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III. Basic Factors Affecting Field Soil Loss

III. Basic Factors Affecting Field Soil Loss

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minute intensities was also generally poor. Good correlation with maximum 30-minute intensity was found only on steep slopes or sandy loam.

At every location for which fallow-plot data were available, both runoff

and soil loss were more highly correlated with rainfall energy than with

rain amount or any short-period maximum intensity. Momentum rated

second, but was well below energy as a predictor of soil loss from fallow.

In further regression analyses of the data, Wischmeier (1959) found

that the rainstorm parameter most highly correlated with soil loss from

fallow was a product term, kinetic energy of the storm times maximum

30-minute intensity. He called this product the “rainfall-erosion index.”

Maximum 30-minute intensity was defined as twice the greatest amount

of rain falling in any 30-minute period. A break between storms was defined as a period of 6 consecutive hours with less than 0.05 inches of rainfall. This index was selected as the most appropriate rainfall parameter

for use in the soil loss prediction equation.

The rainfall-erosion index thus defined explained from 72 to 97 per

cent of the variation in individual-storm erosion from tilled continuous

fallow on each of six widely scattered soils. The percentage of the soilloss variance explained by the index was greater than that explained by

any other of 42 factors investigated and greater than that explained by

rain amount and maximum 5, 15, and 30-minute intensities, all combined in a multiple regression equation.

The erosion index evaluates the interacting effect of total storm

energy and maximum sustained intensity. Thus it is an approximation of

the combined effects of impact energy and rate and turbulence of runoff.

Rainfall energy is a function of the specific combination of drop velocities

and rain amount. The maximum 30-minute intensity is an indication of

the excessive rainfall available for runoff.

The product terms-rain amount times 30-minute intensity, and momentum times 30-minute intensity-were also more precise estimators of

soil loss than was energy alone, although less accurate than the energyintensity product. This supports the conclusion that the erosive potential

of a rainstorm is primarily a function of the interacting effects of drop

velocity, rain amount and maximum sustained intensity. In assembled

plot data, maximum 30-minute intensity was more effective than maximum 15- or 60-minute intensity as the second element of the interaction


The relationship of soil loss to the storm energy-intensity products is

linear. Therefore, the location erosion-index value for a year can be

computed by summing the storm energy-intensity products. The assembled

plot data showed that when all factors other than rainfall were constant,

specific-year soil losses from cultivated areas were directly proportional



to the yearly values of the index. Yearly or monthly values of the erosion

index can be computed on a locality basis from recording rain gauge

records. Return-period values can also be computed.

The rainfall-erosion index provides valid estimates of the effects of

rainfall patterns over long time periods. For prediction of losses from

specific storms, precision was improved by combining with the erosion

index, the parameters: rainfall energy, an antecedent moisture index,

and antecedent energy since cultivation ( Wischmeier and Smith, 1958).


Some soils erode more readily than others. Soil properties that influence soil erodibility by water may be grouped into two types: (1)

those properties that effect the infiltration rate and permeability; and

( 2 ) those properties that resist the dispersion, splashing, abrasion, and

transporting forces of the rainfall and runoff.

Middleton (1930) was one of the first to try to obtain an index of soil

erodibility based on the physical properties of the soil. He considered

the dispersion ratio and what he called the erosion ratio to be the most

significant soil characteristics influencing soil erodibility. His erosion

ratio was the dispersion ratio divided by the ratio of colloid to moisture

equivalent. The dispersion ratio, expressed as a percentage, was the ratio

of the apparent total weight of silt and clay in the nondispersed sample

to the total silt and clay in the dispersed sample. He also suggested that

organic matter, the silica: sesquioxide ratio, and the total exchangeable

bases influenced the erosional behavior of soils. Middleton et al. (1932,

1934) grouped soils of the original ten erosion stations according to the

above criteria.

Peele et al. (1945) modzed Middleton’s criteria in an analysis of

four major soils of South Carolina. Voznesensky and Artsruui (1940) developed a formula for an index of erodibility based on dispersion, waterretaining capacity, and aggregation.

