Agricultural Water Management, 15 (1988) 37-46 37 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Soluble Potass ium Transport in Agricultural Runoff Water

A.N. SHARPLEY, S.J. SMITH and L.R. AHUJA

USDA Agricultural Research Service, Water Quality and Watershed Research Laboratory, P.O. Box 1430, Durant, OK 74702-1430 (U.S.A.)

(Accepted 28 August 1987)

ABSTRACT

Sharpley, A.N., Smith, S.J. and Ahuja, L.R., 1988. Soluble potassium transport in agricultural runoff water. Agric. Water Manage., 15: 37-46.

Potassium transport in runoff from three agricultural soils was investigated in laboratory and field experiments using a kinetic equation describing soil K desorption. In the laboratory, this equation was used to study the effect of rainfall intensity and soil slope, cover, and residue incor- poration on the effective depth of interaction between surface soil and runoff (EDI), an important parameter in the kinetic equation. Using simulated rainfall, EDI increased linearly (from 1.6 to 22.0 ram) with an increase in rainfall intensity (50 to 180 mm h- i ) and soil slope (2 to 20% ). EDI was reduced an average 82 % following incorporation of 5.0 t ha - I of wheat straw (Triticum aes- tivum L. sp.) and 44% by a 0.5-ram 2 mesh screen, simulating crop cover, compared to the control (4.5 ram). This reduction was attributed to a decreased turbulent mixing of water at the soil surface brought about by an increased physical protection of soil. For all soils and treatments, EDI was logarithmically related to soil loss (r 2 = 0.80 ), allowing estimation of EDI under variable rain- fall intensity, soil slope, type, and cover conditions from measured or estimated soil loss. The values of EDI, thus estimated were used in the kinetic equation to predict solution K transport in runoff from eight agricultural watersheds in Oklahoma, U.S.A. Measured and predicted mean annual flow-weighted concentrations of solution K were not significantly different and were strongly correlated (r 2 = 0.92). These results improve our capability to predict K transport in runoff under field conditions. These methods could also be applied to other agricultural chemicals transported in runoff and results used to improve management of soil and water.

INTRODUCTION

T h e release f rom soil and t r a n s p o r t in r u n o f f of adsorbed chemicals , such as agr icul tura l ly appl ied p l an t nu t r i en t s and pest icides, can be of b o th an agro- nomic and e n v i r o n m e n t a l concern . T h e t r a n s p o r t of these chemicals has been the subject of ex tens ive field research and model l ing effor ts (Donig ian et al., 1977; L e o n a r d et al., 1979; Fre re e t al., 1980; McDowel l and McGregor , 1984). Recent ly , several s tudies have a t t e m p t e d to inves t iga te the processes involved

0378-3774/88/$03.50 © 1988 Elsevier Science Publishers B.V.

38

in the release of adsorbed chemicals to runoff from surface and near surface soil (Ingram and Woolhiser, 1980; Ahuja et al., 1981; Sharpley et al., 1981; Heathman et al., 1985).

Desorption of soil potassium (K) by rainfall and runoff occurs by mixing of a thin layer of surface soil with water, as a result of raindrop impact and flow of runoff water. The kinetics of this desorption fbr soil phosphorus (P) have been described by an empirical equation (Sharpley et al., 1981, 1985), which has subsequently been shown also to describe the desorption of exchangeable K (Sharpley, 1987) and has the /orm:

Kr = ( kK~EBt~W/J ) / V (1)

where K,. is the average solution K concentration of runoff (kg 1- J ), Ko the exchangeable K content (kg kg-1 ) of surface soil (0-10 mm), E the effective depth of interaction between surface soil and runoff (m), B the bulk density o f soil (kg m - :~ ), t the mean residence (or contact ) time of runoff water during the rainfall (s), W the water/soil ratio during rainfall (1 kg 1), V the runoff volume per unit area during the event (1 m-2) , and k, c~ and fl are constants for a given soil calculated tbr K release (Sharpley, 1987). The units of k are s ~ (1 kg-1) -/~, and c~ and fl are dimensionless. Equation (1) can be derived assuming desorption is controlled by nonlinear ion diffusion in the surface soil mixing zone (Sharpley and Ahuja, 1983 ). The depth of this zone is represented by the effective depth of interaction (or mixing) and is defined as the depth of soil in which the degree of turbulent mixing is equal to that at the surface. Equation (1) can be rewritten to calculate EDI (E):

K r V E = k K ~ B t ~ W /~ (2)

This paper reports an investigation of the use of equation (2) to determine the effects of rainfall intensity, soil slope, crop residue incorporation, and soil cover on EDI for K transport from three agriculturally important soils in the Southern Plains, U.S.A., under simulated rainfall. Preliminary testing of the prediction of soluble K transport in runoff using equation (1), for eight Okla- homa watersheds, is also presented. The results presented here for K are an- ticipated to have application to the transport and behavior in runoff waters, of other adsorbed agricultural chemicals.

