DIVISION S-6—SOIL AND WATER MANAGEMENTAND CONSERVATION

Interdependence of Water Drop Energy and Clod Size on Infiltration and Clod Stability1

W. C. MOLDENHAUER AND W. D. K.EMPER2

ABSTRACT

The decline of intake rates as successive increments of waterdrop energy were applied to various sizes of soil fragments oftwo Iowa and two Colorado soil types is presented. There wasa wide spread between intake rate of the 8- to 20-mm sizerange and those of all other sizes after the first increment (1.25cm as 5-mm drops) was applied. The intake rate of the 8- to20-mm size declined rapidly after the first or second increment.Final intake rates were not correlated with aggregate stabilityin this study. There was a negative correlation between claycontent and final intake rates. Final intake rates after disinte-gration of large soil fragments in many cases were lower thanwhen initial size ranges were small.

Additional Key Words for Indexing: soil crusting, soil sur-face sealing, runoff, soil erosion.

OFTEN OVERLOOKED in studies of the erosion process arechanges that take place in a tilled soil before runoff

begins (Ellison, 1945). In this process raindrop splashplays a large part even though net movement in one direc-tion may be slight. Before runoff can begin on a tilled soil,large surface pores usually must be filled with soil materialto a point where intake rate is exceeded by rainfall rate.This is accomplished by removal of material from the largeclods by shearing action of the raindrops. The amount ofmaterial removed is increased as the clod becomes weakerdue to wetting. Material detached by raindrops is carriedinto lower pores by infiltrating water. Finally, the materialthus washed in reduces the surface pore sizes to a pointwhere intake rate is exceeded by rainfall rate.

The amount of energy required to initiate runoff is afunction of the size of clods in the tilled soil. The smallerthe initial size of the clods, the less energy will be requiredto break down the clods and the less inwashing will berequired to reduce the pores to a critical size.

In the field of soil and water conservation, one of themost important developments in recent years has been therealization of the benefits of reduced tillage. A roughcloddy surface between rows enhances water intake andcontributes to surface detention of water even after intakehas been reduced by pore sealing (Larsen, 1964). Cumu-

1 Contribution from the Soil and Water Conservation Re-search Division, ARS, USDA; the Iowa Agricultural and HomeEconomics Experiment Station; and the Colorado AgriculturalExperiment Station. Journal Paper no. J-5919, Iowa Project no.1064. Published with the approval of the Director, ColoradoAgricultural Experiment Station as Scientific Series paper 1313.Received Sept. 3, 1968. Approved Oct. 28, 1968.

2 Soil Scientists, USDA, and Professors of Agronomy at Iowaand Colorado State Universities, respectively.

lative water intake up to the time of initial runoff has beenshown to be 20 times higher on rough-plowed soil andmore than three times higher on plowed, disked and har-rowed soil than on untilled soil (Burwell et al., 1966).

The work cited has led to the belief that clods muchlarger than the sizes used to measure stability in wet sievingprocedures are important. These larger soil fragments havebeen described by Chepil (1953), who investigated theirimportance in wind erosion control. We are now convincedthat these units are equally important in delaying the timewhen runoff begins and thus help control the soil and waterloss (Free, 1960). Moldenhauer et al. (1967) suggestedthat the rapid decrease in infiltration resulting from lowstructural stability and consequent surface sealing of Bhorizon material may be largely offset by the greater clod-diness of this material when it is tilled.

This study was carried out to determine the rate of de-cline of intake rates as increments of water drop energywere applied to various sizes of soil fragments, and to dif-ferent soils and under different conditions of structuralstability within soils.

MATERIALS AND METHODS

Two Iowa and two Colorado soil series were used. The Iowasoils were the Marshall—a silty clay loam soil containing 36%clay, 62% silt, 2% sand; and the Ida—a silt loam soil con-taining 12% clay, 80% silt, 8% sand. A general description ofthese soils and the physiographic region in which they occur isgiven by Oschwald et al. (1965). The Colorado soil series wereNunn—a clay loam soil which contained 38% clay, 32% silt,30% sand (the Nunn from meadow contained 31% clay, 41%silt, 28% sand); and the Weld—a loam soil containing 23%clay, 40% silt, 37% sand.

