A PORTABLE RAINFALL SIMULATOR

R. W. Tossell1, W. T. Dickinson2, R. P. Rudra2, and G. J. Wall3

1Land Resource Science Department, and2School of Engineering, University ofGuelph, Guelph, Ontario NIG 2W1;and 3Agriculture Canada, Ontario Institute of Pedology, Guelph, OntarioNIG 2WJ

Received 2 September, 1986, accepted 27 January 1987

Tossell, R. W., W. T. Dickinson,R. P. Rudra, andG. J. Wall. 1987. Aportable rainfall simulator. Can. Agric. Eng.29: 155-162.

Anew portable rainfall simulator has been developed for both field and laboratory soil erosion research attheUniversityofGuelph. Employing several low- to medium-flow-rate, full-jet nozzles in a continuous-spray design, the simulator iscapable of reproducing storm intensities ranging from 17.5 mm/h to inexcess of200mm/h. Simulated rainfall intensityvaries with nozzle size,water pressure atthenozzle, and height ofthe nozzle above theplot surface. Uniformity of rainfallintensity overa 1-m by 1-m plot averages 88.33% overallnozzles andsettings, with many nozzles producing averages over90%. The simulatorand support equipment are portable to the extent that they can be loaded onto a half-ton truck fortransport to the field.

INTRODUCTION

Rainfall simulation has become a veryeffective technique for assessing soil erosion, particle detachment, overland flow,and chemical runoff (Meyer and Harmon1979; Bubenzer and Jones 1971; Turner1965; Beasley et al. 1985). The benefits ofusing a rainfall simulation approach to soilerosion research are well documented byMeyer (1965), Neff (1979), and Molden-hauer (1979), with versatility being theforemost advantage. Rainfall simulationprovides control of precipitation characteristics such as intensity, and spatial andtemporal frequency and duration in bothfield and laboratory studies (Pall et al.1983).

Several types and designs of rainfallsimulators have been proposed to meet theobjectives of researchers. These objectivesin turn have often dictated the type ordesign of simulator appropriate to theresearch studies. For example drop-formersimulators (Ellison and Pomerene 1944;Adams et al. 1957; Chow and Harbaugh1965;Langford 1970; Black 1972) producea relatively uniform, narrow, drop-sizedistribution. These simulators are there

fore suitable for studies of rainsplash orparticle detachment, where known homogeneous drop-size characteristics aredesired. Recent design improvements insprinkler nozzles have greatly improvedthe drop characteristics produced bynozzle-type simulators. A wide range ofrandomly-distributed drops are producedby such simulators, rendering the simulated rainfall more comparable to naturalrainfall. Devices employing nozzlesinclude: F-type simulators producing continuous spray upward (Hart 1984; Meeu-wig 1969), rotating-boom and oscillatingsimulators (Luk and Morgan 1981; Meyer1979), rotadisk simulators (Morin et al.1967; Pall et al. 1983), and continuous

downward flow spray simulators (Lylesetal. 1969).

A number of rainfall and operationalcharacteristics, identified to be importantfor rainfall simulation for soil detachment

and transport studies, have been documented by Bubenzer (1979), Meyer (1965,1979), and Mutchler and McGregor(1979). These include:

1. Drop-size distribution similar to natural rainfall given comparable rainfallintensities.

2. Drop impact velocity approximatingterminal velocity of natural raindrops.

3. Rainfall intensity representing thegeographical region where studies are tobe conducted.

4. Uniform rainfall over the study area.5. Energy characteristics correspond

ing to natural rainfall for comparableintensities.

6. Rainfall intensity continuous over thestorm event.

7. Storm pattern reproduction.8. Sufficient area of coverage.9. Drop impact angle near vertical.10. Site to site portability.PSall et al. (1983) have added that a sim

ulator should be efficient and simple tooperate to facilitate reliability.

The Guelph Rainfall Simulator I (GRS 1)was developed on the basis ofthe above listof criteria for both laboratory and fielderosion research (Pall et al. 1983). It employs a large-capacity wide-angle spray,rotadisk system. The GRS I has proven tobe very useful for laboratory studies; however, the system has not been sufficientlyportable to be readily used at various fieldlocations.