MATERIALS AND METHODS

Soil samples (0-100 mm) of Houston Black clay (fine, montmorillonitic, thermic Udic Pellustert), Pullman silty clay loam (fine, mixed, thermic Tor- rertic Paleustoll) and Ruston fine sandy loam (fine, loamy, siliceous, thermic Typic Paleudult) were collected from locations in Oklahoma and North Texas. Air-dried and sieved ( < 2 mm ) soil was packed in impermeable-bottomed boxes

39

(1.0 m long, 0.3 m wide, and 0.15 m deep) in 1-kg lots to a depth of 0.15 m and a uniform bulk density as in the field (1.25, 1.33 and 1.45 t m -3 for Houston Black, Pullman, and Ruston, respectively). The soil in the surface 0-40 mm layer was mixed with 230 mg K kg- 1 of soil (added as K2HPO4 ) before packing (equivalent to a surface application of 120 kg K ha- l ) . The packed soil was slowly wetted by a drip system to field capacity to allow chemical dissolution and equilibrated for 3 days, prior to rainfall application. Rainfall was applied for 30 min at 50, 70, 90, 110 and 160-mm h - ' intensities to soil at 2, 4, 6, 8, 12 and 20% slopes by a capillary-tube type rainfall simulator (Munn and Hun- tington, 1976 ).

Surface runoff samples (50 ml) were collected at runoff initiation and then once every 5 min for K determination by atomic absorption. Runoff samples were centrifuged at 868 km s-2 to facilitate filtration (0.45 ~m ). The sediment concentration of runoff was determined as the difference in weights of dupli- cate 250-ml aliquots of unfiltered and filtered samples after evaporation to dryness. The exchangeable K content of packed soil was measured prior to each rainfall by end-over-end shaking of 1 g soil with 10 ml 1.0 M NH40Ac (pH 7.0) for 15 min. Total K was determined following digestion with 48% HF and 6 M HC1 (Knudsen et al., 1982). Nonexchangeable K was determined as the difference between total K and water soluble plus exchangeable K. All experimental treatments were duplicated and the results presented are the mean of duplicate treatments.

Ground (2 mm) wheat straw (Triticum aestivium L. sp.) was mixed with Houston Black and Ruston soils at rates of 0.4, 1.0, 2.0, 3.0 and 4.0 g of straw kg-1 of soil (equivalent to incorporations of 0.5, 1.25, 2.5, 3.75, and 5.0 t ha-1 to a depth of 150 mm) and incubated (25 + 2°C) at field capacity soil water content for 182 days. At the end of 182 days, the wheat straw-soil mixture was allowed to dry and packed in runoff boxes, wetted to field capacity, and sub- jected to several rainfall intensities and soil slopes as above. At this time phys- ical breakdown of the wheat straw had not occurred. In other treatments, screens of 0.50, 1.25, 2.50, 3.75 and 5.00-mm 2 mesh were placed 50 mm above the soil surface before application of rainfall at several intensities and soil slopes. The screens were used to simulate varying degrees of rainfall kinetic energy reduction by vegetative soil cover (Ahuja et al., 1982; Sharpley, 1985).

