DIVISION S-6-SOIL AND WATER MANAGEMENTAND CONSERVATION

Influence of Rainfall Energy on Soil Loss and Infiltration Rates: I. Effectover a Range of Texture1

W. C. MOLDENHAUER AND D. Ci LONG2

ABSTRACTSoil and water losses were determined from sieved

air-dry samples of five Iowa soils varying in texture usinga laboratory rain simulator with a 3-m. drop fall, a 4.85to 5.00 mm. drop diameter, and an intensity ranging from3.43 to 6.78 cm. per hour. The soil samples were containedin a pan 15.2 cm. deep, 30.5 cm. wide and 45.7 cm. longand tilted to a 9% slope. Runs were 90 minutes in length.The infiltration rate was the most important factor influ-encing total soil loss. Relative total soil losses for 90minutes of run at high intensity were silty clay > siltyclay loam > silt > loam > fine sand. At the low intensity,the positions of silty clay and silty clay loam and of siltand loam were reversed. Total soil loss varied with in-tensity, but infiltration was essentially constant over theentire range except on the fine sand. With equal waterloss the order of erodibility was fine sand > silty clay >silty clay loam > silt > loam. Total kinetic energyrequired to initiate runoff was constant over a range oftime and intensity.

THE VARIOUS SOIL-LOSS prediction equations usedthroughout the world have included a factor which,

hopefully, expressed the relative erodibility of soils whenall other factors were the same. The reasons soils differ inerodibility are not well established, however. Much hasbeen written concerning the effect of texture, structure,organic matter content, and other factors on erodibility,but this varies from region to region, and reports of studieson the effects of these factors determined independentlyare scarce in the literature.

Wischmeier in developing a soil loss equation (15)refers to the relative erodibility of soils as the K factor.There are thousands of soil types in the United States,and a K factor must be assigned to each soil if Wisch-meier's equation (15) is to be used universally. Since Kvalues have been experimentally derived for only a fewsoils, values for most soils must be assigned on the basisof field observations and from soil physical properties.Guides in assessing the effect of physical properties onsoil erodibility, however, are quite generalized and cannotbe used with confidence over wide areas. The present

Contribution from the Soil and Water Conservation ResearchDivision, ARS, USDA, and the Iowa Agr. and Home EC. Exp.Sta. Journal Paper No. J-4832, Project No. 1064. Presentedbefore Div. VI, Soil Sci. Soc. Am., Aug. 20, 1962, Ithaca, NewYork. Received May 4, 1964. Approved June 30, 1964.

2Research Soil Scientist, USDA, and Associate Professor ofAgronomy; and Graduate Assistant, Agronomy Department,Iowa State University, Ames, Iowa, respectively. The authorswish to express appreciation to Mr. Donald Law for his con-tribution in modifying and improving equipment and proce-dures during the course of the study.

3Moldenhauer, W. C. A procedure for studying soil charac-teristics using disturbed samples and simulated rainfall. Pre-sented at the winter meetings of the American Society ofAgricultural Engineers, Chicago, Dec. 10-13, 1963. Paper No.63-730. (Presented for publication).

study was undertaken to obtain detailed information onthe relationship of physical properties of a soil to itserodibility and to obtain relative erodibility values toguide in assigning K values to soil types.

The wqrk reported here is an attempt to study in detailthe effect of soil texture on infiltration and erosioncharacteristics of five Iowa soil types by using a laboratoryrain simulator and disturbed soil samples.

DESCRIPTION OF SOILSA general description of each soil used in this study is given

in North Central Regional Publication No. 76 (16). All soilsamples used came from sites that had been in meadow con-tinuously for a number of years.

Mechanical analyses and dry aggregate size distributions ofthe soil samples are given in Table 1. Luton silty clay, eventhough it occurs in an essentially nonerodible position, wasused because it has a high clay content and controlled croppingconditions have existed at the site for a number of years. Ofsignificance is the low clay content and high percentage ofcoarse silt in the Ida silt. Dry aggregate size distribution inthe Luton, Marshall and Kenyon soils was not greatly differ-ent. The Ida silt contained fewer aggregates of the 8 to 2.83mm. size and more of the < 0.5 mm. size than the Luton,Marshall, and Kenyon soils. Hagener fine sand contained asmall percentage of aggregates > 0.5 mm.

PROCEDUREDevelopment of the procedures described here has been

reported by Moldenhauer.3 The soil sampling procedure wasadapted from DeLeenheer and DeBoodt (3) in Belgium.Samples Were from areas that had been flooded with 10 cm.of water, covered with polyethylene film, and left for 48 hours(or taken 48 hours after a rain sufficient to bring the surface10 to 15 <im. of soil to field moisture capacity).

