Effects of Stones on Runoff, Erosion, and Soil Moisture1

E. EPSTEIN, W. J. GRANT, AND R. A.

ABSTRACTStudies on a caribou silt loam at Presque Isle, Maine were

conducted to determine the effect of stones on runoff, erosion andsoil moisture. The three treatments consisted of a control,stones over 3.81 cm in diameter removed, and stones over 3.81 cmremoved, crushed and reincorporated into the soil. Removingthe stones increased soil and water losses over a 4-yr period.Infiltration and soil moisture was decreased as a result of stoneremoval. Crushing and returning the stones resulted in soillosses similar to that obtained under normal soil conditions.During 1961-1964, 20 storms produced 75% of the total soil lossand about 55% of the total runoff.

THE GLACIAL TILL soils of northeastern USA character-istically contain large amounts of stones. Increased

mechanization of farming operations along with the decreasein availability of farm labor often necessitates the removalof large stones. Although this removal facilitates mechanizedfarming, its effect on runoff and erosion has been questioned.In much of the cultivated area of the region, soils are leftunprotected over winter; in the spring, the coarse fragmentsappear to cover the surface. This stone surface may act as amulch, decreasing the amount of soil loss and influencing themoisture regime of the soil.

Lamb and Chapman (4) and Lamb et al. (5) found thatsurface stones increased water intake and decreased soilerosion by protecting the surface from the puddling anderosive action of raindrop impact. Lutz and Chandler (6)state that stones tend to make a soil more porous and allowgreater penetration of water and air into the soil. Huberty (3)found that the greatest reduction in water penetrationoccurred on soils having a wide range of particle sizes, such asgravelly loams. Grant and Struchtemeyer (1), using alaboratory infiltrometer, found a decrease in the rate ofinfiltration and an increase in runoff and soil loss as particlesgreater than 12.7 mm were removed. Studies in Israel (8)showed that erosion was increased 12-fold on the average bystone removal from plots with 28 to 62% stone cover.

The objectives of this study were to evaluate the effectof stone removal on runoff and erosion and to determine thevalue of crushing stones and returning the small fragmentsto the field.

lower end, and an earth dike at the upper end. The border plateswere imbedded to a depth of 6 inches and the sill plate to adepth of 22 inches. Soil and water washed from the plots werecollected in metal tanks for measurement (2).

The following treatments were established: (i) Natural soilcondition, (ii) Stones over 3.81 cm in diameter removed,(iii) Stones over 3.81 cm removed, crushed, and returned to thesoil.

The amounts of stones removed from the second treatmentwere 12.43, 5.74, 5.87, and 5.91 tons/acre for 1961, 1962, 1963,and 1964, respectively. In the last treatment, the stones werecrushed by a jaw crusher to pass a 3.81-cm sieve. These crushedstones were returned just prior to tillage and were incorporatedinto the soil. The statistical design was a randomized completeblock with each treatment replicated three times. Potatoes(Solanum tuberosum) were planted in rows 34 inches apart with9-inch seed spacing in the row. All plots were tilled up and downthe slope in a southeasterly direction, and tillage operationswere carried out by tractor-drawn equipment.

Soil moisture was measured gravimetrically. Bulk samples,(35 to 50 g), containing soil material smaller than 1.27 cm indiameter, were taken twice a week at a depth of 5-7 cm. In-filtration was calculated as the difference between precipitationand runoff. Rainfall was measured with a 12-inch tipping bucketrecording raingage. In the spring of 1965, prior to field opera-tions, the weight, size distribution, and surface area covered bystones were obtained for the three treatments. The land surfacecovered by stones was measured by photographing 1-ft2 areasof the soil surface, projecting the images to a square foot on aground glass screen and measuring the stone area by the dotgrid method. Figure 1 shows the soil surface and condition forthe three soil treatments.

RESULTS AND DISCUSSIONAnnual and seasonal precipitation plus the number of

excessive rate storms is shown in Table 1. During most of "thecollection period, May to November, precipitation was aboveaverage for 1961 and below average for 1962, 1963, and 1964.The high seasonal value for 1963 was a result of an excessof 4.75 inches in November. In some years, considerableerosion and runoff in the area results from spring snowmelt.The unevenness of snow cover and the small plot size precludedan accurate evaluation of this type of erosion.

