•Sediment leaving the watershed has decreased 94%
•Gullies—some described as ―big enough to hold a house‖—
were reduced by 77% by the late 1970s.
•Flooding, once common on area farms, has been minimized.
•The watershed is now 44% forested

65. Since 1936,
~ $300 billion has
been spent on
conservation
programs !

66. In the 1985 Farm Bill, Congress decided that as a quid pro
quo for federal farm assistance, farmers receiving taxpayer
support should control soil erosion on highly erodible lands
(HEL) used to grow subsidized crops. The policy principle
was straightforward and widely embraced in conservation
and agriculture policy circles: taxpayer support for
agriculture should not inadvertently subsidize degradation
of natural resources or the environment. Parallel policies
were authorized in the 1985 law to prevent subsidies from
encouraging conversion of fragile lands and wetlands to
crop production.

67. WHAT IS HEL LAND?
According to the USDA, a field is designated
as highly erodible land (HEL) if:
a) RKLS/T for the soil mapping units equals
or exceeds 8.
b) the highly erodible soil mapping units in
the field make up 33 percent or more of the
field’s acreage or
c) the highly erodible soil mapping units in
the field equal 50 or more acres.

68. http://www.ewg.org/book/export/html/22513

69. In order to maintain their eligibility for federal farm benefits
such as commodity crop subsidies and disaster payments,
farmers with subsidized crops on HEL land were required
to develop and implement a government-approved soil
conservation plan specifying soil conservation practices.
Common erosion reduction practices include: rotating
crops, minimizing tillage, leaving soil covered with crop
residue after harvest, and installing grassed buffers, etc.
This program was called the Highly Erodible Land
Conservation (HELC) Compliance provision or
―conservation compliance‖.

70. Corn production on land classified as HEL by NRCS
Acres per county
200 - 12,000
12,000 – 37,000
37,000 - 62,000
> 62,000
https://www.agronomy.org/publications/aj/articles/96/1/1

71. Farmers were given 10 years (until 1995) to fully implement
the soil conservation plans. The U.S. Department of
Agriculture (USDA) attributes the HELC planning and
compliance process with widespread adoption of
conservation systems, which made unprecedented progress
in reducing erosion over these 10 years.
HELC compliance, coupled with the Conservation Reserve
Program (CRP), reduced erosion by about 40 percent (1.2
billion tons) from 3.07 billion tons in 1982 to 1.9 billion tons
in 1997 (national soil survey years which encompass the
1985 to 1995 time period). USDA attributes about 25
percent of that reduction to HELC compliance requirements.
HELC compliance is also credited with a ―technology-
forcing‖ effect that helped reduce erosion on cropland not
subject to HELC plans.

72. Since full implementation of HELC
compliance plans in 1995, there has been
little additional progress in reducing erosion.
According to the National Resources
Inventory (NRI) survey, approximately 100
million acres of cropland in the U.S.—nearly
one-third of the 368 million acres of cropland
nationwide —continue to erode at rates
deemed ―unsustainable.‖

74. Each red dot = 100,000 tons of wind erosion, total = 765 million tons
Each blue dot = 100,000 tons of water erosion, total = 960 million tons

75. HEL Compliance
Is conservation system application required on HEL ground?
Yes, if the land is used to produce agricultural commodity crops.
Responsibility rests with both landowners and operators to implement an
appropriate conservation system or forfeit USDA program benefits.
Conservation systems are specific to each HEL tract on a farm.
What is the most common mistake made by operators out of
compliance?
Working soybean ground is the most common mistake that results in
noncompliance. Working soybean ground even lightly can cause your
operation to be out of compliance, as many conservation systems require
no-till or strip-till on soybean stubble.
Keep a few things in mind about tillage. Using aerators or rotary
harrows on fields scheduled for no-till may result in noncompliance. Strip
tillage with less than 25% row disturbance is equivalent to no-till. In a no-till
system, if ruts occur due to wet conditions at harvest, light tillage to level
the site is acceptable, but only on the affected area. Producers should
contact their NRCS office before performing tillage.