O’Neal (1952) attempted to develop a key for evaluating soil permeability on the basis of certain field conditions. The first step in this procedure was to determine the type of structure. Then the class of permeability was estimated from four principal factors and one or more of

eight secondary factors. Principal factors used were: relative dimensions

(horizontally and vertically) of structural aggregates, amount and direction of overlap aggregates, number of visible pores, and texture. Important secondary factors were: compaction, direction of natural breakage, silt content, cementation, type of clay minerals, character of coatings

in aggregates, degree of mottling, and certain features of climate.



Parr and Bertrand (1960) presented a comprehensive review of past

studies of water infiltration into soils. Instrumentation, procedures, and

results were briefly summarized.

TOdescribe what they considered to be the most important soil properties affecting erodibility, Adams et al. (1958b) made the following field

and laboratory measurement on Iowa soils: runoff, infiltration, wash

erosion, splash erosion, water-stable aggregates < 0.10 mm., dispersion

ratio, per cent silt and clay, bulk density, pores drained by 60-cm.

water tension, and air permeability at field capacity. For soils containing

relatively large amounts of swelling type clay that crack upon drying, the

erodibility has been considered to be significantly influenced by moisture

content (Smith et al., 1953; Adams et al., 1958a).

Browning et al. (1947) developed a conservation guide for soils

mapped in Iowa, to be used in computing field soil loss. The soils were

divided into seven groups on the basis of what was considered to be their

relative erodibility. This procedure was extended to other soils of the

North Central and Northeastern States (U. S. Soil Conservation Service,

1956; Lloyd and Eley, 1952).

Many soil properties appear to influence erodibility. Their effects

may be interrelated. Some of them are influenced by cropping history,

past erosion and management practices. Subjective classifications of soil

into erodibility classes by relating observed erosion to soil survey data

have often been biased by confounding soil effects with those of rainfall

and management. The automatic confounding of rainfall and soil effects

has also complicated efforts to evaluate soil erodibility empirically from

field plots under natural rainfall. As a means of isolating soil effects

If'ischmeier et al. (1958) proposed use of the rainfall-erosion index to

adjust measured soil losses for differences in rainstorm characteristics

and use of slope factors to adjust for differences to topography.

For soil-loss prediction purposes, Wischmeier and Smith (1961) defined soil erodibility as soil loss in tons per acre per unit of rainfallerosion index, measured from tilled continuous fallow with length and

per cent of slope at specified values. The soil erodibility factor thus becomes a quantitative factor. Empirical measurements for its evaluation

include combinations of all primary and interacting factor effects.


Field soil loss is affected by the degree of slope, the length of slope,

and the curvature of the slope. Studies of the first two effects have been

conducted under both natural and simulated rainfall, but little has been

done to evaluate the third.



1. Per Cent Slope

The effect of per cent slope was studied on small plots under

sprinklers by Duley and Hays ( 1932), Neal (1938), Borst and Woodburn

( 1940), and Zingg (1940). Water applied by sprinklers in these studies

did not simulate natural rainfall in drop size distribution, drop velocity,

or energy. Duley and Hays found that the increase in soil loss with each

unit increase in per cent slope became greater as the slope became

steeper. In a comparison of a silty clay loam with a sandy soil, the former

gave greater erosion loss on the flatter slopes and the latter on the

steeper slopes. Borst and Woodburn, using artificial rainfall at Zanesville, Ohio, found soil loss proportional to P 3 0 , where S is per cent of

slope. Neal, working with Putnam soil, found soil loss proportional to

So.7Z1.2,where Z is intensity in inches per hour.

The first comprehensive study of the effect of slope on soil loss was

published by Zingg (1940). He concluded that soil loss varies as the 1.4

power of the per cent slope. He used data by Duley and Hays (1932),

Diseker and Yoder (1936) and from a series of studies he conducted.

For better description of the relationship on the flatter slopes of Midwest claypan soils, and using data of Neal (1938), Smith and Whitt

(1947) proposed the equation:

R = 0.10

+ 0.21 s



where R is relative soil loss in relation to unity loss from a 3 per cent

slope and S is per cent of slope.