The distribution of stones and the surface areas covered forthe three treatments in the early spring of 1965 are shown inTable 2. Under natural conditions, stones greater than 0.64cm in diameter covered 31 % of the surface area. Total areacovered was 38% lower where rocks were removed and 29%

METHODSA field study was established at Presque Isle, Maine, in 1960

on a Caribou gravelly silt loam of 8% slope. Plots were con-structed 12 ft wide and 72.6 ft long (0.02 acre) with metal bordersalong each side, a sill plate and metal collection trough at the

1 Contribution of the Soil and Water Conservation ResearchDivision, ARS, USDA, and the Department of Plant and SoilSciences, Maine Agr. Exp. Sta., University of Maine, Orpno.Supported in part by Regional Research Project NE-11. ReceivedFeb. 11, 1966. Approved May 31, 1966.2 Research Soil Scientist, USDA, Soil Scientist, USDA, andHead, Department of Plant and Soil Sciences, University ofMaine, Crono.

Table 1—Precipitation and excessive-rate storms 1961-1964,Presquelsle, Maine

Year

1961196219631964

Annual*

inches44.4335.4331.1040.02

Seasonal+

inches27.3425.4227.1922.09

Excessive ratestormsj

Number9225

* From US Weather Bureau records.t May to Nov.. during runoff collection period.j Excessive rate storm—A Storni where the maximum intensity for at least

one of the following time periods must equal or exceed the indi cated intensity:5 min, 3.0 inches/hr; 15 min, 1.4 inches/hr; 30 min, 1.10 inches/hr; GO min,0.8inches/hr.

638

EPSTEIN ET AL.: EFFECTS OF STONES ON RUNOFF, EROSION, AND SOIL MOISTURE 639

Fig. 1—Soil surface for the 3 soil treatments: (A) Natural soil condition. (B) Stones removed. (C) Stones crushed.

lower where stones were crushed as compared to the naturaltreatment. The area covered by stones smaller than 3.81 cmwas increased as a result of crushing.

Soil and water losses as affected by stone removal are shownin Table 3. There was a significant difference at the 5% levelof probability for soil loss and at the 1 % level for water lossfor the 4-yr period. The greatest difference in soil lossbetween treatment (i) and (ii) was in 1963. In 1963 treatment(ii) lost 47% more soil than treatment (i), whereas in 1961,1962 and 1964 the losses from treatment (ii) as compared to(i) were 27, 13, and 30%, respectively.

With reference to runoff, the greatest percentage increaseof treatment (ii) over treatment (i) was in 1963. The increasein runoff for the 4 years was 20, 16, 37, and 27% for 1961,1962, 1963, and 1964, respectively. Crushing the stones andreturning them to the soil surface resulted in soil losses similarto those obtained under natural soil conditions. Results witha laboratory rainfall simulator (Author's unpublished data)showed that increasing the stone content from O to 35% byweight decreased soil losses. Increasing the area of soil coverfrom O to 25% artificially with metal plates also decreasedthe soil loss.

Table 2—Weight distribution and surface area covered bystones, May 1965

Weight distribution*% of total soil and stones

stones

0.64-1.271.27-2.542.54-3.81>3.81Total

Natural

9.448.815.249.13

32.62

Stonesremoved

fr*

9.497.383.08

.7720.72

Stonescrushed

9.939.994.67

.7425.33

Surface covered by stones4"% of total area

Natural

5.315.794.82

15.1631.08

Stonesremoved

frr

5.815.823.313.22

18.16

Stonescrushed

5.636.835.204.36

22.02

Table 4 gives individual storm data where more than 0.5ton/acre of soil loss occurred from the treatment where stoneswere removed. For the period studied, 20 storms resulted in75% of the total soil loss and about 55% of the total runoff.These storms had Elsa values ranging from 2.0 to 27.2.(EIW = Total kinetic energy of the storm in feet—tons peracre inch X the maximum 30-min intensity in inches per hourfor the storm.) The correlation coefficient for EI30 vs. soilloss for the natural soil condition was 0.62. This is con-siderably poorer than the relation between Ely, and soil lossfrom cultivated fallow plots reported by Wischmeier andSmith (9). Most of the storms listed in Table 4 occurred afterthe potato crop provided considerable cover, thus reducingthe impact force of the raindrops. Plant cover is essentiallycomplete by the middle of July. Cultivation, surface sealingand antecedent moisture will also influence the amount of soillost. When cultivation precedes a storm the surface crust isbroken, thus enhancing infiltration and reducing runoff.A high soil moisture content will reduce infiltration andincrease runoff.

Removing the stones significantly (5% level) decreasedinfiltration (difference between precipitation and runoff)during 1961 and 1963. For the 4 years, infiltration, as percent

Table 3—Soil and water losses from the 3 treatments

YearSoil loss Runoff

Stones StonesNatural removed crushed

Stones StopNatural removed crui l . d

* Average of 9 samples 12 inches2 and 3 inches deep,*" Average ^f 18 samples 12 inches2.

1961196219631964TotalAvg.

