Development of a Hand Portable 

Rainfall-Simulator Infiltrometer

GEOLOGICAL SURVEY 

CIRCULAR 482
Development of a Hand Portable 

Rainfall-Simulator Inflltrometer

By I. S. McQueen

GEOLOGICAL SURVEY CIRCULAR 482

Washington T963
United States Department of the Interior

STEWART1. UDALL, SECRETARY

Geological Survey

THOMAS B. NOLAN, DIRECTOR

Free on application to the U.S. Geological Survey, Washington, D. C 20242
CONTENTS

Page 

Abstract. ____________________________ 1

Introduction. _________________________ 1

Principles of infiltration measurement. _ 1 

Irfiltrometer requirements ____________ 4

Design of microrainulator

infiltrometer _ ______________________ 4

Reservoir and control unit___________ 5

The rainulator _____________________ 5

Supporting tripod and wind screen ____ 5

Base and splash shield ______________ 5

Runoff-water and sediment measuring 

system __________________________ 8

Page

Tests of infiltrometer performance____ 8

Laboratory tests __________________ 8

Field tests________________________ 8

Field operation procedures ___________ 10

Site preparation and installation of

the base ________________________ 10

Assembly of the rainulator _________ 10

Making the measurement ___________ 10

Data recording and computations ____ 12

Construction notes___________________ 12

Evaluation of the system _____________ lo

References _________________________ 16

ILLUSTRATIONS

Page 

Figure 1. Details of USGS microrainulator infiltrometer _______________________________ 6

2. USGS microrainulator infiltrometer in use___________________________________ 8

3. Typical set of field notes obtained with USGS microrainulator infiltrometer _____ 11

TABLES

Table 1. Comparison of infiltration rates as measured by USGS microrainulator

infiltrometer and U.S. Forest Service "Rocky Mountain" infiltrometer 

2. Comparisons between infiltrometers ________________________________

Page

9 

. 14

III
Development of a Hand Portable Rainfall-Simulator

Infiltrometer

By I. S. McQueen

ABSTRACT

Comparisons of relative infiltration of summer thundershowers into undisturbed and treated western dry lands are 

necessary for guidance of future treatment practices.

A system designed for measuring infiltration of simulated 

rainfall into' smalt plots of undisturbed soil that can be handcarried to sites inaccessible to vehicles has been developed. 

Disturbance of surface soil by driven plot frames or cylinders, a primary source of errors in infiltration measurement, 

has been eliminated in this design. Water requirements have 

been reduced to 1 or 2 gallons per measurement; so, distilled 

water or controlled chemical solutions can be used. Accurate 

measurements can be made rapidly and economically.

INTRODUCTION

Water and accurate inventories of its 

movement and disposition are becoming vitally important in the national economy. 

This importance is especially true in the 

arid and semiarid areas of the western 

States, where economic development is limited by perpetually inadequate water supplies. Of the total water supplied to a drainbasin as precipitation, a large portion, variously estimated to be from 50 to 90 percent, 

enters the soil and is either used by vegetation or lost by evaporation. The entry of this 

water into the soil and its subsequent disposition are subjects of intense study, but progress in the studies is limited by lack of adequate measuring methods.

Precipitation in the drier portions of the 

western States is sporadic and variable; 

therefore experiments in infiltration and runoff cannot be conducted economically by waiting for natural storms of the necessary magnitude. The alternative to natural precipitation is artificial simulated rainfall applied to 

small plots. This report describes an instrument and system for measuring the infiltration of simulated rainfall into the undisturbed 

surface of soil. The instrument is referred

to as the USGS. microrainulator infiltrometer 

in this report.

The cost of building and operating a rainfall simulator for infiltrometer measurements is roughly proportional to the plot size. 

However, errors in infiltration measurements are generally greater for small plots, 

and any infiltrometer system, therefore, must 

be a compromise between the ideal of a plot 

large enough to reduce errors to a tolerable 

minimum and the limitations imposed by economics and practicability. Increased accuracy can be obtained with a small plot by 

eliminating or compensating for sources of 

error and by making duplicate measurements 

on adjacent similar plots.

The instrument described here was designed to measure infiltration with a high 

degree of accuracy and with minimum cost. 

Errors inherent in earlier infiltrometer designs have been eliminated wherever possible.

PRINCIPLES OF INFILTRATION MEASUREMENT

Infiltration is usually measured indirectly 

as the difference between the water applied 

to a given area and that portion of the water 

applied that does not enter the soil (runoff). 

It is expressed in units of depth per unit of 

time in the United States, this is generally 

inches per hour.