The authors (Smith and Wischmeier, 1957) evaluated the per cent

slope-soil loss relationship on the basis of plot data under natural rainfall secured by several investigators. 0. E. Hays, at the Upper Mississippi

Valley Conservation Experiment Station, Lacrosse, Wisconsin, obtained

data that covered 17 years of soil loss measurements from slopes of 3,

8, 13, and 18 per cent on Fayette soil. The plots were cropped to continuous barley for five years, followed by twelve years of corn-oatsmeadow rotation with across-slope tillage. A second-degree polynomial

gave a better least-squares fit to these data than did the logarithmic

relationship suggested by the earlier investigators. The constants of

parabolic equations derived from Hays’ data and from Zingg’s rainfall

simulator data were nearly identical when the latter were adjusted for

cropping effect. Data of Van Doren and Gard (1950) and Borst et al.

(1945), each comparing two slopes, if adjusted to conform with conditions at Lacrosse, also fit the equation derived from Hays’ data. The

combined data for the four studies gave a very good least-squares fit

to the equation:



A = 0.43

+ 0.30s + 0.043S2


in which A is soil loss and S is per cent slope.

In both the Hays and Zingg studies, runoff increased significantly

with increase in per cent slope although the two soils were quite different, one being a deep loess that sealed under raindrop impact and

the other a loam over a clay subsoil.

2. Length of Slope

Zingg (1940) concluded that total soil loss varied as the 1.6 power

of slope length; and the loss per unit area, as the 0.6 power. His conclusion was based on data from Bethany, Missouri; Guthrie, Oklahoma;

Clarinda, Iowa; Lacrosse, Wisconsin; and, Tyler, Texas. A group study

in 1946 under Musgrave (1947) proposed 0.35 as the average value for

the slope length exponent for soil loss per unit area.

In 1956, the results of statistical analysis of data for 532 plot years,

involving simultaneous measurements on two or more lengths of slope

under natural rainfall from 15 plot-study locations in 12 States, were

published by Wischmeier ef nl. (1958). The analysis showed that the

relationship of soil loss to slope length often varied more from year to

year on the same plot than it varied among locations. The magnitude

of the slope-length exponent appeared to be influenced by soil characteristics, rainfall pattern, steepness of slope, cover, and residue management. However, the data were not adequate to provide quantitative

evaluations of the factor-interaction efTects.

Average values of the slope length exponent for the different locations

varied from 0 to 0.9. Magnitude of the exponent appeared definitely to

be related to the effect of slope length on runoff. At Hays, Kansas, and

Temple, Texas, runoff decreased significantly with slope length. From

these data the over-all average value of the exponent did not differ

significantly from zero. However, in the final 7-year period of the 15year study at Temple, soil loss was proportional to LO3. At Guthrie,

Oklahoma, and for corn following bluegrass sod at Bethany, Missouri,

where runoff showed a significant increase with increased slope length,

soil loss varied as Lo.’ and Lo.9, respectively. At the other 11 locations

studied, slope length had no significant effect on runoff. In these studies,

the magnitude of the slope length exponent ranged from 0.27 to 0.60.

In seven of the studies cropping was continuous corn or cotton. In the

other four it was rotational, including row crops.

Average values of the length exponent computed for ten locations in

the Corn Belt and the Northeastern States did not differ significantly at

the 10 per cent level. The mean of the ten exponents was 0.46. A group



study at Purdue University in 1956, which included the authors (Smith

and Wischmeier, 1957), concluded that for field use the value of the

length exponent should be 0.5 2 0.1.


The greatest deterrent to soil erosion is cover. Cover and management

influence both the infiltration rate and the susceptibility of the soil to

erosion. The most effective vegetative cover is a well-managed, dense sod.

Fields easily eroded are usually those in poorly managed, cultivated

crops. The severest erosion occurs when erosive rainstorms coincide with

periods in the rotation when the soil surface is essentially bare. Sodbased rotations have played a predominant role in runoff and erosion

control. Also, many new tillage practices have been very effective.

Baver (1956) classified the major effects of vegetation on runoff and

erosion into five distinct categories: ( 1 ) interception of rainfall by the

vegetative cover; ( 2 ) decrease in the velocity of runoff and the cutting

action of the water; ( 3 ) root effects in increasing granulation and

porosity; ( 4) biological activities associated with vegetative growth and

their influence on soil porosity; and, ( 5 ) the transpiration of water

leading to subsequent drying out of the soil.