The 5- to 10-cm. layer was sampled, broken down by hand,and sieved within 24 to 48 hours through an 8-mm. square-hole sieve. Samples were air dried before runs were made.

Samples were gently placed in 30.5-cm. by 45.75-cm. metalpans to a depth of 15 cm. (Fig. 1A). Filling pans this wayresulted in average bulk densities of 0.94 g. per cc. for Lutonsilty clay, 1.00 for Marshall silty clay loam, 1.10 for Kenyonloam and Ida silt and 1.50 for Hagener fine sand. The surfacewas leveled to correspond with the lip of the runoff concen-trator. Pans were tilted to a 9% slope. Splash boards 46 cm.high were;placed over the pans. (Fig. IB).

Simulated rain was applied with an applicator described byMutchler and Moldenhauer (10). Drop diameter varied be-tween 4.84 and 5.00 mm. over the range of intensities used,

Table 1—Mechanical analysis and dry aggregate sizedistribution of soil samples.

___Fraction size, mm.___2. O- 0. 05- 0. 02- < 0. 0020.05 0.02 0.002

Aggregate size, mm.8-

2. 832, 83- < 0. 50. 5

Organiccarbon*

____ t

Luton silty clayMarshall silty

clay loam:

Ida siltKenyon loamHagener fine sand

1

12

3290

18

4366261

28

242219

2

53 43

321123

7

502746

2

52

45424213

5

5311285

2.73

1.881.162.150.86

By method 0f Meblus (9).

813

814 SOIL SCIENCE SOCIETY PROCEEDINGS 1964

Fig. 1—(A) Soil pan ready for a run. (B) Soil pan with splash interceptor, discharge measuring pan on left, and splash-collecting apparatus from 7.6-cm.-diameter samples in foreground.

LOWER COLLAR

H

O O00

O OO O

°oS<

-7.6cm. UPPERCOLLAR

fT ROUGH

ILES FORSURFACE DRAINAGEDRAINAGE ——

TUBE

(-7.6 cm. CYLINDERWITH SOIL

"<— SCREEN

Fig. 2—Cross section view of splash apparatus.

and intensity was constant during each run. Height of fallwas 3 m. Samples of runoff material were taken at 5-minuteintervals after runoff began. Material from the splash boardswas returned to the back of the sample with a squeegee at5-minute intervals after the first runoff sample was taken.Alum was used to settle the soil material in the runoff water.After standing overnight, the runoff water was decanted andmeasured. The soil material was dried and weighed.

Relative splash losses were determined from sieved soilplaced in 7.6-cm. cylinders around which 10.2-cm. cylinderswere fitted (Fig. 2). The soil surface was relatively free fromwater and suspended material during the run because smallholes were drilled near the top and down the sides of the7.6-cm. cylinders. Splashed soil was collected on the sides

and in a trough at the bottom of the larger tube. Excess ma-terial from the trough drained into an Erlenmeyer flask. Atthe end of the 90-minute run, splashed material was washedinto a weighing can, dried, and weighed.

In runs in which the energy was removed from the sim-ulated rainfall, three screens (1/16-inch mesh) spaced 3.8 cm.apart were set level just over the soil surface. Exploratorywork showed that three screens arranged in this way willdissipate most of the kinetic energy of the raindrops beforethey strike the soil surface.

RESULTSSoil loss and infiltration rate curves with time are

typical of rate curves for similar soils studied by Schmidtet al. (13), Adams et al. (1), and others. These curvesshow a rapid decline in infiltration rate and rise in soil-loss rate as soon as runoff begins, after which an equi-librium rate is soon reached. Final soil loss and infiltrationrates at two intensities and peak rate at the highest in-tensity are shown in Table 2. Elapsed time before runoffbegan is given in Table 3. Peak and final soil loss rateswere highest from Luton silty clay with Marshall siltyclay loam ranking second. Final infiltration rates were

Table 2—Final and peak soil loss rates and final infiltra-tion rates for 5 Iowa soil types at 2 rainfall intensities.

Soil type 3. 43 cm. /hour IntensityFinal

soil lossg.

Luton sllty clayMarshall sllty

clay loamIda siltKenyon loamHagener fine sand

. /5 min.7.2

6.02.14.00.0

FinalInfiltrationcm. /hour

0.65

0.581.941.333.43

6. 78 cm. /hour IntensityPeak

soil lossg. /5 min.

21.3

17.012.312.816.1

Finalsoil loss

g. /5 min.19.2

13.49.49.7

10.7

Finalinfiltrationcm. /hour

0.85

0.731.481.102.55

Table 3—Kinetic energy of raindrops applied at high andlow intensity before runoff began, l,300-sq.-cm. area.