Data from small infiltrometer plots have 

notbeen successfully correlated with rainfall 

and runoff or other field infiltration data. 

This lack of success may be because of the 

variability of infiltration rates with time and 

location, but it is also partly a result of
DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR INFILTROMETER

measuring errors that are inherent in the 

design of the instruments.

Factors in the design of an infiltrometer 

and in the infiltration process that may influence measurements are not clearly defined in the literature. However, the following are known to be important and should be 

considered in the design of an infiltrometer 

system:

1. Disturbance of soil structure during the 

installation of a plot frame or infiltrometer 

ring.

2. Lateral movement of water past the plot 

boundary.

3. Changes in the geometry or relative 

effectiveness of the system of forces that induce or control water movement.

4. Movement of soil particles, dislodged 

during water application, into the pores of 

the surface strata.

5. Hydration and swelling of expansive type 

clay minerals.

6. Moisture content of the soil matrix prior to application of water to the surface.

7. Prior disturbance of soil structure by 

plants and animals.

8. The effects of temperature on soil characteristics and water viscosity.

9. Plot size and shape. 

10. Heterogeneity of soils.

Any ring, tube, or plot frame forced into 

the soil surface to delimit an infiltrometer 

plot will disturb a part of the plot area. 

Water movement into the disturbed zone may 

be increased to several times the natural infiltration rate. On agricultural soils that are 

plowed or cultivated the additional disturbance 

of the driven plot frame is not considered 

important, and many infiltrometer designs 

have disregarded it. On undisturbed soils 

where infiltration is limited by impeding 

zones at or near the surface, any disturbance 

or the surface structure or condition should 

be avoided if accurate measurements are to 

be made. This disturbance is more critical 

on small plots because the disturbed area is 

a larger percentage of the total plot area.

Lateral movement of water past the plot 

boundary is generally considered to be a primary source of error inartificial infiltration 

measurements. Attempted methods to minimize it have included (1) a deeper penetration of longer infiltrometer rings, (2) larger 

ring diameters, (3) buffer zones (double ring 

infiltrometers), and (4) prewetting of the plot 

area. These methods have been used extensively on agricultural soils, but because they 

involve the use of driven plot frames or rings 

that inevitably disturbs the soil and increases 

infiltration, they are unsuitable for untilled 

or virgin soils. An alternative suggested by 

Marshall and Stirk (1950) involves a correction factor based on the relative volume of 

moistened soil inside and outside of the plot 

area following a measurement. If this correction is valid, then lateral movement past 

the plot boundary can be tolerated where the 

disturbance of a plot by a driven ring or 

frame cannot.

The direction of movement and velocity of 

a drop of water at the soil surface are determined by the system of forces acting on the 

water. As infiltration into dry soil starts, the 

capillary force gradient, established at the 

wetting front (Bodman and Coleman, 1943) is 

the primary force inducing movement. This 

movement is opposed by viscous frictional 

resistance in the water. As the length of the 

flow paths increases with movement of the 

wetting front, the frictional resistance increases. If no forces other than the capillary gradient and frictional resistance are 

involved, the quantity of water infiltrated 

would be prpportional to the square root of 

time (x = kt 2, where y is the depth of water 

infiltrated in time t, and k is a constant depending on the units used and the permeability of the soil).

An infiltration equation commonly appearing in the literature is in the form 1 = A t B , 

where I is the depth of infiltration, A is a 

constant, t is the time since the start of infiltration, and B is a dimensionless positive 

exponent smaller than unity. In this equation, 

A has been defined as a measure of the average infiltration rate for the first unit time interval (Swartzendruber and Huberty, 1958). 

The exponent B is an indicator of the changes 

in the system of force with time, and AB is a 

combination parameter indicating the instantaneous rate at the end of the first unit of 

time. If B is close to 0.5, it may be assumed
PRINCIPLES OF INFILTRATION MEASUREMENT

that capillary forces are controlling infiltration. The deviation of B from 0.5 is a measure of the relative importance of other forces 

and factors in controlling infiltration. Gravity forces and hydraulic pressures, such as 

ponding, cause higher values for B .

Natural raindrop impact dislodges soil particles and moves some of them into the pores 

of the soil surface. In order to include the 

effects of this action in an infiltration measurement or to obtain a measure of soilerodibility, the water must be applied as simulated rain. The simulated rainfall should duplicate the type and intensity of storms characteristic of the area where measurements 

are being made.

The energy of natural rainfall is extremely variable during a storm and between 

storms. Even if a rainfall simulator could 

be programed to duplicate exactly the energy 

distribution of a given storm, there is no insurance that the chosen program would be 

representative of the most common occurrence. Also, computation of infiltration from 

a variable application rate would be cumbersome. A constant rainfall intensity having a 

constant kinetic energy, if it is within the 

range of energy levels commonly occurring, 

is more satisfactory than a changing rate.