Bertoni et al. (1958) observed that final infiltration rates of a soil

varied with season of the year. They suggested that the higher infiltration rates during July were due to increased vegetal cover which protected the soil surface against sealing, to lowered surface moisture, and

to high soil and water temperatures. Higher infiltration rates during the

summer months than during other seasons also were observed by Beutner

et al. ( 1940), Horner and Lloyd ( 1940), and Borst et al. ( 1945). Woodward (1943) also found a linear relation between vegetal cover and

infiltration rate.

In a study of the relation of plant cover to infiltration and erosion in

Ponderosa Pine forests of Colorado, Dortignac and Love (1960) concluded that large pore space of the upper 2 inches of soil and the

quantity of dead organic materials were the two properties that accounted

for most of the variation in infiltration rates among cover types. The case

of soil dislodgement by rainfall impact varied with types of soil cover,

but soil origin and the amount of exposed bare soil were the main factors

affecting erosion.

In 15 years of soil-loss measurements, Horner (1960) found that the

kind and amount of cover provided during the winter season was the

dominant factor affecting runoff and erosion on Palouse silt loam at

Pullman, Washington, where large summer storms with high intensities



were few. Land seeded to winter wheat was more vulnerable to erosion

than any other winter cover condition common to the area. Sod-based

rotations provided more effective erosion control and soil organic matter

maintenance than did cropping systems without the meadow. Summer

fallowing caused the largest erosion losses and the most rapid depletion

of organic matter. Melting snow on frozen soil contributed significantly

to erosion hazards in this area. Exceedingly high soil losses from a gentle

rain falling on soil thawed to a depth of 4 inches were also reported by

Bay et nl. (1952) in \%‘isconsin.

Taylor and Hays (1960) found that a heavy mulch of chopped

cornstalks and manure provided excellent erosion control on corn following corn on Fayette silt loam of 16 per cent land slope. Whitaker

et nl. (1961) found that fertilization adequate to produce high crop

yields and large quantities of plant residues greatly reduced the formerly

serious soil and water losses from sloping claypan soils. Seedbed

preparation by subtillage, which left shredded cornstalks on or near the

surface, significantly reduced erosion losses even from very high intensity


Shredding the cornstalks increases their wintertime erosion control

value. In studies on Warsaw loam and Russell silt loam with about 4

per cent slope, hleyer and Mannering (1961b) found that soil loss

associated with shredded cornstalks was slightly less than half that from

cornstalks as left by mechanical pickers when rainfall was applied

artificially at 2.4 inches per hour for 60 minutes. However, one trip over

the shredded stalks with a disk significantly increased the soil content

of the runoff.

The importance of crop residues, cover crops, and sod-based rotations

in control of runoff and erosion in the Southern Piedmont soil area has

been shown by studies at Watkinsville, Georgia (Carreker and Barnett,

1949; Barnett, 1959). Analyzing data from the blackland prairie of

central Texas, Adams et al. (1958a) found that both runoff and soil

loss from corn managed by subsurface tillage methods were significantly less when the corn followed fescue than when it followed another

year of corn.

Krall et al. (1958) found that stubble mulch fallow provided better

erosion control than did other methods of summer fallowing in the

semiarid areas. In Wyoming, Barnes and Bohmont (1955) found that

land in grass as commonly left after haying operations absorbed water

at a rate 25 per cent lower than did land with “trashy” fallow. Both

conditions absorbed from 30 to 75 per cent more water in an hour than

did bare fallow land. Raking and baling loose straw from a stubble field

reduced water intake rate by more than 30 per cent. Burning the



residue reduced water absorption by nearly 50 per cent. Duley (1960)

reported that in a three-year grain rotation, plowed land lost 2.6 times

as much water and 4.8times as much soil in runoff as did stubble mulched

land over a 20-year period. The amount of water stored in the soil

during summer fallow depended on the amount of residue present on

the soil.

In a study by Mannering and Meyer ( 1961), 6% inches of simulated

rain were applied at 2% inches per hour on various quantities of straw

mulch spread over freshly plowed and disked wheat stubble on Wea

silt loam with 5 per cent slope. With no mulch, soil loss was 12 tons

per acre, but with 2 tons mulch per acre no runoff and erosion occurred.