Soil type

Ida siltMarshall sllty clav loamKenyon loamLuton sllty clayHagener fine sand

Time and energy before runoff began3.43 cm.

min.33.536465690»

/hourjoules57.361.678. 795.8

154.1

6. 78 cm.min.1617.5232646t

/hourjoules

54.559.678.388.6

156.6* Total for storm; no runoff occurred,t Total for 90-mfnute storm was 303. 9 joules.

250r

200

S.

MOLDENHAUER AND LONG: INFLUENCE OF RAINFALL ENERGY

A HAGENERO IDAA KENYON• MARSHALLn LUTON

4.0 4.5 5.0 5.5 6.0 6.5INTENSITY (CM. PER HOUR)

7.0

Fig. 3—Total soil losses in 90 minutes from sieved samplesof 5 Iowa soils over a range of intensities.

10-

9

8

§6

ON SOIL LOSS AND INFILTRATION: I.

t HAGENERq IDA* KENYON«j MARSHALLp LUTON

•—•"

815

-••-M

4.0 4.5 5.0 5.5 6.0INTENSITY (CM. PER HOUR)

6.5 7.0

Fig. 4—Total infiltration in 90 minutes into sieved sam-ples of 5 Iowa soils over a range of rainfall intensities.

somewhat lower from Marshall than from Luton. At the6.78-cm.-per-hour intensity, peak and final soil losses werehigher from Hagener fine sand than from Kenyon loamand Ida silt. At the 3.43-cm.-per-hour intensity, no soilor water loss occurred from Hagener. Final soil loss waslower from Ida than from Kenyon at the low intensity butessentially equal at the high intensity.

The relationship between total soil loss and rainfallintensity for each of the five soils is given in Fig. 3. Theorder of soil loss at low intensities was silty clay loam> silty clay > loam > silt > fine sand, whereas, athigh intensities, the order was silty clay > silty clay loam> silt > loam > fine sand. The relationship betweensoil losses and intensity was markedly curvilinear for theIda silt and Hagener fine sand.

The relationships among runoff, infiltration, and in-tensity are given in Fig. 4. Infiltration rate was essentiallyconstant with different intensity on all soils except Hagenerfine sand. The order of infiltration rates was fine sand

250

20O

(O•s.ccO

150-

<ngiooh

oU) 50-

A HAGENERo IDAA KENYON• MARSHALLa LUTON

2 3 4 5RUNOFF (CM.)

Fig. 5—Relationship between total soil loss and runoff fora 90-minute period for sieved samples of 5 Iowa soils.

> loam > silt > silty clay > silty clay loam. Runoffwas greater from Kenyon loam than from Ida silt at in-tensities below 4.70 cm. per hour but at higher intensitiesit was less.

A very high correlation between soil loss and runoffis shown in Fig. 5. These curves show also that, for equiv-alent amounts of runoff, the order of erodibility was finesand > silty clay > silty clay loam > loam > silt. TheIda silt and Kenyon loam reversed positions as theamount of runoff increased.

Detachability of soil particles as indicated by the rela-tionship between splashed material and intensity is shownin Fig. 6. These curves show greatest detachment fromHagener fine sand and Luton silty clay. Curves for theother three soils show Marshall silty clay loam and Idasilt to be lowest at low intensities. Splash from Ida siltincreased more rapidly than that from Kenyon loam orMarshall silty clay loam as intensity increased.

Kinetic energy of water drops applied to each sampleup to the point at which runoff began is shown in Table3. The energy applied was nearly the same at high and

343230-28

22S 20

A HAGENERo IDA* KENYON• MARSHALLo LUTON

3.5 4.0 5 5X) 5 ^ 5 6 ^ 0 6 ~ ! 5 ~INTENSITY (CM. PER HOUR)

7.0 7.5

Fig. 6—Detachment from a 7.6-cm. diameter sievedsample during a 90-minute run over a range of intensi-ties.

816 SOIL SCIENCE SOCIETY PROCEEDINGS 1964

Table 4—Soil loss with full energy from a 7.37-cm.-per-hour rain and with energy removed with screens.

Soil type Soil lossFull energy_____Energy removed

- g. /90 minutesIda siltMarshall sllty clay loam

140160

low intensity for each soil. Although no runoff occurredat the low intensity, kinetic energy applied to the Hagenerfine sand at this intensity was nearly equal to that appliedat the 6.78 cm.-per-hour intensity up to the point at whichrunoff began.