The kinetic energy of a falling body is equal 

to one half the mass times the square of the 

velocity. For a given unit of precipitation 

(for example, one inch), the energy is a function of the velocity. The data of Laws and 

Parsons (1943) indicates that the average 

diameter of rain drops in a storm having an 

intensity of 1 inch per hour is 2.25 mm. A 

drop of this size would require a free fall of 

10 meters to reach a terminal velocity of 7 

meters per second (Laws, 1941). A drop of 

5 mm in diameter falling 3 meters would 

have the same velocity and therefore the 

same energy per unit of rain. Both the drop 

size distribution and the energy of natural 

rainfall can not be practically duplicated, but 

a larger drop falling a shorter distance has 

the same energy per unit of rain and therefore a similar erosive force.

Soil particles are removed either suspended in the excess water accumulating on 

and running off of the soil surface or carried 

by drops of water splashed from the plot. 

For studies where erodibility of the soils is 

important, measurements of the quantity of

soil particles removed should be made. 

Methods of collecting and measuring sediments should be compatible with the use to 

be made of the data.

If the information desired is the contribution of a given soil to the rate of silting of a 

reservoir, the sediments may be measured 

as wet volumes. If the weight of soil removed is required, it may be necessary to 

retain all the runoff water and sediments 

and take them to a laboratory for analysis.

Expansion of clay minerals during hydration reduces the volume of soil pores and, 

therefore, the infiltration rate. Ions, from 

salt dissolved in water may influence the 

expansion of clay minerals. In general, sodium will increase swelling and thus decrease infiltration, whereas calcium may 

have the opposite effect. In order to simulate natural conditions, the water applied by 

an infiltrometer should be of nearly the 

same chemical composition as natural rainfall. Distilled water is similar to natural 

rainfall, but its cost is prohibitive in most 

infiltrometer tests.

Infiltration into moist soil is generally 

slow because capillary force gradients are 

small and clay minerals are expanded, thus 

constricting flow channels.

Insects, rodents, and plants alter the 

structure of soil and provide channels for 

flow in excess of natural rates through the 

undisturbed soil mass. The channels may 

be opened or closed during a measurement 

by raindrop impact. This alternation causes 

abrupt changes in infiltration rates.

Water temperature influences the infiltration rate in two ways: (1) Warm water has 

a lower viscosity and therefore produces 

higher infiltration rates; (2) increased soil 

temperature increases the volume of the soil 

grains, and therefore the volume of the voids 

decreases, resulting in lower infiltration 

rates. The effect of temperature, however, 

is so slight that it can be disregarded for 

most infiltration measurements.

The accuracy of infiltrometer measurements is influenced by the size and shape of 

the plot. Plot sizes have ranged from as 

much as several acres in infiltration ponds 

to as small as rings of 2 or 3 inches in diameter. Shapes most often used are rectangular
DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR INFILTROMETER

or circular. Increased infiltration leading to 

erroneously high measured rates generally 

occurs at the plot boundary either as capillary underflow or as accelerated flow rates 

through the disturbed soil. The relative 

magnitudes of these errors are a function of 

the ratio of boundary length to the plot area; 

to minimize the errors, this ratio should be 

as small as possible. It can be shown that 

the ratio is smallest for an equilateral and 

regular polygons. For a given area, the ratio 

becomes smaller as the number of sides increases. This relation indicates that the 

ratio is smaller for a circular plot than for 

a polygon of the same area. The ratio decreases as the plot size increases. The plot 

having the best shape, therefore, is a circle. 

The size will be controlled largely by economics and the requirements of portability.

Because soils are heterogeneous, the infiltration rates for the soils within a given basin have a wide range of values. In order to 

obtain an accurate average value, either the 

entire basin must be measured or a statistically adequate sampling of point values distributed throughout the basin must be obtained.

INFILTROMETER REQUIREMENTS

In setting up requirements for a new infiltrometer system, the purpose of the measurements and the conditions existing that 

might influence them must be considered. 

The USGS microrainulator infiltrometer described herein was designed for research on 

wild dry lands in the western United States. 

These lands are not tilled, and any disturbance of the surface structure introduces errors into the measurements.

Most measurements made with this instrument are comparisons between adjacent 

treated and untreated or vegetated and nonvegetated plots. A measurement of differences is more important than a determination of actual natural rates.

The methods of conducting research on the 

wild lands and remote locations of some research sites impose rigid portability and 

water-use requirements that could only be 

met with an extremely small instrument.