A second rainfall simulator, designated as the Guelph Rainfall Simulator 11(GRS II), has been developed. This simulator incorporates many of the goodqualities of the original simulator, with a

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

number of new features to provide animprovement over the rotadisk system.The GRS II is completely portable and canbe assembled at field locations with a mini

mum of site disturbance. The continuous

spray generated by the small nozzle designis also more physically realistic than theintermittent spray produced by the rotadiskand some other simulator systems.

As noted briefly above, each rainfallsimulator type meets only a subset of thedesired criteria. Drop formers have a narrow range of drop sizes, and a nonrandom,biased, drop-size distribution. F-typedesigns are usually nonportable and aresusceptible to wind-sorting effects. Portability is also a problem with many rotatingand oscillating simulators. A noncon-tinuous spray, combined with a narrowrange of drop sizes at all intensities are themajor drawbacks of the rotating diskdesign. On the other hand, the continuous-spray, single-nozzle simulators have notbeen very useful for reproducing lowintensity storms, and do not readily allowthe changing of intensity over the durationof a simulated storm. However, a widerange of nozzles are now available to reproduce a variety of storm intensities.

The purpose of this paper is to provide asummary of the design and development ofthe GRS II rainfall simulator, includingpreliminary calibration results regardingsimulated intensities and spatial uniformity. A thorough analysis of drop-size,drop velocity, and energy and momentumcharacteristics will be presented in a subsequent paper.

DEVELOPMENT AND

DESCRIPTION OF THE GUELPH

RAINFALL SIMULATOR II

In light of the merits and shortcomingsof the various simulator types, a new continuous-spray simulator design has been

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selected and developed for both field andlaboratory soil erosion studies at the University of Guelph. Special attention hasbeen given to the portability ofthe system,simple operational features, and cost. Aschematic ofthe GRS II system is shown inFig. 1, and a photograph of the field set-upis given in Fig. 2.

The GRS II has a downward-oriented,single-nozzle, continuous-spray design,employing a selection of low- to medium-flow-rate, fulljet nozzles (Spraying Systems Co., Wheaton, 111.). Six nozzles witha full-cone spray angle of 120° (at 70 kPa),including the 1/8GG 2.8W, 1/8GG 4.3W,1/4GG 10W, 1/4GG 14W, 3/8GG 20W andthe 1/2HH 30W nozzles (Spraying Systems Industrial Catalog 27), have beenselected to provide a range of rainfallintensities. Table I illustrates the nozzle

specifications. Water is supplied through a1.25-cm (1/2-inch) galvanized pipe, by a1475 L/h rotary gear pump.

A high-pressure gauge (1400 kPa) measures water pressure originating at thepump, while a second gauge (200 kPa)monitors the pressure at the nozzle. Thepump is driven by a 0.75 kW, 220-V, 1725-rpm motor, which, in turn, is driven by a3000-W gasoline-powered generator.

156

Figure 1. Rainfall simulator assembly.

TABLE I. SPECIFICATIONS FOR NOZZLES EMPLOYED IN GRS II DESIGN

Flowrate capacity atspecific pressure (kPa)

1 jM ^

Figure2. Photograph of the Guelph Rainfall Simulator II in the field.

sheet-metal strips are used to delineate theplotboundary and to provide adequate protectionagainst any rainsplash entering thestudy area from outside the plot. At thebase of the plot, a galvanized steel trough(with cover) channels runoff water andsediment through a sieve system into acollection apparatus (Fig. 3).

The simulator, water supply system,generator, and all related facilities are sufficiently portable to be loaded onto a half-ton truck and transported from the laboratory to field sites.

AREAL UNIFORMITY AND

INTENSITY CALIBRATION

Performance tests regarding rainfallintensity and areal rainfall uniformity overthe plot area were conducted as describedby Pall et al. (1983). The procedureinvolved placing collection apparatus orgauges throughout the target area ofapplication, and taking a representativesample of simulated rainfall. In the case ofthe GRS II, the 1-m by 1-m plot was thetarget area. The collection gauges were

placed throughouttheplotareaas showninFig. 4. The volume of simulated rainfallcollected in each gauge was recordedaccording to its position within the plotarea (recorded by gauge number). Averagerainfall intensity over the plot was determined by the following equation:

/„ =10 [(2 VjAf)/n) x60/rj (I)where /p is the plot-average intensity(mm/h); Vt is the volume in the ith gauge(cm3); Ag is the gauge collection area(cm2); / is the time of each run in (min);and n is the number of gauges; the coefficient 10 converts measurements from cm/h

to mm/h.