RESULTS AND DISCUSSION

The three soils varied widely in physical and chemical properties (Table 1 ), and should provide a rigorous test of the EDI concept in predicting K transport. The EDI for K was calculated for each soil and rainfall-runoff event using equation (2), where the mean flow-weighted soluble K concentration of runoff, Kr, was calculated for each event. Mean contact time between runoff and sur- face soil, t, was measured as the time from the start of rainfall to runoff initi-

40

TABLE 1

Physical and chemical properties of the soils

Property Houston Black Pullman Ruston

Clay (%) 50 30 10 pH 7.9 6.8 5.6 Organic C (g kg- 1 ) 22.1 8.3 14.1 Cation exchange capacity

(mmol ( + ) kg- 1 ) 552 221 22 Water soluble K (mg kg- 1 ) 11 89 8 Exchangeable K (mg kg- 1 ) 309 264 12 Nonexchangeable K (mg kg- ~ ) 6410 8057 1850 Total K (rag kg- 1) 6730 8410 1870 Kinetic constants

k ( s -~( lkg-~) -B) 0.523 0.309 0.698 0.058 0.074 0.221

fl 0.361 0.442 0.190

ation, and water/soil ratio, W, was determined as the ratio of' runoff volume (V) and soil loss in runoff. Values of constants k, a and fl for K are presented in Table 1 and were calculated from cation exchange capacity (CEC) of the soil, using regression equations derived for each constant (Sharpley, 1987). Al- though runoff volume and actual EDI values will differ between simulated and field conditions, processes involved in solution and transport of K in runoff will be comparable.

Rainfall intensity and soil slope

The relationship between calculated or apparent EDI of bare surface soil for soluble K transport increased with an increase in rainfall intensity and soil slope (Table 2 ). The values of EDI ranged from 1.6 mm for Houston Black to 22 mm for Ruston at 2% slope and 50 mm h -1 rainfall intensity. The increase in EDI for soluble K transport in runoff with increasing rainfall intensity and soil slope is a result of more turbulent mixing of rainfal l-runoff and soil. The relative increase in EDI with increasing rainfall intensity, was negatively re- lated to soil clay content. In other words, rainfall intensity had a greater effect o n E D I for coarse compared to fine textured soils.

Residue incorporation and soil cover

A decrease in E D I was measured with an increase in the amount of wheat straw mixed with the surface soil (Fig. 1 ). Houston Black and Ruston soil are presented as examples of fine and coarse textured soils, respectively. The Pull-

41

TABLE 2

Depth of surface soil-runoff interaction for potassium at several rainfall intensities and soil slopes

Rainfall Soil slope intensity (%) (mmh -l)

Depth of interaction (mm)

Houston Black Pullman Ruston

50 2 1.6 2.7 3.7 4 2.0 4.0 4.9 6 2.1 4.5 5.2 8 2.3 5.8 5.9

12 2.9 6.9 7.3 20 3.7 9.5 10.4

70 2 2.0 4.3 5.0 4 2.9 4.8 5.9 6 3.1 6.3 6.7 8 3.8 6.9 7.5

12 4.0 9.1 9.1 20 5.8 12.4 12.9

110 2 3.5 5.3 7.8 4 4.0 6.3 8.9 6 4.6 7.7 9.9 8 5.5 8.8 11.3

12 6.8 11.2 13.0 20 7.4 16.2 19.0

160 2 4.0 6.5 10.9 4 4.8 7.7 11.3 6 5.3 9.3 13.1 8 6.1 10.4 14.1

12 7.5 13.1 17.0 20 9.7 19.1 22.0

m a n soil b e h a v e d s imi la r ly (da t a no t p r e s e n t e d ) . T h e m a g n i t u d e of the de- crease in EDI wi th w h e a t - s t r a w incorpora t ion was grea te r a t lower residue rates. For example , 5.0 t s t r aw ha -1 reduced EDI by 79, 81 a n d 84% for H o u s t o n Black, P u l l m a n a n d R us t on , respect ive ly , c o m p a r e d to no s t r aw at 70 m m h - 1 ra infa l l i n t ens i t y a n d 4% soil slope, whe reas respec t ive reduc t ions of on ly 48, 53, and 64% were obse rved a t 160 m m h - 1 ra infa l l i n t e n s i t y a n d 20% soil slope. In addi t ion, the reduc t ion in EDI by s t r aw (5.0 t ha -1 ) was g rea te r for the coarse r t e x t u r e d R u s t o n (84 a n d 64% ) t h a n H o u s t o n Black c lay soil (79 a n d 48% ).