The following specific requirements were 

set up to guide selection and design of the 

USGS infiltrometer:

1. The accuracy and constancy of measurements should be equal to or better than 

other infiltrometer systems currently in use.

2. It should be portable. It is to be transported in field vehicles such as pickup 

trucks, carryalls, or sedan delivery trucks. 

It should be portable by hand to inaccessible 

sites if necessary.

3. Water requirements should be as small 

as possible, consistent with accuracy requirements.

4. It should not disturb the surface structure of the soil during installation or operation.

5. It should provide a means for preventing 

lateral movement of water past the plot 

boundary, or, for measurement of and correction for such lateral movement.

6. It should provide for collection and 

measurement of water and sediments 

splashed out of the plot separately from 

water and sediments accumulating on the 

plot. Water should be allowed to accumulate 

on the plot only to the depth necessary to 

cause runoff under natural storm conditions.

7. Energy of simulated rain should correspond to the average energy level of natural 

storms.

8. Rainfall intensity should be controlable 

over a wide range and should be stable at 

any intensity chosen for a given test.

9. Installation and operation of the instrument should be rapid and convenient. Data 

obtained should require little or no computation.

10. The instrument should operate satisfactorily under adverse conditions, such as 

high winds.

DESIGN OF MICRORAINULATOR INFILTROMETER

Several types of infiltrometers currently 

in use elsewhere were investigated to determine their adaptability for use on undisturbed plots of dry wild lands in the western 

United States. An infiltrometer developed at 

Iowa State College and described by Adams, 1

1 See also Adams,J.E., 1956, A rainfall simulator and the credibility of some Iowa soilsttowa State College, Unpub.. Ph. D. thesis.
DESIGN OF MICRO RAINULATOR INFILTROMETER

Kirkham, and Nielson (1957) has several features that were adaptable to our requirements. Preliminary design and development 

of the USGS instrument, using some of these 

features, were started in 1957. The first instrument was completed and subsequently 

tested in the laboratory and field in 1958, 

and a second instrument having an improved 

design was constructed for use in 1959.

Details of the USGS microrainulator infiltrometer in its present form are shown in 

figure 1. The instrument comprises several 

units: (1) A reservoir and control unit; (2) a 

rainulator; (3) a supporting tripod and wind 

screen; (4) a base unit containing a splash 

shield; and (5) a system for measuring runoff 

water and sediment. The units are described 

in the following paragraphs.

RESERVOIR AND CONTROL UNIT

The reservoir and control unit performs 

three functions: (1) It supplies water to the 

rainfall simulator (rainulator), (2) measures 

the quantity of water supplied, and (3) controls the hydraulic head and consequently the 

rate of application of the rainulator. It is 

essentially a calibrated Mariotte bottle and 

an added bubbler tube to extend the controlled level below the reservoir and to provide for more sensitive control. The unit is 

calibrated to measure the depth of water applied to the plot area either in inches or in 

centimeters. The spacing of the calibration 

marks is computed from the ratio of the 

square of the plot diameter to the square of 

the diameter of the reservoir. If the reservoir is 3.5 inches in diameter and the plot is 

5.75 inches in diameter, the ratio is 

5.752 /3.52 =2.698. Therefore, 2.698 inches 

on the reservoir represents one inch of precipitation or 2.698 centimeters on the reservoir represents one centimeter of precipitation.

THE RAINULATOR

The rainulator utilizes the same principles 

of flow control and drop formation described 

by Adams, Kirkham and Nielson (1957), but 

the materials and methods of construction 

are different. In the design described by 

Adams, Kirkham, and Nielson, drops were 

formed on the ends of glass capillary tubes 

set in a plastic matrix, and flow was limited 

by a pin made of re-drawn chromel wire inserted through each tube. However, capillary

tubing and wire of the proper size and uniformity to give the needed range of application rates was not available here. As an alternative, capillary-size holes drilled 

through a block of plastic with pins made of 

standard size chromel wire were tested and 

adopted. Adams reported (written communication, 1958) application rates of 4.03 to 4.32 

inches per hour with minimum hydraulic 

head of 6 mm. With careful drilling, we achieved uniform application rates as low as 

0.8 inch per hour with a minimum hydraulic 

head of 1 inch. A final drilling with a No. 57 

drill provided available rates of 1 inch to 15 

inches per hour.

Drop size is controlled by the geometry of 

the bottom surface of the simulator head and 

by the properties of the plastic from which 

the head is machined. Adequate drop size 

control has been obtained by three different 

methods: (1) projections simulating the ends 

of glass capillary tubing have been machined 

in plexiglas by use of a spot-face tool; (2) 

similar projections have been produced by 

casting plastic in a mold; and (3) countersinking the capillary holes as shown in the detail of figure 1, which allows an easy control 

of drop size that is proving very effective.