76. Additional Practices
Depending on your land, additional practices may be
required, such as gully and/or concentrated flow erosion
control through structural practices. This may include
establishment and maintenance of practices, such as:
Grassed waterways
Water & sediment control basins
Terraces
Grade control structures
Diversions
Other NRCS approved conservation practices

77. Is your farming system in compliance?
Conservation cropping systems for HEL

79. None of the 10 Mississippi River border states reviewed sufficient numbers
of tracts to achieve the one percent NRCS goal from 2000 to 2006
Number of tracts reviewed
# to
review
each
year to 2000 to
State 2000 2001 2002 2003 2004 2005 2006
achieve 2006
1%
review
goal
Arkansas 815 322 266 273 545 338 430 367 2,541
Illinois 2,883 1,184 1,162 1,030 1,061 1,803 2,257 1,977 10,474
Iowa 2,535 1,512 1,430 1,542 1,516 2,387 2,205 1,707 12,299
Kentucky 2,367 762 938 823 1,017 1,248 1,934 1,612 8,334
Louisiana 606 242 244 242 247 423 349 285 2,032
Minnesota 1,912 572 505 514 506 1,382 1,049 960 5,488
Mississippi 853 426 423 421 465 482 356 297 2,870
Missouri 1,723 838 881 1,069 922 1,283 1,103 1,185 7,281
Tennessee 1,775 361 440 440 435 584 1,059 861 4,180
Wisconsin 1,620 625 835 827 791 1,430 1,428 1,239 7,175
TOTAL 17,089 6,844 7,124 7,181 7,505 11,360 12,170 10,490 62,674

80. Erosion continues to be a serious
issue in Western Illinois
(#s = % of sample points)
County < 1 *T 1-2*T > 2*T
Adams 85 12 3
Brown 75 17 8
Hancock 91 6 3
Henderson 91 7 2
McDonough 85 12 3
Pike 70 18 11
Schuyler 83 13 4
http://www.agr.state.il.us/darts/References/transect/transect06.pdf
T = tolerable level of erosion according to NRCS

81. ~ 50% of the crop acres eroding at rates > T
in the 10 Mississippi River border states are
not subject to conservation compliance
http://www.ewg.org/book/export/html/22513

82. Crop residue is not trash – its cover for the soil !!

83. This is soil pornography !

84. Photo comparison method of estimating residue cover
25% 50% 75% 90%

85. One pass with a disk
~60 % residue cover

86. Two passes with a disk
might still be considered
conservation tillage

87. A single disking of less
abundant and more fragile
soybean residues is likely to
leave less than 20% residue
cover and not qualify as
conservation tillage.

88. Impact of field
operations on
residue cover can
also be estimated
using tables such
as the one to the
right

89. Relationship between yield and residue cover

90. Line transect method of estimating residue cover

91. Line transect method of estimating residue cover
The line-transect method is an easy, reliable way to determine residue cover.
It involves stretching a 50-foot measuring tape, line or rope (knotted, beaded
or otherwise marked at six inch intervals) diagonally across crop rows.
Percent cover is determined by counting the number of marks that intersect
or lie directly over a piece of residue.
The key to accuracy with this method is avoiding over- or underestimation.
Look straight down on each mark and take all readings on the same side of
the tape or rope, asking yourself, ―If a raindrop falls at this point, would it hit
residue or bare soil?‖ In general, residue should be 3/32 inch (roughly the
size of a healthy wheat straw) in diameter or larger. If there is any doubt at
all, do not count it.
At least five measurements should be taken and averaged at each site !