With 1 ton of mulch per acre, soil loss was reduced to % ton per acre;

and with

ton of mulch, to about 1ton per acre.

Minimum tillage practices in which the corn-planting operation

coincides with or immediately follows plowing with moldboard plows,

and fitting operations with disk and harrow are omitted, have gained in

popularity in recent years. Minimum tillage provides erosion and runoff

control because of larger aggregate or clod size and decreased compaction

(Free, 1960a). Idltration is increased and erosion is reduced during

the highly vulnerable seedbed and crop establishment periods. Quantitative data on the magnitude of erosion-control benefits from minimumtillage practices are too limited to permit evaluating the interaction

effects of soils, slope, cover, and row direction.

Meyer and Mannering (1961a) compared plow-plant as a single

operation with planting on a seedbed fitted by two diskings with a

trailing harrow on Russell silt loam of 5 per cent slope that previously

had been in meadow. Runoff and soil losses were measured from three

5.2-inch applications of simulated rain at 2.6 inches per hour. In a test

about 2 weeks after seeding, losses from the minimum-tillage plots

averaged 63 per cent of the runoff and 52 per cent of the soil loss from

the fitted seedbeds. When the corn in both treatments was cultivated

to prevent surface crusting, reductions in both runoff and soil loss by

the minimum-tillage practice were still apparent even after corn harvest,

but the magnitude of the benefits decreased significantly with successive

cultivations and increased vegetative growth. When corn cultivations

were omitted on the minimum-tillage areas, both runoff and erosion

were greatly increased as a result of surface crusting. Soil loss from this

treatment was greater than that from the corn planted on fitted seedbed

and cultivated after emergence.

Swamy Rao et al. (1960) found that minimum tillage for corn

resulted in a higher rate of infiltration, less soil resistance to penetration,

lower bulk density, and less soil compaction due to tractor and implement



traffic. The data for fallow periods in rotations measured in other studies

(Wischmeier, 1960) show that the magnitude of the benefits attainable

with minimum-tillage practices may be expected to depend upon crop

sequence, quality of meadows in the rotation, and quantity of residues

plowed under.

Hays (1961) reduced total soil and water losses from rotations by

spacing corn rows 60 inches apart and interseeding legumes in the

corn after the second cultivation. Rains soon after the interseeding caused

increased soil and water losses due to the smoothing and packing of the

soil by the seeder, but after the seeding became established losses

were significantly reduced. The quality of meadows established by this

procedure was good.

Effects of specific cover, sod crop sequences, tillage practices, and

residue managements on field soil loss have been investigated in

cooperative USDA and State Agricultural Experiment Station plot studies

under natural rainfall at more than 45 locations in 23 States and Puerto

Rico. The data have generally been analyzed and reported by the study


Cover and management data have usually been reported either

on a crop-year basis or as rotation averages. Crop-year losses reflect the

effect of different crops or crop sequences, but do not show the cause of

favorable or unfavorable results. These can be more readily discerned

if the individual-storm data are analyzed on the basis of different stages

of crop growth.

To study the relations of cover, crop sequence, productivity level,

and residue management to soil loss, Wischmeier divided each crop row

into five crop stages, defined for relative uniformity of cover and residue

effects as follows: (1) rough falIow-turnplowing to seedbed preparation; ( 2 ) seedbed--first month after crop seeding; ( 3 ) establishment

-second month after crop seeding; (4)growing cover-from 2 months

after seeding until harvest; ( 5 ) stubble or residue-harvest to plowing

or new seedbed.

On these bases, soil losses from the cropped plots were compared with

losses from tilled continuous fallow under identical rainfall, soil, and

topographic conditions. Ratios of these losses, expressed as percentages,

were published in the form of a ready-reference table (Wischmeier,

19600).About a quarter million individual-storm soil loss measurements

were available for this study.

Highly sigdicant inverse correlations between crop yields and

erosion losses were apparent in the data. In gcncral, crop yields appeared

to provide a fair indication of the combined ef€ects of such variables as



density of canopy, rate of water use by the growing crop, and quantity

of crop residues.

Specific-year erosion losses from corn after meadow ranged from

14 to 68 per cent of corresponding losses from adjacent continuous corn.