Collection containers of the same sizewere used (i.e., 108-mm-diameterby 178-mm-high cylindrical cans); however, thenumber of cans was reduced to 17 from 49,and they were placed in strategic positionswithin the calibration area, as shown inFig. 4. The reduced number of containershelped to decrease the time needed forcalibration by a factor of 6 for each run,

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

thus greatly speeding up the calibrationprocess. The only drawback to decreasingthe number of containers is the accuracy ofthe estimate for overall plot average rainfall intensity. Covering the entire plot areawith gauges would yield the most accurateestimate of rainfall intensity; however,time efficiency necessitated reducing thenumber to a manageable figure. Thereduction in the number of gauges from 49to 17 represented a difference in measurements of 1and 3% for intensity and uniformity, respectively.

Uniformity of rainfall application wasdetermined using a procedure developedby Christiansen (1942). The Christiansenmethod is somewhat of a unwritten stan

dard procedure for determining the overalluniformity of rainfall application over thestudy area (plot area). As is the case withthe intensity measurements, the numberand also the size of the gauges will ultimately affect the results of the uniformitytrials. An in-depth look at uniformity ofapplication would involve several smallgauges covering the entire plot area. Thiscan be extremely time consuming, and theamount of information obtained is offset

by the time involved. Conversely, a fewlarge gauges covering the entire plot areamay lead to misleading results. Therefore,it is more informative to use small gaugesequally spaced throughout the plot area,than it is to use the same number of largegauges covering the whole plot. Uniformity of rainfall application, often referredto as uniformity coefficient, was determined by the following Christiansen(1942) equation:

UC = 100 (1.0 - X \x,\/mn) (2l

where UC is the uniformity coefficient as apercentage; jc, is the deviation from themean value m; and n is the number ofobservations or gauges employed. Tworeplications wereconducted for each intensity/uniformity trial, and a third replicatewas added when observations from the firsttwo replicates were dissimilar. Simulatedrainfall intensity and uniformity measurements were determined for simulation runs

of 20-min duration.

RESULTS AND DISCUSSIONRainfall intensity and uniformity were

observed to be dependent upon nozzlesize, water pressure at the nozzle, and theheight of the nozzle above the plot surface.Each nozzle was tested using operatingpressures of 48.3, 69.0 and 96.5 kPa;while the distance from the nozzle to the

plot was varied from 0.8 m to 1.7 m inincrements of 0.1 m or 0.2 m. The 1/8

157

RAINFALL SIMULATOR PLOT SYSTEM

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2.8-W nozzle was not tested at the 48.3

kRa pressure because the specified minimum operating pressure was 69.0 kRa. The1/2 30-W nozzle was not tested at 96.5 kRabecause rainfall intensities were considered to be unrealistically high for Ontarioconditions. Not all heights were tested forevery nozzle type because of several overlapping rainfall intensities. In such cases acoarser increment of nozzle height (i.e.0.2 m) was used. Results ofthe intensity/uniformity trials are shown in Table II.

With the exception of the 1/8 2.8-Wnozzle, the rainfall intensity increasedwith increasing nozzle size. The smallerflow-rate nozzles (i.e. the 1/8-inchnozzles) showed the smallest range inintensity as a function of nozzle height.

158

The larger nozzles (i.e. the 3/8-, 1/2-inchnozzles) provided the largest range ofintensity with changes in height. Thiseffect was more evident at the lower

heights with the rate of change in intensitydecreasing with increasing height. Intensity tended to level off with nozzle heightsabove 1.7 m for the 1/8 2.8-W and 1/8 4.3-

W nozzles, while the intensities generatedby the larger nozzles leveled out at greaterheights. Results were not presented fornozzle settings above 1.7 m in heightbecause of the leveling out of intensitieswith some nozzles. Furthermore, operation of the simulator above the 2.0-m

height becomes functionally more difficult, and the spray distribution becomesmore susceptible to wind-sorting effects.

