Pertanika 13(2), 239-246 (1990)

Tests on a Small Rainfall Simulator for Evaluating Soil Erodibilityl

WAN SUlAIMAN WAN HARUN, ISNAN MARSOMand MOHD. MOKHTARUDDIN A. MANAN

Soil Science DepartmentFaculty of Agriculture

.Universiti Pertanian Malaysia43400 UPM Serdang, Selangor Darul Ehsan, Malaysia

Key words: soil erodibility, rainfall simulator, soil loss, runoff, soil conservation.

ABSTRAK

Beberapa tanah bukit, in situ diperlakukan dengan hujan tiruan jangka pendek untuk menentukan kaedahyang sesuai bagi penilaian erodibiliti tanah di Malaysia. Alat hujan tiruan menghasilkan sejumlah 18mmhujan dalam tiga minit dan titis-titis hujan menimpa ke atas 0.0625m2 permukaan petak ujian padakecerunan 20 % dari ketinggian purata 0.4m. Hasil daripada kedua-dua jenis tanah permukaan dantanah bawah menunjukkan variasi pada kehilangan tanah di antara petak-petak adalah lebih kecilberbanding dengan variasi bagi simulasi yang berturut-turut pada petak tetap. Kehilangan tanah puratabagi tanah-tanah yang diuji ialah di antara 5.42g hingga 12.66g setiap petak. Kehilangan tanah didapatitidak begitu peka terhadap perubahan kecil pada kandungan air asal petak ujian dalam lingkunganmuatan medan. Kaedah hujan tiruan ini kelihatan berupaya membezakan gerak balas di antara tanah­tanah dan dengan itu sesuai untuk kajian erodibiliti tanah bUkit di Malaysia.

ABSTRACT

A selected number of upland soils were subjected, in situ, to simulated rain of short duration in order toestablish a methodology for evaluating erodibility of Malaysian soils. The rainfall simulator produces an18mm shower in three minutes from an average height of OAm, falling onto a 0.0625m2 test plot area at20 % slope. Results from both the surface and sub-surface horizons show less variability in the soil lossbetween plots than between successive runs on the same plot. Mean soil loss ranged between 5A2g and12. 66g per plot. Soil loss was found to be not very sensitive to small variations in antecedent soil moisturearound the field capacity value. The method appears capable of discriminating responses between soils andis thus, suited to the study of erodibility of upland soils in Malaysia.

INTRODUCTION

The inherent susceptibility of a given soil towater erosion is quantitatively expressed by itserodibility, namely, the K-factor of the universalsoil loss equation (USLE) (Wischmeier andSmith, 1960) which represents the integratedeffect of particle detachability, transportabilityand infiltration characteristics. El-Swaify andDangler (1977) categorized three approachesto determining soil erodibility, i.e., actualmeasurements of soil loss form standard bare

IProject funded under MPKSN Program 01-07-05-23

plot (Wischmeier 1976), measurements undersimulated rainfall and, finally, estimations usingpredictive equations or nomographs. The firsttwo approaches, particularly the actualmeasurements from bare plots, are costly andtime consuming. In the third approach,however, the predictive equations ornomographs based on them have to be firstderived from experimental data obtained byeither one of the first two approaches.

Quantitative and qualitative assessments of

W.H. WAN SULAIMAN, M. ISNAN AND A.M. MOHD. MOKHTARUDDIN

soil erosion by water in Malaysia are well­documented (Douglas, 1968; Wan Sulaiman etal., 1981). Yet very little is known about theerodibility of the soils inspite of its importancein conservation planning, the main reasonbeing the excessive cost and effort in obtainingthis parameter. An early attempt was made byWong (1974) who, basing on fieldobservations, classified soils with clay contentsexceeding 27% and sand contents less than45% as 'less erodible' and soils with more than45% sand and less than 27% clay as 'moreerodible'. Static laboratory rainfall simulatorshave also been used to compare erodibilitiesof several soils under disturbed conditions(Maene et aI., 1975; Abdul Rashid, 1975).Erodibility factors of two soils, the Durian series(an Orthoxic tropudult) and Padang Besarseries (a Petroferric tropudult) have beenmeasured using unit plots and reported to be0.08 and 0.02 respectively (Soil ScienceDepartment, 1979 and 1980). The presentstudy is part of a systematic effort in evaluatingerodibility of Malaysian soils. A selectednumber of upland soils were subjected, in situ,to short duration simulated rain from a smallportable rainfall simulator. The objective wasto evaluate their response and reproducibilityin the soil loss and runoff collected so as toestablish a methodology for using the simulatoron Malaysian Soils.