SUPPORTING TRIPOD AND WIND SCREEN

The instrument is supported over the infiltration plot by adjustable tripod legs such 

as are used in levelling or planetable mapping. The windscreen is a length of 6-inch 

OD plastic tubing hung in an aluminum ring 

that is supported by the tripod legs. A bullseye-type level mounted on the support ring 

indicates when the wind screen is vertical. 

A steel rod half an inch in diameter extends 

up from the support ring to hold the reservoir and control assembly. The rainulator 

fits in the top of the wind screen. An alternate method of mounting the infiltrometer 

utilizing a standard planetable tripod is 

shown in figure 2B'. This method was designed for easier positioning over the plot, 

but the instrument was found to be sensitive 

to wind disturbance.

BASE AND SPLASH SHIELD

Two forms of base units used with the infiltrometer are shown in figure 1. Both 

bases are designed to be sealed to the soil 

surface with bentonitic clay. The plastic 

base is used for special studies where the
DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR 1NFILTROMETER

Reservoir and 

control unit.

91 drop formers 

on 1/2 in. centers

Rainulator

Level

Tripod

RAINULATOR 

Scale 1:4

\

Alternate base 

(separate splash 

and runoff)

Figure 1. Details of USG!
DESIGN OF MICRORAINULATOR INFILTROMETER

PIN: 18 gage (0.0403) or 19 gage (0.0359) chromel wire. 

Hook formed on end with remainder of wire perfectly straight

CAPILLARY HOLES: For 18 gage pin, drill with No 60 

and ream with Nos 59, 58 and 57 drills. Withdraw drill 

bit each 1/8 in. of hole to clear chips and prevent heating. 

For 19 gage pin, start with No 65 drill. Bottom of hole 

is countersunk to form drop control surface. 

Note: Water must adhere to plastic but should not be 

absorbed by it. Acrylic (lucite), acryiate (plexiglas) and 

polystyrene plastics have been used successfully

ALTERNATE RAINULATOR: Machined with modified 

spot-face tool which leaves projections that simulate 

ends of capillary tubing. Drop size is slightly 

smaller and more uniform

ALTERNATE CAST RAINULATOR: Precast core 

disc, 5-1/2 in D, prevents distortion by shrinkage 

forces during curing of plastic. Drilling preceedure 

and pins are the same as those on machined units

Core supported by 

3 needle points

ALUMINUM MOLD

PRODUCTION DETAILS 

Scale 1:2

1/4 in. 0 D Acrylic tubing 

1 in. I D Acrylic tubing

i.9- -.7 !

.4 I

Scale indicates water applied to plot area 

in inches or centimeters

Scale Factor (SF)= Plot diameter 2 

Reservoir diameter2

= 2- 351

2.351 in on reservoir = 1 in of precipitation

/

Scale indicates accumulated runoff from plot 

in inches or centimeters

1/8 in. drain cock

=10.796

Calibration of lower conical section determined 

by introducing known volumes of water or 

by precise volume computation 

1 cm runoff = 25.967 cc 

1 in runoff = 65.957 cc

No 5 1/2 Stopper 

No 10 Stopper

Runoff and sediment 

measuring tube 

scale 1:4

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11-

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RESERVOIR 

Scale 1:4

microrainulator infiltrometer.
DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR INFILTROMETER

Figure 2. USGS microrainulator infiltrometer in use. A, near 

Carrizo, Ariz., showing preferred support assembly." B, near 

Fort Peck, Mont., showing alternate tripod assembly.

water and sediments splashed from the plot 

are measured separately from the water accumulating on the soil surface as runoff. 

Where separation of the splash and runoff is 

not necessary, the metal base is more economical and easier to use. A base similar to 

that described by Adams may be used if the 

soil is sandy and structureless or has been 

tilled.

RUNOFF-WATER AND SEDIMENT MEASURING SYSTEM

Excess water and suspended sediments are 

withdrawn from the plot to calibrated accumulator tubes through flexible plastic tubing 

by applying a partial vacuum to the accumulator tubes. The vacuum is produced by rubber gas-sampling pumps. The accumulator 

tubes are 2-inch OD rigid plastic tubing 

having bottoms machined from 2-inch OD 

plastic rod. In order to provide for accurate 

measurement of sediment volumes, the tube 

bottoms are accurately machined cones. The 

accumulator tubes are calibrated to measure 

the depth of water and sediment removed 

from the plot in centimeters or in inches.