92. In 2006, no-till rose to
33.2% of Illinois cropland,
while conventional tillage
dropped to 31.2% !

93. In 2006, for the first
time, more than half
of the soybeans
grown in IL were
planted no-till !

94. WET SPRINGS CAUSING ILLINOIS PRODUCERS TO
INCREASE FALL TILLAGE
Wet springs the past two years have caused an increase in crop tillage,
according to a new study from the Illinois Department of Agriculture.
The 2011 Illinois Soil Conservation Transect Survey revealed that no-till crop
production has fallen five percentage points since 2009.
The farmers who switched production systems have not abandoned soil
conservation practices entirely, however. While the use of conventional tillage
increased during this period, so did the use of mulch-till, a practice that
leaves at least 30 percent of the residue from the previous crop on the
ground and, much like no-till, protects soil from erosion.
No-till farming still is the conservation practice of choice among Illinois
farmers. However, the gap between it and mulch-till has narrowed
considerably. The survey shows 21.4 percent of fields now are planted using
mulch-till, up from 20.7 percent in 2009 and 16.4 percent in 2006. No-till
usage has declined during the same span from a record-high of 33.2 percent
in 2006 to the current 24.2 percent. http://www.agrimarketing.com/ss.php?id=71271

95. Criteria used to rate soil suitability for no-till in PA
Factors More suitable Less suitable
Temperature regime Warmer: > 2800 GDD Cooler: < 2800 GDD
Soil drainage Good: Most soils are well or Poor: Most soils are
moderately well drained very poorly, poorly or
somewhat poorly drained
Water holding Low: < 6‖ High: > 6 ―
capacity of root zone
Slope High: > 8 % slope Low: < 8% slope
Rock fragment High: Most soils are loamy Low: Most soils are not
content and sandy skeletal skeletal
Duiker et al., 2001

96. No-till is much more challenging on some soils but
innovative farmers are making no-till work on most soils

98. Conservation tillage is just one piece of a
comprehensive approach to soil and water
conservation

99. Contour strip
cropping
http://allamakeeswcd.org/wp-content/uploads/2010/04/contour-farming.jpg

100. Contour terraces capture sediment and transform
long slopes into a series of shorter slopes.
http://www.ia.nrcs.usda.gov/news/brochures/terraces.html

101. Grassed waterways

102. Steve
Nebel
Cover crops

103. Grassed filterstrip

104. Grade stabilization structure

105. Tile Drainage - it is well established through both
scientific investigation and farmer observations that
improved drainage reduces surface runoff and
transport of sediment.

106. Impact of the 2008 floods on IA soils
20 tons per acre average soil loss across
2,284,000 ac!
Conservation structures needing repair
12,157 Grassed Waterways
8,137 Terraces
3,375 Water and Sediment Control Basins
800 Grade Stabilization Structures
Fields with combinations of two or more
conservation practices (e.g., no-till + cover
crops) performed much better than fields with a
single practice

107. Erosion
is more
than an
agricultural
issue

108. Major efforts
are being
made to
control
erosion
in urban
areas

109. Silt fences are
intended to trap
sediment while
allowing water
to slowly flow
through.
The bottom of the fabric
should be buried at
least six inches under
the soil to prevent
sediment from escaping
under the fence

110. Sediment retention pond

111. Diversion of water from trails prevents washouts

112. Waterway stabilized with rock and geotextile

113. Wind erosion is a serious problem
in the Western US

114. Wind Erosion Equation (WEQ)
E=f(IKCLV)
E is the estimation of average annual soil loss in tons per acre
f indicates the equation includes complex relationships rather than
just multiplication as in the USLE/RUSLE.
I is the soil erodibility index.
K is the ridge roughness factor.
C is the climatic factor. All climatic factor values are expressed as a
percentage of the value established at Garden City, Kansas.
L is the unsheltered distance across an erodible field, measured
along the prevailing wind erosion direction.
V is the vegetative cover factor.

115. http://www.weru.ksu.edu/weps/wepshome.html

116. Lots of discussion about dust
storms on Ag Talk recently:
―I want to say it doesn't look that
bad, but I know every pic I've
taken "doesn't look that bad."
This is definitely the worst I've
ever seen. Even no-tilled ground
is blowing on top of the hills.‖