MATERIALS AND METHODS

The Rainfall SimulatorThere are many rainfall simulators of varyingsizes, complexity and cost depending on theirpurpose. Some are the static laboratory typedesigned for studying mechanisms while othersare portable, intended for field use. Amongthe latter category is a small unit that can easilybe carried around and operated in a very shorttime (Kamphorst, 1987). It produces an 18mm rainshower in three minutes, thus givingan intensity equivalent to 36 cm/h. Theraindrops fall from an average height of O.4m,onto a 0.0625 m 2 surface area of the test plothaving a 20% slope. Details of the design,operational procedure and field plotpreparation are fully described by Kamphorst(1987).

Assessing Intra- and Inter-plot VariationsFour soil series common to the upland areaswere selected for the test. The soils wereMunchong (Tropeptic Haplorthox), Serdang,Bungor and Padang Besar' (all TypicPa1eudult). For Bungor series soil, two siteswere chosen, one at the UPM Serdang Farm(designated Bungor UPM) and the other atthe Puchong Farm (designated BungorPuchong). Simulation runs were made on boththe surface (0-15 cm) as well as sub-surface (15­30 cm) horizons. For each soil, eight plots(replications) were selected on sites undersimilar vegetation or landuse. Each of the firstfour plots was subjected to four successivesimulated rainstorms at three-minute intervals,giving sufficient time to collect the erosionsample and reset the simulator. The remainingfour plots were subjected to single rainstormsonly. Erosion samples were taken back to thelaboratory where the amounts of runoff andsediment were determined by weighing anddrying.

As part of the standard procedure, all thesimulation runs were made at a time whenthe soils were at or very near field capacity,with the exception of the consecutive runs onthe same plot. A simple rule of thumb is that,the soil would be at field capacity one dayafter a moderate rainfall or two days after aprolonged downpour. But if there was no rainor very little of it for several days, thecurrent water content was determined bygravimetric sampling and with previouslymeasured values of field capacity (volumetricwater content at 10 kPa), the amount neededto bring the top 5 cm of the test area to fieldcapacity was computed and then added toplot.

Effect of Antecedent Water ContentAdditional runs were conducted on the surfacehorizons of Munchong and Bungor Puchongsoils at two moisture conditions, namely, wetterthan field capacity (wet run) and drier thanfield capacity (dry run). For the wet case,measurements were made on the day followinga heavy overnight storm and for the dry case,after 5 consecutive days without rain. No pre­wetting was done in either case.

240 PERTANIKA VOL. 13 NO.2, 1990

TESTS ON A SMALL RAINFALL SIMULATOR FOR EVALUATING SOIL ERODIBILITY

RESULTS AND DISCUSSION

Variations between Consecutive Runsand between PlotsVariations in soil loss and runoff between rep­licates (plots) and between consecutive runson the same plot are illustrated by results forthe surface horizons of four soils (Figs. 1 and2). The soil losses recorded, mostly less than10g and none exceeding 20 g were comparableto the riverine soils and fine loess in theNetherlands (Kamphorst, 1987) but muchlower than the aeolean soils and the coarserloesses. Successive runs on the same plotyielded a decreasing trend in the soil loss. Forinstance, a loss of between 8.6 g and 10.8 gfor the first run on the Serdang series soil fellto 2.4-4.6 g on the fourth run. In the case ofrunoff, a slight increasing trend was seen withthe successive runs. The observed trends werealso true for the other cases tested with someshowing more pronounced effects than others.Consequently, the sediment concentration (soilloss/runoff volume) decreased with successiveruns.

The variation in soil loss with successiveruns could be attributed partly to thedifference in antecedent soil moisture,whereby, the soil that was at field capacity priorto the first run became wetter with eachsucceeding run. However, it is unlikely thatincreasing antecedent soil moisture wouldcause a reduction in the soil loss. Dangler andEl-Swaify (1976) reported lower soil loss fromdrier soils than from wetter ones wheresi'mulated rain was applied to a much biggerarea and for a longer duration. A moreprobable cause is redeposition of the erodedmaterial. After the first run the originalsmooth surface became rough withinterspersed cavities and mini-rills created bythe raindrop impact and channel flow. Duringsubsequent runs these cavities and mini-rillsprovided sites for redeposition of erodedmaterial from upslope. On the other hand,the increasing trend in the runoff volume isto be expected due to a reduction in theinfiltration rate with increasing soil saturation.Partial suface sealing by fine particles resultingfrom splash could also contribute further to

the decrease in the rate of infiltration. Indeed,fig. 2 shows that for Padang Besar, Bungor andSerdang, runoff on consecutive runs could beas much as the applied shower (1125 ml fallingon 0.0625 m 2) and this could be explainedonly by the infinite impedance of the surfaceseal. That such a condition (when infiltrationrate essentially became zero) was achieved ina very short time would have a profound effecton the surface runoff from large exposed soilsurfaces.