TESTS OF INFILTROMETER PERFORMANCE

The individual units of the USGS microrainulator infiltrometer were thoroughly 

tested in the laboratory during development. 

The complete system was tested in the field 

for two years. As a result of the tests, some 

minor modifications were made in the instrument and the operating procedures.

LABORATORY TESTS

The rainulator unit was tested in the laboratory for uniformity of rainfall pattern, 

drop size, and range of application rates.

The uniformity of the rainfall pattern was 

checked by counting the drop rates from randomly selected capillaries, by observing the 

wetting pattern on absorbent paper, and by 

observing the erosion pattern on a series of 

test plots. In all tests the uniformity was 

judged to be good to excellent. The drop 

size, measured by weighing a given number 

of drops and computing their average volume, was found to be equivalent to a sphere 

5.61 mm in diameter. Calibrations of all the 

measuring units was checked in the laboratory. A final test of the calibrations was 

made by setting the instrument up on an impermeable floor and by operating it at a low 

application rate. In the test the water applied 

equaled the runoff plus a small correction 

for evaporation.

FIELD TESTS

Field tests included a series of infiltration 

measurements on a variety of wild natural 

undisturbed soils in Colorado, Wyoming, and 

Montana. During the measurements operating procedures and accessories were developed and tested.
TESTS OF INFILTROMETER PERFORMANCE 9

In September of 1958, the U.S. Forest Service conducted a series of infiltration measurements in connection with the Badger Wash 

cooperative hydrologic study using a "Rocky 

Mountain" type F infiltrometer. The study 

provided an excellent opportunity to make 

comparative measurements with an instrument that has been in use for several years. 

Measurements were made on 16 plots near 

those of the Forest Service tests and in the 

same soil types. Results are summarized in 

table 1. The application of a t test to the 

paired data indicates that no significant difference in the mean values of results was 

obtained on soils that were measured by both 

instruments. ( t = 0.258; with 15 degrees of 

freedom, a value of t greater than 2.947

would have to be obtained in order to define 

a significant difference at the 1 percent 

level.)

As a further check on the comparative performance of these infiltrometer s, the data 

from the various plots were separated according to soil type. Four pairs of measurements were made in sandy soil; 4 pairs of 

measurements, in a mixed sandy shale soil; 

and 8 pairs of measurements, in a shale-derived soil. The t values for the soil groupings 

indicate the following: (1) On the sandy soil, 

there is a 30 percent chance that the difference in the mean values is not due to the 

characteristics of the instruments (t^ = 1.19), 

(2) on mixed soil, there is no statistically

Table 1. Comparison of infiltration rates as measured by USGS microrainwlator infiltrometer and U.S. Forest Service "Rocky

Mountain" infiltrometer

[The infiltration rate is the average rate for a 40- to 60-minute interval]

Plot

Particle size distribution 

(millimeters)

Gravel 

2.0

Sand 

2.0-0.05

Silt 

0.05-.002

Clay 

0.002

Infiltration rate 

(inches per hour)

USGS microrainulator

USFS "Rocky 

Mountain"

Difference

Sandy-soil type

12-1____

12-2____

33-l_ __

33-2_ __

Mean_

9

4.6

9.6

5.5

7.2

42

42.4

42.9

56

45.8

38

42

31.5

29.5

35.2

11

11

16.0

9.0

11.8

3.45

4.53

2.37

4.23

3.64

2.76

2.21

3.08

3.57

2.90

0.69

2.32

-.71

.66

Mixed-soil type

7-l_ __

7-2 ___

35-1 ___

35- 2_ __

Mean_

0.7

10.9

12.3

21.2

11.3

38.3

32.1

31.7

27.7

32.4

43.3

40.9

38.0

32.9

38.8

17.7

16.1

18.0

18.2

17.5

0.81

1.38

1.29

1.35

1.21

0.68

1.88

1.11

1.06

1.18

0.13

-.50

.18

.29

Shale-soil type

B-l 1 ....

8-2 __

9-1 ___

9-2 __

18- 1_ __

18-2_ __

32- 1_ __

32-2 __

Mean_

7.8

4.2

1.1

1.2

2.1

4.1

2.6

1.0

3.0

17.2

17.3

17.9

12.8

15.7

14.6

13.0

11.2

15.0

51.7

54.2

54.0

56.5

53.2

53.8

54.2

55.3

54.1

23.3

24.3

27.0

29.5

29.0

27.5

30.2

32.5

27.9

0.30

.60

.81

.39

.81

.84

.51

.42

.58

0.99

.84

1.01

.86

.98

.98

.87

.45

.87

-0.69

-.24

-.20

-.47

-.17

-.14

-.36

-.03

Mapped as mixed-soil type but grain-size distribution is closer to shale-soil type.
10 DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR INFILTROMETER

significant difference in the mean values 

(t = 0.14), and (3)on the shale soil, there is a 

significant difference at the 1 percent level 

(t = 3.83).