Soil fragments in the various size ranges were placed in7.6-cm diameter lucite cores 7.6 cm in depth. The top wasbeveled so a water drop hitting the edge would splash away fromthe sample. Small holes were drilled down each side of the lucitecore to drain away surface water that would otherwise collectand affect the drop energy (Palmer, 1963). The drop applica-tor used was built and described by Adams et al. (1957). The16.5-cm iron pipe they used for a windshield was extended togive a drop fall of 2.44 m. After 0.0256 joules/cm2 of energyhad been applied as 1.25 cm of water in 5-mm diameter drops,the sample in the cylinder was placed on a bed of sand and thedrainage holes at either side of the lucite core were sealed withelectricians' tape. Water intake rate was determined by keepinga constant 3- to 5-mm head of water on the sample and periodi-cally measuring the amount of water that had moved throughthe soil surface. Samples were then allowed to stand for 24hours on a sandbed which maintained approximately 25 cm ofsuction at the bottom of the sample. During the next three days,0.0256, 0.0512, and 0.1023 joules/cm2 of water drop energywere added on the 1st, 2nd, and 3rd day, respectively, and thesame procedure for measuring infiltration was repeated eachday. Several variations in the above procedure were carried out.

297

298 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

In one samples were dried thoroughly between increments; inanother 0.2047 joules/cm2 was applied as one continuousapplication rather than as increments; in another 8- to 20-mmclods were placed to a depth of 2.5 cm (1 inch) over < 0.5,0.5 to 2, and 2 to 4.7-mm size clods, respectively. Stability ofsmall aggregates was determined by the method of Kemper(1965).

RESULTS AND DISCUSSION

The importance of large soil fragments in keeping intakerate of the soil surface high has been emphasized by Larsen(1964) and Burwell et al. (1966). In western Iowa theamount of energy in the average rainfall between cornplanting and complete canopy cover approximates 0.2joules/cm2. Results from the present study (Fig. 1-5)show a high intake for all soils for the 8- to 20-mm sizerange through the first 0.0256-joule/cm2 application, andthrough the second 0.0256-joule/cm2 application for theIda silt loam and Weld loam from corn and Nunn clayloam from meadow. By the time 0.1024 joules/cm2 hadbeen applied, intake rate of the 8- to 20-mm size rangevaried from 0.4- to 3.0-cm/hour among the soils. The rain-fall expected for a 1-year frequency in 1 hour is 3.3 cmfor Iowa and 1.8 cm for east central Colorado (5-year fre-quency—5.3 cm for Iowa. 3.3 for Colorado) (Hershfield,

• 0.5-2mm.±2-4.7mm.•4.7-8mm.08-20mm.

1961). The only soil with a 3.0-cm/hour intake rate afterthe third increment had been applied was the Ida (at the8- to 20-mm size range); all others were below a 1.5-cm/hour intake rate.

Intake rate was not consistently related to size of aggre-gates smaller than 8 mm. There was much crossing overof curves and actually a completely inverse relation be-tween intake rate and beginning size of fragments by thetime 0.2047 joules/cm2 of drops had been applied onsome soils. The greatest difference due to size in the earlystages was between the 8- to 20-mm and all other sizeranges. The one exception was the 4.7- to 8-mm size inthe Nunn clay loam from meadow.

Higher intake rates after 0.2047 joules/cm2 had beenapplied were found when water drops were applied con-tinuously using the apparatus designed by Mutchler andMoldenhauer (1963). Results are given by Moldenhauerand Koswara (1968). The continuous application of 90min may not have allowed the soil to wet as much andthus it remained more stable than where water was usedto measure the intake rate between applied increments.A higher intake rate after 0.2047 joules were appliedwas found where samples were dried thoroughly be-tween applied increments (Table 1) and also where theentire 0.2047 joules/cm2 was applied continuously (Table

.0256 .0512 .1023 .2047CUMULATIVE ENERGY (joules per sq cm.)