Table 1 gives a comparison between themeans of first simulation runs from fourreplicates and the average of the means of thefour successive runs within each plot togetherwith their respective coefficients of variation(CV). The mean soil loss for first runs onlywas greater than that for the successive runsin every case with the exception of Serdangsubsurface soil, where, both were virtuallyidentical. The difference is a necessaryconsequence of the decreasing trend seen withsuccessive runs on the same plot.

The strengh of a particular methoddepends very much on its reproducibility. Thecoefficients of variation in the soil loss for firstruns only seem generally smaller than thosefor averages of successive runs. The larger CVin the latter could be due to the uneven soilsurface and material redeposition mentionedearlier. The range of 6% to 24% for first runsonly can be considered acceptable for erosionmeasurements. In the casse of runoff, thecoefficients of variation for single runs areslightly larger than for averages of consecutiveruns and in overall terms, runoff data appearmore reproducible than the soil loss figures.However, considering that soil loss is moreimportant than runoff in erodibilitydetermination, measurements of single runsonly offer a better alternative for assessingrelative erodibilities. For greater confidence,the number of replicates could be increasedto between 6 and 8. Notwithstanding these,special attentian must be given to the selectionof plot sites, avoiding obvious dissimilarities.The influence of local heterogeneities whichare usually masked in large plots like the uniterosion plot, can appear in their entirety withthe current method. In the present study there

PERTANIKA VOL. 13 NO.2, 1990 241

W.H. WAN SULAIMAN, M. IS AN AND A.M. MOHD. MOKHTARUDDIN

Padang Besar (Surface)

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Fig. 1: Soil losses from rainfall simulator test plots offour soils. Points connected fly a line represent soil lossesfrom consecutiveruns on one plot.

were several instances when measurements(plots) had to be discarded because runoff

differed from those of other replicates by morethan 100%. The cause was traced to differences

242 PERTANIKA VOL. 13 NO.2, 1990

TESTS ON A SMALL RAINFALL SIMULATOR FOR EVALUATING SOIL ERODIBILITY

1000 Munchong (Surface) Padang Besar (Surface)

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Run NumberFig. 2: RunojJjivln min/all simulator test plots 0/ JOUl" soils. Points connected by a line represent runojJflD1I! consecutive

runs on one plot.

in texture, colour (related to organic mattercontent) and abundance of roots.

Table 2 presents the means and co­efficients of variation based on eight replicates

of single runs only. The coefficients ofvariations are observed to be of the samemagnitude and perhaps slightly smaller thanfor the case of four replicates only. The

PERTANlKA VOL. 13 NO.2, 1990 243

W.H. WAN SULAIMAN, M. ISNAN AND A.M. MOHD. MOKHTARUDDIN

TABLE 1Comparison of plot replications and means of successive runs

First run only Avg. of 4 successive runs

Soil Series Mean soil loss (g)or runoff (ml)

cv(%)

Mean soil loss (g)or runoff (ml)

cv(%)

Munchong (s)Munchong (ss)

Bungor UPM(s)Bungor UPM (ss)

Bungor Puchong (s)Bungor Puchong (ss)

Serdang (s)Serdang (ss)

Padang Besar (s)Padang Besar(ss)

Munchong (s)Munchong (ss)

Bungor UPM (s)Bungor UPM (ss)

Bungor Puchong (s)Bungor Puchong (ss)

Serdang (s)Serdang (ss)

Padang Besar (s)Padang Besar (ss)