From the results of this comparison with 

the "Rocky Mountain" type F infiltrometer, 

certain relative characteristics of the USGS 

microrainulator infiltrometer can be deduced. On sandy soil the USGS microrainulator infiltrometer may indicate a higher infiltration rate because of greater capillary 

flow past the plot boundary. A correction 

can be made for this rate by excavating the 

wetted soil following a measurement and by 

measuring the relative volume of wetted soil 

outside of the plot boundary. On shale-derived soils, the USGS microrainulator infiltrometer consistently indicated a lower infiltration rate than the "Rocky Mountain" infiltrometer.

Infiltrometers generally indicate infiltration rates that are higher than actual, because of soil disturbance at the plot boundary, lateral movement past the plot boundary, 

or failure to form pneumatic back pressures 

in the limited area covered. The consistently lower rate indicated by the USGS microrainulator infiltrometer shows that it is 

more accurate on fine textured soil. If corrections are made for lateral movement past 

the plot boundary, the instrument is more 

accurate on all types of undisturbed soils. 

Disturbance of the soil structure by a driven 

plot frame is a primary source of inaccuracies in infiltration measurements.

FIELD OPERATION PROCEDURES

In use of this instrument for measurement 

of infiltration, runoff, and erosion, some procedures may be modified to speed up operations or to provide specific information that 

may not always be required. Figure 2A and 

B shows two models of the instrument in 

operation. The basic procedure for use of 

the instrument is presented in the following 

paragraphs.

SITE PREPARATION AND INSTALLATION OF THE BASE

The first step is to locate precisely and 

mark the plot according to requirements of 

the given study. A plastic cylinder is positioned on the plot to facilitate preparation. 

Vegetation and litter are cleared from the 

area outside the plot for a distance of 3 or 4

inches by clipping the vegetation at the soil 

surface and around the plot boundary. The 

infiltrometer base is positioned on the plot 

and sealed to the cleared soil surface with 

bentonite. The plot is normally not prewetted 

so that the test may simulate the effect of a 

thunder shower on dry soil; however, when 

measuring infiltration resulting from gentle 

long-continued rains, prewetting of the soil 

or long term continuous or intermittent application of simulated rain may be used.

ASSEMBLY OF THE RAINULATOR

With the base in place, the supporting tripod and wind screen are positioned so that 

the wind screen is vertical and is centered 

directly over the plot. The reservoir, control unit, and rainulator head are installed 

on the supporting tripod, and the reservoir 

is filled with water. For the first measurement made, the rainulator unit should be 

prewet by setting it on a clean surface and 

pouring water into it. This should be done 

prior to preparation of the plot in order to 

give a longer time for stabilization of the 

flow rate.

The pickup tubes for excess water are installed in the base so that water will be withdrawn from the soil surface on the downslope side of the plot. The measuring tubes 

can be placed anywhere that is convenient 

for easy reading, commonly a few feet downslope from the plot.

MAKING THE MEASUREMENT

To start an infiltration measurement, water is poured into the rainulator unit to a 

level higher than required to give the desired 

application rate. The timing is started, and 

the water level in the unit is read when the 

first drops fall. When the water in the rainulator unit drops to the level that previous 

experience has shown will produce the desired application rate, the reservoir flow 

control unit is adjusted to maintain this 

level. As water accumulates on the plot, it 

is pumped into the measuring tubes. As each 

5-minute interval ends, the pumping of excess water is interrupted, and the water levels in the reservoir and the rainulator unit 

are read and recorded. Before pumping is 

resumed, the water and sediment levels in 

the accumulator tubes are read and recorded.
FIELD OPERATION PROCEDURES

Infiltrometer Data Sheet

11

Date '7, I960 Operator 5

Project ^e/g,f/'»r?s Soil Moisture - rs/V?

Location Sa.e.- Con?nnonl-fcv Cover On plot A/O|

Time

Scale Reading c

Roinulator Reservoir Runoff

H 2

Water Applied

Increment Cumulative

AA

Infiltration

i 

(A-H3)

Sediment

Remarks

0940

0945- .10 0 Ql 0. 81

09 so 1.35- /. 12. 0. 30 . 96 t- 77 t .47

09 56 / 39 2.^9 t. 84 0.02.