With the exception of a few anomalies(i.e. the 1/8 2.8-W nozzle and 1/8 4.3-Wnozzle at 48.3 kRa), an increase in waterpressure at the nozzle increased rainfallintensity. The smaller 1/8-inch nozzles,however, showed a small change in intensity with pressure. In fact, the 1/8 2.8-Wnozzle at 96.5 kR* and the 1/8 4.3-W

nozzle at 69.0 kRa produced lower intensities than the corresponding lower pressures. Originally it was hoped that the 1/82.8-W nozzle would produce intensitiesdown to 10.0 mm/h, but the lowest intensity attained was 20.7 mm/h. The 1/84.3-W nozzle produced the lowest intensity of 17.5 mm/h.

The various combinations of nozzle

size, nozzle height, and water pressure

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

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provide a broad range of simulated rainfallintensities. Figure 5 summarizes therange: the 1/8 4.3-W, 1/4 10-W, 1/4 14-Wand 3/8 20-W nozzles provide a range ofintensities from 17.0 mm/h to 110.0 mm/h;and the 1/2 30-W nozzle produces higherrainfall intensities, ranging from approximately 140.0 mm/h to 215.0 mm/h. A gapexists between 110.0 mm/h and 140.0

mm/h, where the production of rainfallintensities for the 3/8 20-W and 1/2 30-W

nozzles did not overlap. However, this gapis not a major concern with this simulatordesign, because new nozzles with intermediate flow rates are available (e.g.3/8GG 24-W and 3/8GG 27-W nozzles)and can be calibrated quickly. The widerange of rainfall intensities possible withthe GRS II provides a great deal of flexibility in the development stages of erosionresearch employing rainfall simulation.

Uniformity of simulated rainfall application revealed no distinct relationshipwith either nozzle waterpressure or nozzlesize, as shown in Fig. 6. This result is incontrast to the observations found by Railet al. (1983) which revealed that pressurewas the primary factor in determining theuniformity coefficient for the GRS I. Onan individual nozzle basis, only the 3/820-W nozzle indicated any increased uniformity with an increase in pressure. The

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CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

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larger nozzles (i.e. the 1/2 30-W and 3/820-W)exhibited a slight increase in uniformity with nozzle height, whereas the remaining nozzles showed no distinct relationship between rainfall uniformity andheight.

The 1/4 14-W nozzle had the highestaverage uniformity coefficient of90.78%,and the 1/2 30-W nozzle produced thelowest at 81.49% (averaged over allheights andpressures). Otheraverage uniformity coefficients were 89.93, 89.47,89.34 and 85.47% for the 1/8 4.3-W, 1/410-W, 3/8 20-W, and the 1/8 2.8-Wnozzles, respectively. The most inconsistent nozzle proved to be the 1/8 2.8-W at69.0 kPa, producing uniformities whichranged from 80.45 to 92.80% (Table II).All other nozzles had uniformity coefficientranges of less than7%. Average uniformity for all nozzles and settings was88.33%.

In terms of uniformity of rainfallapplication, the GRS II outperformed itspredecessor, the GRS I, by approximately4% on average. The uniformities of approximately 90%areamong thehighest ofany presented in the literature.

The GRS II rainfall simulator has thepotential to accommodate a wide range of

research needs. Toprovidea widerangeofrainfall intensities, three nozzles willcover many research requirements. The1/8 4.3-W nozzle will provide relativelylow rainfall intensities, while giving reasonably consistent uniformity coefficients. Intermediate intensities can becovered with nozzles such as the 1/4 14-Wor 3/8 20-W. Higher rainfall rates, such asthunderstorms, can be produced with the1/2 30-W nozzle. If there is an option,choosing between one of two nozzles, thelarger nozzle is recommended because ofthe widerdropsizedistributions produced.

The simulators nozzle height above thestudy area, should be kept within functional limits, such as 1.0-2.0 m. It issuggested, however, that if drop velocitiesare of importance, the greater heights arerecommended. The increase in fall height,will increase individual drop velocities.

Operational nozzle water pressureshould also be kept within certain limits.Pressures ranging from 45 to 100 kPa arerecommended. The smaller nozzles (1/82.8-W and 1/8 4.3-W) will not provide afull 120° spray angle and may yield lowuniformity coefficients if nozzle pressuresare below 45 kPa. Conversely, pressures inexcess of 100 kPa will produce a drop size

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987

distribution which is too narrow to be considered as a representation of natural rainfall for the same given intensity based onpreliminary investigations into drop sizemeasurements. Even with an increase indropvelocities associated with an increasein nozzle water pressure, the increasedvelocity effect is offset by a reduction intheproportion of larger drops which makeup a large percentage of the rainfalls erosive characteristics.