5.718.28

5.686.08

14.088.20

9.875.43

7.537.62

558884

808828

1052856

980785

985939

SOIL LOSS

21.59.7

6.222.7

17.315.0

9.217.9

13.419.8

RU TOFF

12.07.1

15.718.9

3.324.5

12.415.5

6.0ILl

5.607.14

5.524.28

13.006.69

5.955.48

6.536.81

771984

970972

1090981

1090944

10801037

23.917.3

10.119.5

34.322.6

20.527.4

18.751.2

13.58.3

7.99.0

4.016.7

11.88.6

2.75.7

s= surface horizon; ss= subsurface horizonCV= coefficient of variation

discriminating power of the method can beassessed from results of the Duncan's multiplerange test. There were significant differencesin the response of the soils, both in terms ofsoil loss and runoff. Soil loss of 5.50 g fromBungor UPM, for instance, was found to besignificantly different (p=0.05) from a loss of7.40 g from Padang Besar soil. It is interestingto note that for the riverine soils and fine loessin the Netherlands (Kamphorst, 1987) lossesof 4.lg and 8.3g were found not to differsignificantly from each other suggesting greatervariability in their measurements. The latterwere also based on eight measurements butmade on unploughed fallow plots during thewettest season in the cultivation cycle. In the

present study, the sites chosen were all underpermanent crops ranging from pasture to ba­nanas and, thus, had experienced less distur­bance. Significant differences are also observedbetween surface and subsurface horizons ofMunchong, Bungor Puchong and Serdangsoils. Another important feature is the largedifference between soil loss occurring on thesame soil series (Bungor series) but at twodifferent localities (both incidently were underpasture). From the above observations themethod appears to be sensitive to the intrinsicsoil properties and hence well-suited to uplandsoils of Malaysia. Report on the relationshipbetween soil loss measured with the samerainfall simulator and several soil properties isunder preparation.

244 PERTANlKA VOL. 13 NO.2, 1990

TESTS ON A SMALL RAINFALL SIMULATOR FOR EVALUATING SOIL ERODIBILITY

TABLE 2Soil loss and runoff from different soils for 8 replications

Soil loss Runoff

Soil Series Mean (g) CV (%) Mean (ml) CV (%)

Munchong (s) 5.95 de 18.9 639 d 19.0Munchong (ss) 8.23 c 7.9 828 be 12.6

Bungor UPM(s) 5.50 e 7.5 758 cd 17.2Bungor UPM (ss) 5.94 de 18.4 822 be 10.5

Bungor Puchong (s) 12.66 a 23.4 989 a 11.3Bungor Puchong (ss) 8.52 be 12.6 864 abc 18.4

Serdang (s) 9.95 ab 14.4 975 a 6.8Serdang (ss) 5.42 e 14.9 812 c 13.1

Padang Besar (s) 7.39 cd 14.9 957 ab 4.7Padang Besar(ss) 7.40 cd 16.8 885 abc 12.3

s =surface horizon; ss =subsurface horizon;CV = coefficient of variation;Values within a column having a letter in common do not differ significantly at the 5% probability level according to Duncan'smultiple range test.

TABLE 3Mean erosion losses from Muchong and Bungor Puchong soils for different

antecedent moisture conditions

Munchong Bungor Puchong

Antecedent Soil Runoff Antecedent Soil Runoffmoisture (v/v) loss (g) (ml) moisture (v/v) loss (g) (ml)

0.37 5.71 a 660 a 0.41 16.74 a 1070 a

0.32 5.95 a 639 a 0.29 12.66 b 989 b

0.29 5.52 a 546 a 0.22 10.77 b 770 c

Values within a column having a letter in common do not differ significantly at the 5% probability level according to Duncan'smultiple range test.

Effect of Antecedent MoisureSoil loss and runoff collected for Munchongand Bungor Puchong surface soils at differentantecedent soil moisture contents arepresented in Table 3. For Munchong soil,there was no significant difference in theresults at the three antecedent moistureconditions tested. The result is consistent withthe soil loss pattern shown in Fig. 1. In thecase of Bungor Puchong, soil loss for the 'wetrun' was significantly higher than either the'field capacity run' or 'dry run'. The soil loss

from the dry run was lower than from the fieldcapacity run by 1.89g, being just smaller thanthe critical range of 1.90g for significantdifference according to Duncan's multiplerange test. It is not the intention of this studyto explain the difference in response betweenthe two soils but it is worth noting that theBungor series is an ultisol of moderate to gooddrainage while Munchong is an oxisol havinga higher infiltrability, better drainage andpresumably higher structural stability. What ismore relevant is that while acknowledging the

PERTANIKA VOL. 13 NO.2, 1990 245

W.H. WAN SULAIMAN, M. ISNAN AND A.M. MOHD. MOKHTARUDDIN

importance of antecedent moisture in theprocess of soil erosion, the rainfall simulatormeasurements do not appear to be verysensitive to small variations in antecedent soilmoisture around the field cspacity value. Largedifferences, however, can occur if the initialsoil condition is too wet or too dry.