The effectiveness of grass and legume meadow sod plowed under

before corn in reducing soil loss from the corn was, in general, directly

proportional to meadow yields. Its erosion-control effectiveness was

greatest during the fallow and corn-seedbed periods. The residual effect

of grass and legume mixtures was greater than that of legumes alone.

Direct comparisons of corn foIIowing first, second, and third years of

meadow, though limited, indicated that second-year meadow, when

allowed to deteriorate, was less effective than one year of meadow.

When succeeding meadows were more productive than first-year, they

were usually more effective in reducing erosion from corn in the following


When the corn residues were removed at harvest time, soil losses

from corn after corn were high and yields were ususally low. Soil

losses from the growing corn under these conditions were from 35 to

50 per cent of those from adjacent continuous fallow. Leaving the

cornstalks and plowing them under in spring significantly decreased

erosion during the following corn year as well as during the winter

period. Effectiveness of the corn residues turned under was directly

related to the quantity of residues available and was greatest during

the fallow and seedbed periods.

Erosion from clean-tilled cotton during the growing period appeared

to be about 50 per cent more than from corn under similar management,

soil, and rainfall. Soybean data were too limited to reveal a significant

difference in average annual erosion from beans in 42-inch rows as

compared with corn in comparable sequence. Erodibility of fallow soil

occurring for brief periods in crop rotations was influenced more by crop

sequence and the nature and quantity of residues turned under than

by the inherent characteristics of the soil itself.

The erosion control effectiveness of winter cover seedings depended

upon time and method of seeding, time of plowing, rainfall distribution,

type of cover seeded, and density of cover produced. Covers such as

vetch and ryegrass seeded early enough to attain good fall growth

and turned in April were effective in reducing erosion not only in the

winter months, but also in the following crop year (Uhland, 1958).

Small grain alone seeded in corn or cotton residues and plowed under

in the spring showed no residual erosion-reducing effect after the next

year’s corn or cotton planting.





Contour tillage and planting, strip cropping, terracing, waterways,

and gully control structures are generally included under erosion control

practices. Sometimes they are referred to as supporting practices.

Tillage practices, sod-based rotations, fertility treatments, and other

cropping-management practices discussed in the preceding section are

not included in this group, although it is recognized that they contribute

materially to erosion control and frequently provide the major control

in the farmer’s field. This discussion will be confined to the first three

of the listed practices.

Contour planting of crops has been in general an effective practice.

It functions, however, only to control runoff or scour erosion and then

only for those storms that are low to moderate in extent or until the

capacity of the rows to hold or conduct runoff is exceeded. In field

practice, key rows are either level or have a grade toward a waterway.

Because of land slope irregularities, row breakage is frequent with the

larger runoff storms. When this occurs losses may equal or exceed those

from up- and downslope planting (Smith et al., 1945; Moldenhauer and

Wischmeier, 1960).

The effectiveness of contour planting and tillage in erosion control has

varied with slope, crop, and soil (Smith and Whitt, 1947; Van Doren

et al., 1950; Van Doren and Bartelli, 1956; Tower and Gardner, 1953).

Its maximum effectiveness in relation to up- and downhill rows is on

medium slopes and on deep, permeable soils that are protected from

sealing. The relative effectiveness decreases as the land slopes become

either very flat or very steep. Row shapes as secured with listing increase

the channel capacity and , therefore, increase the average annual

effectiveness of farming on the contour. However, when row breakage

occurs, the results are disastrous because of the sudden release of large

quantities of impounded water. The ratio of soil loss with contouring

to that from up- and downhill rows is generally considered to be 0.5 for

slopes of from 2 to 7 per cent, 0.6 for slopes down to 1 per cent and up

to 12 per cent, 0.8 for 12 to 18 per cent, and 0.9 for 18 to 24 per cent.

With the development of land-forming machinery and techniques,

controlled row grades and shapes, designed to handle those storms

causing the bulk of runoff and erosion, became possible for those soils

amendable by management practices applied after reshaping.

Strip cropping-a practice in which contour strips of sod alternate

with strips of row crops-has proved to be a more effective practice

than contouring alone. The sod acts as filter strips when row breakage

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III. Basic Factors Affecting Field Soil Loss

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