CONCLUSIONSThe Guelph Rainfall Simulator II em

ploys a continuous, downward-flow wide-angle full-jet nozzle spray system which issuitable for both field and laboratoryresearch. A selection of low- to medium-flow-rate nozzles provides a goodrange ofrainfall intensities and uniformity characteristics. The simulator is completely portable for field experimentation, and can betransported ina half-ton truck. Thecontinuous-spray, random-drop distribution pattern is superior to the intermittent spraypattern providing acloser representation ofthe natural rainfall phenomenon.

The simulator design provides a flexibility that renders it suitable for a widevariety of research applications. To sup-

161

plement the great range of nozzle sizes,combinations of nozzle heights above thestudy surface and nozzle water pressureswill provide other desirable rainfallcharacteristics. However, the combinationof such variables will ultimately depend onthe objectives of individual researchprojects.

ACKNOWLEDGMENTS

The authors would like to thank D. Beals for

his constructive suggestions with the initialsimulator design. Special appreciation isextended to B. K. Hohner, I. J. Shelton,E. E. Vaughan and B. G. Johnson for theirhelpful suggestions regarding the developmentof the simulator and the calibration procedure.Development funds were provided by theOntario Ministry of Agriculture and Food, theNatural Sciences and Engineering ResearchCouncil and Agriculture Canada (Great LakesWorking Group).

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BLACK, P. E. 1972. Hydrographresponses togeomorphic model watershed characteristicsand precipitation variables. J. Hydrol. 17:

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BUBENZER, G. D. 1979. Inventory of rainfallsimulators. Rainfall Simulator WorkshopTucson, Arizona., Agric. Review ManualARM-W-10. pp. 120-130.

BUBENZER, G. D. and B. A. JONES, Jr.1971. Drop size and impact velocity effectson the detachment of soils under rainfall sim

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CHOW, V. R. and T. E. HARBAUGH. 1965.

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CHRISTIANSEN, J. E. 1942. Irrigation bysprinkling. University of California Agric.Exp. Sta. Bull. 670.

ELLISON, W. D. and W. H. POMERENE1944. A rainfall applicator. Agric. Eng. 25:220.

HART, G. E. 1984. Erosion from simulatedrainfall on mountain rangeland in Utah. J.Soil Water Conserv. 39: 330-334.

LANGFORD, K. J. 1970. A review of designsfor simulators. University of Melbourne.Agric. Eng. Rep. 17.

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LYLES, L., A. DISRUD, and W. P. WOODRUFF. 1969. Effectsof soil physicalproperties, rainfall characteristics and windvelocity on clod disintegration by rainfallsimulation. Soil Sci. Am. Proc. 33:195-205.

MEEUWIG, R. O. 1969. Infiltration and soilerosion as influenced by vegetation and soil

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MEYER, L. D. 1965. Simulation of rainfall forsoil erosion research. Trans. ASAE (Am.Soc. Agric. Eng.) 8: 63-65.

MEYER, L. D. and W. C. HARMON. 1979.Multiple intensity rainfall simulation for erosion research on row sideslopes. Trans.ASAE (Am. Soc. Agric. Eng.) 22:100-103.

MEYER, L. D. 1979. Methods of attainingdesired characteristics in rainfall simulation.

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MORIN, J., D. GOl DBERG, and I. SEGINER.1967. A rainfall simulator with a rotatingdisk. Trans. ASAE (Am. Soc. Agric. Eng.)10: 74-79.

MUTCHLER, C. K. and K. C. McGREGOR.1979. Geographical differences in rainfall.Rainfall Simulator Workshop Tucson, Arizona Agric. ReviewManual ARM-W-10. pp.8-16.

NEFF, E. L. 1979. Why rainfall simulation?Rainfall Simulator Workshop Tucson, Arizona.,- Agric. Review Manual ARM-W-10.pp. 3-7.

PALL, R., W. T. DICKINSON, D. BEALS,and R. McGIRR. 1983. Development andcalibration of a rainfall simulator. Can.Agric. Eng. 25: 181-187.

TURNER, A. K. 1965. The simulation of rainfallsfor studiesinoverland flow. J. Inst.Eng.Austr. 37:9-15.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 29, NO. 2, SUMMER 1987