To assess the effect of runoff on soil loss independentof raindrop energy the energy was removed from thedrops with a set of screens. To get runoff under theseconditions a reduced suction was used. This suction wasmanipulated on a 1-inch deep sample to give the runoffnormally obtained with a 7.37-cm.-per-hour intensity rainwith full energy. Results are shown in Table 4.

DISCUSSIONSmith and Wischmeier (14) point out that "soil proper-

ties which influence soil erodibility by water may begrouped into two types: (a) those properties that affectthe infiltration rate and permeability; and (b) those prop-erties that resist the dispersion, splashing, abrasion, andtransporting forces of the rainfall and runoff." Significanceof surface sealing caused by drop impact in reducinginfiltration has been investigated and discussed by Duley(4) and Ellison (6) in past years and more recently byMcIntyre (7), Pereira (11) and Rose (12). The effect ofsurface sealing on erosion was investigated by Ellison (6)and McIntyre (8).

Evident in the present study of small-area disturbedsamples is the relationship between soil loss and infiltra-tion rates of the soils when they are subjected to raindropaction. Soil losses from these five soils of widely varyingtexture were, in most cases, in reverse order of infiltration.There was evidence, however, that the second factor ofSmith and Wischmeier, noted above, accounted in partfor soil loss differences between soils. Losses from Lutonsilty clay and Hagener fine sand, for example, increasedmore rapidly than those from the other soils as runoffincreased. In Fig. 5, soil loss due to detachability-trans-portability is shown independently of the infiltration (orrunoff) effect. These curves indicate that Luton andHegener have the highest detachability-transportabilityindices, while the Marshall silty clay loam is intermediateand the Ida silt and Kenyon loam are lower.

Splash results indicate that relative detachability ac-counted for much of the detachability-transportabilityindex of Hagener fine sand and Luton silty clay. ForMarshall silty clay loam, detachability accounted for lessof this index. Peak soil loss rates shown in Table 2 appearto represent a quantity of loose material still present whenthe surface had been compacted enough for runoff tostart. This material had usually disappeared by the timean equilibrium soil loss rate was reached.

Luton silty clay and Hagener fine sand had very similardetachability and early run infiltration characteristics. Thevery stable aggregation of Luton, resulting from a highclay content and the fact that the sample was taken frommeadow, caused it to act like a sandy soil, especiallyearly in the run. As raindrops continued to act on theLuton, however, a seal formed from the very small par-ticles released as the aggregates were broken down. Thisresulted, ultimately, in a high runoff rate with a corre-sponding high soil loss rate. Detachment remained highfrom the Luton partly because of the swelling of the clay,which caused particles to be released throughout the run,

and partly because the aggregates from the meadow soildid not break down completely and were available fordetachment and transport.

From a practical standpoint, of course, the fact thatHagener fine sand is easily detached and highly erodibleis of little consequence since the infiltration rate remainshigh and keeps the runoff low. Thus, in most cases, thereis not enough runoff to cause serious erosion of this soilin the field. If conditions were such that infiltration intoa loamy sand were restricted by a clay horizon very nearthe surface, however, the highly erodible nature of soilsof this texture could become important.

The results presented here, generally, and those of Fig.3 especially might seem to indicate that the soil losseswere actually the result of runoff and that raindrop actionwas not involved except in soil sealing. It was shown byBorst and Woodburn (2) and emphasized by Ekern (5)that shallow flow of water is ineffective in transportingsoil unless raindrops are present to impart turbulence tothe sheet of water. This was verified when most of theenergy was removed from the raindrops with a set ofscreens. Under these conditions a simulated rain of 7.37cm. per hour for 90 minutes on a 4% slope producedlittle soil loss from Marshall or Ida soils (Table 4). Theseresults reemphasize the role of raindrops in acceleratingsoil erosion.

The present study emphasizes the overriding importanceof the infiltration rate in determining what the soil lossrate will be even from small areas where only sheet flowis involved. A soil that has been sealed once by raindropaction and then dried will seal again rapidly when rainbegins and will reach equilibrium soil loss and infiltrationrates quickly unless the seal has been broken up betweenrains (12, 13).

The infiltration rate-time relationship is complex. Rose(12) pointed out that two factors are responsible fordecrease in the infiltration rate with time during a rain-storm. One is the decrease in vertical hydraulic gradientwith wetting of the soil profile. The decrease takes placewhether the soil is wetted by rain or by flooding. Thesecond factor is surface sealing, which is of great impor-tance only when the energy of water drops is involved.In the present study, when the energy was removed fromthe drops with a set of screens, no runoff occurred fromany of the soils during a 90-minute run at highest intensity.Thus, the decrease in infiltration rate was from surfacesealing which formed, in effect, a thin surface layer of asoil with very different characteristics from the one pres-ent at the beginning of the run or from the one whichwould have been present had the water been added witha minimum of disturbance to the soil surface.