Limitations fo the Rainfall SimulatorThere are several features of the rainfall simu­lator that need to be reemphasized. Firstly, theaverage height of fall is O.4m only, too smallto allow the raindrops to attain terminal velo­city. Secondly, the plot size is very small, beingonly 0.0625 m 2• Finally, to offset these smalldimensions, a very high intensity shower (18mm in three minutes, equivalent to 36 em h-1)

is used so as to produce measurable erosionlosses within a short duration that can also bediscriminated from one soil to another. Thesoil loss recorded during the test runs variesfrom 5.42 g to 12.66 g. If expressed in termsof per hectare per annum, the loss will beenormous. In the light of these, the simulatorcannot be used to obtain absolute values oferodibility. Rather, it was designed witth thespecific purpose of evaluating relative erodibi­lity among soils, hence the fixed slope and pro­cedures of plot preparation. Nevertheless, itmight be feasible to obtain indirectly the abso­lute erodibility, if soil loss with the simulatorcan be calibrated against known K values. Thisimplies that simulation runs must first be madeon soils whose K values are already known.

CONCLUSION

The rainfall simulator described by Kamphorst(1987), by virtue of its small size, was designedfor use on soils where spatial variations inproperties have been reduced throughprevious cultivation. However, with judicioussite selection the method can be equallyeffective on the less disturbed upland soils inMalaysia. Its small size makes it possible toassess differences in erodibility with differentlanduse even on a small scale.

ACKNOWLEDGEMENTS

The authors wish to thank Dr. Ahmad HusrriMohd. Hanif for his critical comments on the

draft of the manuscript.

REFERENCES

ABDUL RASHID, F.D. 1975. Studies on Correlationbetween the Erodibility of Standard Sand andFive Malaysian Soils. Unpublished B. Agr. Sc.Project Paper, Univ. Malaya, Kuala Lumpur.

DANGLER, E.W. and SA EL-SWAIFY 1976. Erosion ofSelected Hawaii Soils by Simulated Rainfall. SoilSci. Soc. Am. J 40: 769-773.

DOUGLAS, I. 1968. Erosion in the Sungai GombakCatchment, Selangor, Malaysia. J trop. Geog. 26:1-16.

EL-SWAIFY, SA and E.W. DANGLER. 1977. Erodi­bilities of Selected Tropical Soils in Relationto Structural and Hydrologic Parameters. InSoil erosion: Prediction and Control. Proc.National Corif. on Soil Erosion, p. 105-114. WestLafayette, Indiana, 1976.

KAMPHORST, A. 1987. A Small Rainfall Simulator forthe Determination of Soil Erodibility. Nether­lands J Agr. Sci 35: 407-415.

MAENE, L.M., C.K. MOK and K.F. CHEAH 1975. TheApplication of a Rainfall Simulating Method inErosion Studies on Three Peninsular MalaysiaSoils. Proc. Third Asean Soil Can! p. 331-430.Kuala Lumpur.

SOIL SCIENCE DEPARTMENT. 1979. Joint Soil ResearchProject UPM-Belgium Annual Report: Vol 1,Sub-Project Soil Physics, UPM, Serdang.

SOIL SCIENCE DEPARTMENT. 1980. Joint Soil ResearchProject UPM-Belgium Annual Report: Vol 1,Sub-Project Soil Physics, UPM, Serdang.

WAN SULAIMAN W.H., L.M. MAENE and A.M.MOKHTARUDDIN. 1981. Runoff, Soil and Nut­rient Losses from an Ultisol under DifferentLegumes. In SCYUth-East Asian Regional Symposiumon Problems of Soil Erosion and Sedimentation p.275-286, Bangkok, Thailand.

WISCHMEIER, W.H. 1976. The Use and Misuse of theUniversal Soil Loss Equation. J Soil Water Cons.311: 5-9.

WrscHMEIER, W.H. and D.D. SMITH. 1960. A Uni­versal Soil Loss Equation to Guide ConservationFarm Planning. Trans. 7th Congr. Soil Sci. p. 418­425.

WONG, I.F.T. 1974. Soil Crop SuitabilityClassification for Peninsular Malaysia. Soils andAnalytical Services Bull. No.1 Min. of Agric. andFisheries Malaysia, Kuala Lumpur.

(Received 2 December, 1989)

246 PERTANlKA VOL. 13 NO.2, 1990