Fig. 1—Infiltration rate decrease with applied water-drop en-ergy to fragments of Marshall silty clay loam from continuous

.0256 .0512 .1023 .2047CUMULATIVE ENERGY (joules per sq. cm.)

Fig. 2—Infiltration rate decrease with applied water-drop en-ergy to fragments of Ida silt loam from continuous corn.

MOLDENHAUER AND KEMPER: WATER DROP ENERGY, CLOD SIZE, AND INFILTRATION 299

Table 1—Infiltration rates after each increment of energyadded when samples were dried thoroughly between appli-

cations. (Initial aggregate size 2- to 4.7-mm, 0.0256joules/cm2 — 1.25 cm rain)

Total energy appliedjoules/cm2

0.0260.0510.1020.205

Infiltration ratesMarshall A horizon

cropped

4.903.822.140.72

Nunn A horizoncropped

3.961.180.380.28

2) rather than in four increments. Final rates in Tables 1and 2, however, were far below those found by Molden-hauer and Koswara. This leads to the speculation thathaving a 9% slope and allowing material to be carried overthe surface and off the area in their study may have playeda large part in keeping their intake rates high compared tothose in Fig. 1 and 3 and in Tables 1 and 2.

Final intake rates were not related to aggregate stabilityin this study. Table 3 shows the 1- to 2-mm aggregates ofIda and Weld to be less stable than those of Marshall andNunn, but final intake rates of the 0.5- to 2-mm range offragments were considerably higher for Ida than for Mar-shall and Nunn, as shown in Fig. 1, 2, 3, and 5. There isa reasonably good negative correlation between clay con-tent and final intake rates. We have always assumed that

.0256.0512 .IO23 .2047CUMULATIVE ENERGY (joules per sq. cm.)

Fig. 3—Infiltration rate decrease with applied water-drop en-ergy to fragments of Nunn clay loam from corn.

Table 2—Final infiltration rate when all energy (0.205joules/cm2) was applied at one time

Size of aggregates, mm20-8 2-0.5

Marshall croppedNunn cropped

0.420.36

- cm/hour-0.980.40

Table 3—Stability of aggregates by a wet sieving method

_____Soil type_____________________________Percent aggregationMarshall silty clay loam

A horizon-croppedB horizon-croppedA horizon-continuous meadow

Nunn silt loamA horizon-croppedB horizon-croppedA horizon-meadow

Ida silt loamA horizon-cropped

Weld loamA horizon-cropped

949797

838179

64

74

greater aggregate stability meant a priori higher infiltrationrates. Our findings show that while clay content tends toincrease wet sieve aggregate stability (Kemper and Koch,1966), it apparently decreases final infiltration rates. Theimpact forces of raindrops falling on the wetted soil frag-ments destroy the normal soil structure almost completelyand leave mainly dispersed silt particles or silt size clay

.0256 .0512 .1023 .2047CUMULATIVE ENERGY (joules per sq. cm.)

Fig. 4—Infiltration rate decrease with applied water-drop en-ergy to fragments of Weld loam from corn.

300 SOIL SCI. SOC. AMER. PROC., VOL. 33, 1969

aggregates on the surface. This phenomena has beendescribed in detail by Moldenhauer and Koswara (1968).

If we assume that the median size silt particle is 0.006mm in diameter, the effective pore size through such amaterial is 0.003 mm in diameter, and the hydraulic forceacting across the sealed layer is four times the gravitationalfield, then the intake rate should be approximately 1cm/hour. Consequently, it would seem that rates lowerthan 1 cm/hour must be associated with plugging of poreswith particles smaller than silt size.

One important consideration here is that the soil surfaceshown as a very thin seal in photomicrographs is in a stateof constant disturbance during rainfall. It reaches the ob-served state only after rain has stopped and surface waterhas drained downward into the soil.