T h e reduc t ion in EDI wi th res idue i n c o r p o r a t i o n m a y be caused by an in- c reased phys ica l p r o t e c t i o n of the soil sur face a n d s u b s e q u e n t decrease in the effect of r uno f f ene rgy on t u r b u l e n t mix ing in the surface soil. T h i s is subs t an -

42

25

E E

2o O_ I - 0 15

n~ UJ

I.L

0 " r 5

n

~0

I I I I I

0 HOUSTON BLACK CLAY • R U S T O N F I N E S A N D Y LOAM

" \ - - 7 0 m r n h - t , 4 % S l o p e \ ~ - - -160ram h- l ,20 % Slop e

\

. . . . . . . . . . . Q . . . . . . . O -EL

05 1.25 2.5 5.75 5.0

STRAW INCORPORATED (Mg ho -t 150 rnm soil depth -~)

Fig. 1. Effective depth of interaction between surface soil and runoff for soluble K transport f rom Houston Black and Ruston soils as a function of wheat straw incorporation in surface soil (150 mm ) at several slopes and rainfall intensities.

E Z o_

W I - _z ~ 8 I

W O

4 LU > t -

,,u. 0o W

i ' i I I t I ~ I

S L O P E - o - HOUSTON BLACK CLAY - 4 - - RUSTON FINE SANDY LOAM (%)

20

i I , I , I , t i I 2 4 6 8 I 0

SCREEN SIZE (ram 2)

Fig. 2. Effective depth of interaction between surfhce soil and runoff for soluble K transport from Houston Black and Ruston soils as a function of screen size at several slopes and 70 mm h - rainfall intensity.

tiated by an observed reduction in soil loss with straw incoraportion (data not presented). It should be noted, however, that the magnitude of the residue effect will differ from that in the field, where residue size and mixing of surface soil and residue will be less uniform. Even so, similar response should be ob- served under field conditions.

Effective depth of interaction increased with an increase in the mesh size of a screen placed 50 mm above the soil surface {Fig. 2 ). The different mesh sizes simulate different degrees of vegetative soil cover. In contrast to the effect of wheat straw incorporation, the effect of the mesh was greater at higher runoff energies. For example, the 0.5-mm 2 mesh reduced EDI by 66, 70 and 75% for Houston Black, Pullman and Ruston soil, respectively, compared to no mesh

43

at 160 mm h-1 rainfall intensity and 20% soil slope, whereas respective reduc- tions of 38, 43 and 47% were observed at 70 mm h -1 rainfall intensity and 4% slope.

The effect of rainfall intensity, soil slope, residue incorporation, and mesh size on EDI was greater for the Ruston fine sandy loam than for Houston Black clay. These differences are consistent with the results of Heathman et al. (1985), who also observed that the relative effects of initial soil water content and sorghum mulch cover on solution K transport in runoff were greater in a sandy loam than in a clay loam soil.

Relationship to soil loss

The EDIS for all rainfall intensities, soil slopes, residue and cover t reatments were related to soil loss for each soil and could be approximated by a single relationship (Fig. 3). This is consistent with the fact that rainfall and runoff energy and soil texture influence both EDI and soil loss similarly. Although field measurements of EDI are not readily available, soil loss is usually mea- sured or can be est imated using the Universal Soil loss Equation (Wischmeier and Smith, 1960, 1978). It is suggested, therefore, that EDI may be estimated from soil loss (S) for field application of the kinetic equation to the prediction of the release of exchangeable K or other chemicals to runoff. Equation (2), thus, becomes:

Kr = ( kKoeBt~W P) / Y (3)

where e is an est imate of E calculated from soil loss (s) as 10(-o.139+o.3ss loglo s) (or e=0.726S °'3ss) from the equation of Fig. 3. EDI is a

~ 100

E 6 0

Z 0 4 0 I ' -

~ 2o F- z_ h I0 0

I 6

U.I 4 O

W >_ 2 F-

W u. li h W 10

• HOUSTON B L A C K C L A Y

O P U L L M A N S ILTY C L A Y L O A M

+ R U S T O N F I N E S A N D Y L O A M ÷

÷ * + o ~ ÷

o 4o +o o * * + O + o •

r == O.BO

I 1 I I I I I I I I I I I I 20 4 0 I 0 0 2 0 0 4 0 0 I 0 0 0 2 0 0 0 4 0 0 0 BOO0

SOIL LOSS (kg ha-')

Fig. 3 Relationship between soil loss in runoff and effective depth of interaction for soluble K transport from Houston Black, Pullman and Ruston soils.

44

function of soil cover, roughness, slope, management and rainfall intensity and represents the depth of surface soil from which the release of chemical to runoff occurs (Ahuja et al., 1981, 1983 ). Replacing EDI by S, however, allows a general grouping of factors affecting EDI for different watersheds, runoff events, and soil conditions.