The nearly constant infiltration as intensity increasedcan be explained by the fact that a certain amount ofkinetic energy as rainfall was required to bring aboutsurface sealing and to initiate runoff under the conditionsof this experiment (Table 3). In this procedure, the dropsize was nearly constant so the number of drops requiredto cause sealing was apparently the same whether theycame within the first 15 minutes of the run or did notcome for 30 or more minutes. Thus, the bulk of the infil-tration took place before runoff began. At the higherintensities, infiltration rate dropped faster than at lowerintensities, and this tended to further equalize the amountof infiltration occurring at the low and high intensitiesafter runoff began. Only in the Hagener fine sand wasinfiltration at any intensity limited by the amount of wateravailable for infiltration. In the other four soils, even thelowest intensity used was high enough to cause runoff.

MCHENRY AND DENDY: MEASUREMENT OF SEDIMENT DENSITY i BY ATTENUATION OF GAMMA RAYS 817

Measurement of Sediment Density by Attenuation of Transmitted Gamma Rays1

J. ROGER MCHENRY AND PARRIS E. DENDYS

ABSTRACTThe measurement of density by observing the attenua-

tion of transmitted gamma rays was adapted to the meas-urement of sediment density. A dual probe, utilizingtransmission techniques, was evaluated for use in measur-ing the density of sediments collected from runoff of plotsor small watersheds. The equipment consists of a 7-mc.cesium137 source placed in one access tube and a probecontaining a Nai scintillation crystal, photomultiplier tube,and preamplifier in the second tube. The high-voltagepower supply and sealer ratemeter for readout are suppliedin a portable unit powered by silver-cadmium recharge-able dry cells. Cesium137 gamma radiation is measured byemploying electronic discrimination to eliminate all pho-tons of energies below about 0.65 Mev. Sediment densitieswere measured in silt boxes installed on a small watershedat the North Mississippi Branch Experiment Station, HollySprings, Mississippi, following laboratory calibration tests.The dual probe employed measured densities of sedimentsto within 0.01 absolute density units. The vertical resolu-tion of the apparatus is less than 1 inch so that densitymeasurements of thin layers and of materials at interfacescan be made. In one field test sediment weights, of theorder of 8,500 pounds, computed for a silt box on thebasis of dual probe density determinations varied lessthan 1% from a gravimetric determination. Changes insediment density with time, with additional increments ofsediment, and with storms were effectively measured.

THE PRODUCTION or SEDIMENT from small agriculturalwatersheds has been investigated at many locations in

recent years. These studies are implemented by the instal-lation of sediment-retaining structures, known as silt boxes

or sediment basins, to trap the major portion of thecoarser erosion products. Associated with these structuresare various runoff measuring devices and proportionalsampling instruments (3). Complete measurement andremoval of the sediment accumulated in the silt boxes isa laborious and time-consuming task. Furthermore, inorder to evaluate the sediment produced on a storm basis,measurements must be made and the silt boxes cleanedafter each major storm. If the sediment collected in thesesilt boxes could be measured accurately in situ, cleaningoperations Could be maintained on a prearranged schedule.Additional; accumulations of sediment could be readilymeasured should a particular silt box not be cleanedbefore the next storm event. Moreover some idea of thenature of the erosional and transportational processesinvolved might result if accurately measured density pro-files of accumulated sediments were available.

Nuclear itechniques have been used in industry for thenondestructive measurement of density and thickness ofvarious materials (4). Two types of instruments for thispurpose haye been developed utilizing the attenuation ofgamma rays. One type of instrumentation uses a reflec-tion technique; the other, transmission.

The reflection, or scatter, instrument contains the radio-active source and detection device in a single unit, orprobe, and! measures the radiation reflected back to thedetector. This instrument is commonly called a singleprobe or gamma probe (1). It is inherently less sensitivethan the transmission method because of the shieldingnecessary to prevent the direct transmission of the gammarays from the radioactive source to the detector and it isunable to distinguish densities of layered sediments orof materials near an interface (5, 6).

The transmission instruments contain the radioactivesource and!the detector in individual units (probes) sepa-rated by the material to be tested. Van Bavel (8) describesthe use of the transmission technique to measure thedensity of soils. The transmission method also has beenapplied to measuring the in situ densities of reservoir sedi-ments (7).

The purpose of this paper is to evaluate a portablegamma ray transmission instrument for the measurementof in situ densities of sediments accumulated in silt boxesor similar catchment basins.