Evidence is presented here to show that equilibriumintake rates after disintegration of large soil fragments maybe lower than those when starting size fragments weresmaller. While evidence of this is shown in Fig. 1 through5, the most conclusive evidence is shown in Fig. 6 and 7.There was little difference in final intake rate when materiallying below the 8- to 20-mm size was less than 0.5 mmor from 0.5 to 2 mm, but the rate was considerably lowerwhen the material below was 2- to 4.7-mm. These data

200r-

indicate that the degree of discontinuity between the dis-integrated surface layer and the aggregated soil below it isan important factor in determining infiltration rate. Thelayer below the surface is apparently unsaturated, and largepores reduce the effective cross-sectional area throughwhich water can be pulled from the bottom of the disinte-grated layer. Consequently, the hydraulic pressure gradientpushing the water through the disintegrated layer is low-ered by any soil structure that contains pores larger thana few tenths of a millimeter in diameter. Amemiya (1965)found the capillary conductivity of 3- to 5-mm aggregatesto be lower than that of 0.5- to 1-mm aggregates at thesame moisture content.

PRACTICAL APPLICATION

This work reemphasizes some practical points in keep-ing water intake at a high level. Isolating large clods at thesurface which will persist over the period from plantingto full crop cover is a good possibility. A combination ofporosity and surface roughness needs to be found that willdelay runoff for approximately 2 months in the Corn Belt.This is the concept of designing a soil condition to with-stand a design storm or storms (Free, 1960).

SEE BELOW 8-20mm• 05mm.A 0.5—2mm.D 2 -4.7mm.

J__L

.0256.0512 .1023 .2047CUMULATIVE ENERGY (joules per sq. cm.)

Fig. 5—Infiltration rate decrease with ap-plied water-drop energy to fragmentsof Nunn clay loam from meadow.

.0256.0512 .1023 .2046CUMULATIVE ENERGY (joules per sq. cm)

Fig. 6—Infiltration rate decrease in a2.54-cm layer of 8- to 20-mm materialoverlying 5.08 cm of finer material onMarshall silty clay loam from continu-ous corn.

SIZE BELOW 8-20mn• 0.5 mm.A O5-2mm.D 2-47mm.

.0256.0512 J023 .2046CUMULATIVE ENERGY (joules per sq.cm.)

Fig. 7—Infiltration rate decrease in 2.54cm layer of 8- to 20-mm material over-lying 5.08 cm of finer material on Nunnclay loam from continuous corn.

MOLDENHAUER AND KEMPER: WATER DROP ENERGY, CLOD SIZE, AND INFILTRATION 301

topography steeply sloping surfaces do not seal as readily asflat surfaces. If this microtopography is maintained throughtillage or mulching, it should be very effective in watercapture and erosion control.

Fig. 8—Surface seal with undisturbed fragments beneath—Idasilt loam from continuous corn.

Soil fragments below the surface seal of all soils used inthis study preserved their identity even after repeated wet-ting and drying (Fig. 8). This also was shown by Chepil(1953). This is significant in considering methods of till-age, such as mulch tillage, that keep the direct impact ofthe raindrop from reaching the soil surface.

Tillage of any kind influences the microtopography.Rough plowing leaves a surface composed of large clods.This produces a configuration composed of many steepsloping surfaces. Listing and ridge planting produce slopesas great as 50% between rows. Larsen (1964) has pointedout that dispersed particles from soil peaks are eroded intodepressions, causing most severe crusting in the depressions.This was also brought out by Burwell et al. (1966) as areason for greater water intake over a longer period ontheir "plowed only" than on their "plow-disked-harrowed"treatment. McIntyre (1958) in a laboratory study alsoshowed the effect on intake rate of an eroding versus anoneroding surface. Mihara (1951), using a simulatedrain of 18.0 cm/hour, obtained an intake rate of 10.2cm/hour on a 50% slope (60 cm long), which was nearlydouble that on a 9% slope of the same length after 45 min.He explains that the action of raindrops on steep slopes ismainly shearing, with little compaction. On lower slopes,considerable compaction results from drop action. Molden-hauer & Koswara (1968) in a laboratory study found an in-take 6-10 times that observed in the present study. Theirsoils were on a 9% slope and wash erosion was allowed totake place. The foregoing indicates that with rough micro-