Prediction of potassium transport in small watersheds

The transport of solution K in individual runoff events from eight agricul- tural watersheds (1.6 ha area) at E1 Reno, Oklahoma, was predicted using equation (3) for 1984 and 1985. The watershed management, soil loss, runoff volmne, and exchangeable K content for 1985 are given in Table 3. The mean contact time, t, was approximated as 30 min and Wwas calculated as the ratio of' runoff volume and soil loss in runoff. Exchangeable K content of surface soil (0-10 mm) was determined at monthly intervals and the value preceding each runoff event used in equation (3). The value of S was calculated for each runoff event. Values of equation (2) constants, k (0.053), c~ (0.146) and fl (0.250) were estimated by separate equations from CEC of the dominant soil type (Kirkland silt loam) on the watershed (Sharpley, 1987 ).

Measured and predicted mean annual flow-weighted concentrations of so- lution K in runoff from the eight watersheds in 1984 and 1985 were closely related for runoff events exceeding 3 mm depth (y = 1.16 + 0.88x, r ~ = 0.92, sig- nificant at the 0.1% level) (Fig. 4). Runoff events of less than 3 mm depth were over estimated (Fig. 4). These events consituted only 0.5-8.8% of the

TABLE3

Watershed management, soil loss, runoff volume and surfhce soil (0-10 mm) exchangeable K content for eight watersheds (1.6 ha area) at E1 Reno, OK for 1984 and 1985

Water- Crop type Fertilizer K Soil loss Runoff Exchangeable K shed applied b (kgha ~) (ram) ( m g K k g ~)

(kg K ha L) Mean Range

El Reno. OK FR1 Native grass ~' - - 45 2086 280 210-384 FR2 Native grass - - 44 2066 306 255 332 FR3 Native grass - - 25 1170 299 248-359 FR4 Native grass - - 46 1410 338 271-417 FR5 Wheat 12 8356 2291 416 352-512 FR6 Wheat - - 14880 1550 451 292-637 FR7 Wheat - - 1272 1437 490 359-745 FR8 Wheat --- 1497 1413 525 315 631

~'Native grass is dominantly little bluestem - - A n d r o p o g o n scoparius ichx. )'Fertilizer K added as K,O.

45

20

i 8

~ 16

E ~ 1 4 Z _o

I-- Z bJ I 0 U Z

8 s

I-"

O 10 4 O.

I I I I I I I I I / / ,

EL RENO o /

/ o /

0 0 0 - /

- - o x / / ~ L e a s t - s q u a r e s o o " / / / ~-regression l ine

o o o , / f (for • and x data)

o ,/7., 0 0 ~ • X •

• X° X

x / / ~ . ~ " " - - - - - - I: I Relat ionship - l ' /

x x x / / . ~ STORM RUNOFF DEPTHS -

v x~/"~/.~_ • - • < 3 m m o

~ / ~ " " > 3 ram • N a t i v e g r a s s /

/ x W h e a t /

/ /

/

/ " I I I I I I i I I 2 4 S e I0 12 14 16 is 2c

MEASURED K CONCENTRATION (rag t - I )

Fig. 4. Relationship between measured and predicted soluble K concentrations in individual runoff events from eight native grass and wheat watersheds, using equation (3).

measured amount of K transported from the eight watersheds during the 2- year study. Consequently, these low flow events may be neglected when deter- mining annual K loss, unless they constitute more than a specified volume of runoff. Poor predictions for the low flow events may result from a greater in- filtration of K compared to high-flow events. A similarly poor prediction was observed for soluble P transport in runoff from these watersheds for events less than 0.75 mm depth (Sharpley et al., 1985).

Comparison with an earlier study of P transport and EDI (Sharpley , 1985) indicates that wheat straw incorporation, mesh size and soil loss, had a similar effect on EDI for both K and P transport, even though the ionic nature and solubilities of K and P in soil differ. Consequently, the process of EDI initiation by raindrop impact and subsequent turbulent mixing of soil and water in this thin zone is determined by physical factors, rather than soil-chemical interaction.