LSD .05LSD .01

14.487.013.835.83

31.357.84

1

18.427.905.647.56

39.529.88

.86

13.896.594.816.07

31.367.84

8.065.246.684.10

24.086.02

01.

9 . 636.059.145.22

30.047.51

.8517

8.674.868.204.57

26.306.58

640 SOIL SCI. SOC. AMEH. PROC., VOL. 30, 1966

Table 4—Comparison of soil loss and runoff from stone treatments for storms contributing over 0.5 tons of soil loss/acre

Date ofstorm

7/10/618/7/618/26-27/619/1/619/11/619/15/619/24/619/25/619/26/617/9-10/627/12-13/627/14/628/7/627/2-3/637/7-9/6311/7-10/637/3-6/647/7-8/647/11-12/648/31/64Total for 20

major storms4-yr. total .% lost in major

storms

EIK

6.69.2

14.77.69.5

22.39.02.1

23.45.84.16.64.52.97.2

11.92.03.9

27.2

Amountof

rainfall

inches0.80

.921.241.20

.60

.821.021.62

.802.49

.562.171.40.68

1.293.142.14

.61

.692.05

Natural

1.861.78

.49

.471.11

.872.151.15

.433.671.36.68.34

1.17.55.58

2.39.35

1.09.98

23.3731.55

74.6

Soil loss

Stonesremoved

—— tons/acre ——3.002.00

.61

.681.231.022.961.52.74

3.641.76.58.60

1.61.84.93

2.62.66.94

1.50

29.4439.52

74.5

Stonescrushed

1.991.54

.48

.521.091.112.551.17.51

3.011.42

.45

.421.48

.60

.702.17

.461.07

.93

23.6731.36

75.5

Natural

0.60.37.40.58.43.58.77

1.43.58

1.05.39

1.23.39.26.29

2.09.85.22.30

1.22

14.0324.08

58.3

Runoff

Stonesremoved

—— inches ——0.51

.41

.49

.63

.43

.611.231.54.83

1.13.35

1.19.70.33.43

3.14.80.25.21

1.40

16.6130.04

55.3

Stonescrushed

0.46.33.41.69.39.58.94

1.41.72.95.37.80.49.36.31

3.14.84.19.29

1.14

14.7126.30

56.0

of rainfall for storms over 1 inch was 64.2, 54.5 and 61.6% forthe natural, stone removed and stone crushed treatments,respectively. One of the factors affecting infiltration iscompaction. Wheel track compaction measured by an impactpenetrometer (number of times an 8-lb weight was dropped ona probe, a distance of 12 inches, until the probe penetrated6 inches into the soil) was slightly higher during most of theseason where stones were removed. (Author's unpublisheddata.) Another form of surface compaction is through theimpact of raindrops where a thin compact layer is formed onthe soil surface which seals the pores and reduces infiltration.The surface cover of stones acts as a mulch, dissipating anappreciable portion of the impact energy of the raindrops andpossibly decreasing the extent of this compact layer.

For each treatment, the soil moisture regime was plotted,and the area bounded by the soil moisture curve and the valuefor the wilting percentage of the soil was determined. Table 5shows that there was significantly less soil moisture for thetreatment where stones were removed. This may be due tohigher runoff and thus lower infiltration.

Antecedent soil moisture and El of a storm are two of themost important factors contributing to soil loss. On July 3-6,1964, the high initial soil moisture in conjunction with a stormhaving an El of, 11.9, resulted in considerable soil loss. OnAug. 31, the high soil loss was due primarily to an intense

Table 5—Evaluation of soil moisture as influenced by stonetreatments

Year* Natural Stones removed Stones crushed

Soil moisture evaluation, Cm2f1961196219631964Avg.

LSD .01 1.87LSD .05 1.24

17.1016.6416.5116.5016.69

15.7315.6614.9314.5015.21

17.1016.5516.8216.5916.77

* May 24 to Oct. 20.t Area between curves for actual soil water content and wilting percentage.

storm having an El of 27.2, whereas on July 11-12 the highsoil moisture content was probably the primary factor sincethe storm was of low intensity.

The effect of stones on runoff, erosion and infiltration is aresult of a number of factors. Coarse particles on the surfaceact as a stone mulch by intercepting and dissipating theenergy of the falling raindrop. This action may decrease theamount of surface sealing, thereby providing greater infiltra-tion. Where stones are removed, infiltration decreases andmay result in the presence of a thin surface water layer.Palmer (7) showed that a thin surface water layer canincrease the force of impact of waterdrops thereby increasingsoil losses. Less compaction is found in soils with largeamounts of coarse fragments. These soils will have a greateramount of noncapillary pore space which provides for greaterinfiltration.