The kinetic equation used in this study can describe transport of both P (Sharpley et al., 1981, 1985) and K under differing rainfall and soil manage- ment conditions. Consequently, the results of this study improve our under- standing of the processes involved in the movement of differentially sorbed chemicals in runoff as a result of turbulent mixing of rainfall-runoff and soil in a thin surface soil layer. The kinetic equation has potential use, therefore,

46

in describing the release to runof f and t r anspo r t of o ther agricul tural chemi- cals, such as pesticides, as a func t ion of different m a n a g e m e n t practices. The use of the simplified equat ion (3), which provided good es t imates of K t rans- por t in runof f for mos t condit ions, should be useful in improving soil and water

management .

REFERENCES

Ahuja, L.R., Sharpley, A.N., Yamamoto, M. and Menzel, R.G., 1981. The depth of rainfall-runoff- soil interaction as determined by p32. Water Resour. Res., 17: 969-974.

Ahuja, L.R., Sharpley, A.N. and Lehman, O.R., 1982. Effect of soil slope and rainfall character- istics on phosphorus in runoff. J. Environ. Qual., 11: 9-13.

Ahuja, L.R., Lehman, O.R. and Sharpley, A.N., 1983. Bromide and phosphate in runoff water from shaped and cloddy soil surfaces. Soil Sci. Soc. Am. J., 47: 746-748.

Donigian, A.S., Jr., Beyerlein, D.C., Davis, H.H. and Crawford, H.H., 1977. Agricultural runoff management (ARM) model, version II: refinement and testing. EPA 600/3-77-098, Environ- mental Research Laboratory, U.S. Environment Protection Agency.

Frere, M.H., Ross, J.D. and Lane, L.J., 1980. The nutrient submodel. In: W.G. Knisel (Editor), CREAMS: a field scale model for chemicals, runoff, and erosion from agricultural management systems. USDA Conserv. Res. Pep., 26: 65-87.

Heathman, G.C., Ahuja, L.R. and Lehman, O.R., 1985. The transfer of soil surface-applied chem- icals to runoff, Trans. ASAE, 28: 1909-1920.

Ingrain, J.J. and Woolhiser, D.A., 1980. Chemical transfer into overland flow. In: Proc. ASCE Symp. Watershed Management '80, 21-23 July 1980, Boise, ID. American Society of Civil Engineers, New York, pp. 40-53.

Knudsen, D.A., Peterson, G.A. and Pratt, P.F., 1982. Lithium, sodium, and potassium. In: A.L. Page, R.H. Miller and J.R. Keeney (Editors), Methods of Soil Analysis, Part 2 (2nd Edition ). Agronomy, 9: 403-429.

Leonard, R.A., Langdale, G.W. and Fleming, W.G., 1979. Herbicide runoff from upland Piedmont watersheds-data and implications for modeling pesticide transport. J. Environ. Qual., 8: 223- 229.

McDowell, L.L. and McGregor, K.C., 1984. Plant nutrient losses in runoff from conservation tillage corn. Soil Tillage Res., 4: 79-91.

Munn, J.R. and Huntington, G.L., 1976. A portable rain simulator for erodibility and infiltration measurements on rugged terrain. Soil Sci. Soc. Am. J., 40: 622-624.

Sharpley, A.N., 1985. Depth of surface soil-runoff interaction as affected by rainfall, soil slope, and management. Soil Sci. Soc. Am. J., 49: 1010-1015.

Sharpley, A.N., 1987. The kinetics of soil potassium desorption. Soil Sci. Soc. Am. J., 51: 912- 917.

Sharpley, A.N. and Ahuja, L.R., 1983. A diffusion interpretation of soil phosphorus desorption. Soil Sci., 135: 322-326.

Sharpley, A.N., Ahuja, L.R. and Menzel, R.G., 1981. The release of soil phosphorus to runoff in relation to the kinetics of desorption. J. Environ. Qual., 10: 386-391.

Sharpley, A.N., Smith, S.J., Berg, W.A. and Williams, J.R., 1985. Nutrient runoff losses as pre- dicted by annual and monthly soil sampling. J. Environ. Qual., 14: 354-360.

Wischmeier, W.H. and Smith, D.D., 1960. A universal soil-loss equation to guide conservation farm planning. In: Trans. 7th Int. Congr. Soil Sci., August 1960, Madison WI. Dijkstra, Gro- ningen, The Netherlands, Vol. 1, pp. 418-425.

Wischmeier, W.H. and Smith, D.D., 1978. Predicting rainfall erosion losses. USDA Handbook, 537. Government Printing Office, Washington, DC, 58 pp.