/ooo / 3! 3. oo /. 40 / . 10 3.69 2. 29 . 025

/OO5" /. 32. 3.98 .2 . // . 97

10(0 1 . 33 4. 88 .89 . 75" .045

: 1015- S--75- 3. .90 Z. 9?

(020 /. 33 £.7 . // 99 3 . 33

Ji

L

uJ -fi 1026- 7. 70 S .38 3. . 070

8.64 S~. 60 3 . 07

?. o 3

/OB' OB5-

I

a. LU 

o

Storm.

10:00 io'lo 10:2.0 

TIME

10:4*

Figure 3. Typical set of field notes obtained with USGS inicrorainulator infiltrometer.
12 DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR INFILTROMETER

DATA RECORDING AND COMPUTATIONS

Typical field data are shown in figure 3. 

Measurements are recorded at the end of 

each 5-minute interval, and the infiltration 

is computed and plotted on a cumulative 

basis. Rates of application, runoff, and infiltration are obtained from accumulative 

curves, or they may be computed as average 

rates for any given time interval.

The data were recorded and computed as 

follows (columns are counted from left to 

right):

Column 1. The time is recorded to the 

nearest minute. However, the readings of 

the levels in the rainulator (H,) and reservoir (EL ) should be made within 10 seconds 

for accurate work.

Column 2. Depth of water in the rainulator is recorded to the nearest 0.1 mm or 

0.01 inch if conditions will permit.

Column 3. The volume of water removed 

from the reservoir is recorded to the nearest 0.1 mm or 0.01 inch of water on the plot 

area. If the level in the rainulator remains 

constant the change in the reservoir is the 

depth of water applied to the plot during the 

interval.

Column 4. The total water accumulated 

in the runoff measuring tubes (Hg) is recorded to the nearest 0.1 mm or 0.01 inch 

of water on the plot area. If splash and runoff are collected separately, this value is the 

sum of the two measurements.

Column 5. The water applied as simulated rainfall during the preceding 5-minute 

interval (A A) is computed from columns 2 

and 3 as the change in the reservoir minus 

the change in the level in the rainulator.

Column 6. The total water applied since 

the start of the test (A) is computed either 

from column 5 or from columns 2 and 3.

Column 7. The total infiltration since the 

starf of the test (I) is computed as the total 

water applied minus the total water accumulated in the runoff measuring tubes. For 

precise work this value may be corrected 

for evaporation.

Column 8. The accumulation of wet settled particles in the runoff measuring tube

is measured to the nearest 0.001-cm depth 

on the plot area. If the weight of eroded material is needed, it may be estimated from 

the wet-depth data, or the water an,d sediments may be retained for accurate weight 

determination in the laboratory.

Correlation of depth of sediments accumulated in the measuring tube with the weight 

of the settled particles for 47 samples obtained near Fort Peck, Mont., indicated the 

average unit weight of the sediments (dryweight)to be 0.906 gm/cc (or 56.6 Ibs/cu ft). 

The coefficient or correlation is 0.901, and 

the relationship isW = 153.8d<- 3.6. where W is 

the weight of sediment in grams and d is the 

depth of sediment in the accumulation tube 

in centimeters. By introducing the proper 

conversion factors into the foregoing relationship, a reading of 1 cm of sediments in 

the measuring tube represents 57 tons of 

sediment per acre.

Column 9. Additional observations made 

during the test are recorded to aid in the 

evaluation of data.

As all calibrations are in units of depth of 

water on the plot area, the rate of application 

can be computed immediately for any given 

time interval, and the instrument can be adjusted to apply the desired or prescribed 

rate in the succeeding intervals.

When a test is completed, the flow from the 

reservoir is stopped, and the rainulator unit 

is removed from the infiltrometer. Postrun 

measurements and sampling, as required in 

the project plans, may be performed. The 

wetted soil may be excavated and volume 

measurements may be made to define the 

relative movement of water past the plot 

boundary. The excess water may be retained 

for chemical analyses. The sediments in the 

excess water may also be retained for laboratory studies and gravimetric measurements. When all postrun measurements are 

completed, the instrument is then moved to 

the next plot or disassembled for transport.

CONSTRUCTION NOTES

During the development and testing of the 

infiltrometer system, certain methods and 

materials were found to produce the desired 

results. These are discussed here for the 

benefit of those who may want to construct 

the instrument.
EVALUATION OF THE SYSTEM 13

The capillary holes in the simulator head 

must be drilled with extreme care. Drills 

as small as the No. 60 when working in plastics will not clear the chips from a hole, and 

they cannot be cooled adequately. To prevent binding of the chips or melting of the 

plastics, do not drill more than an eighth of 

an inch of hole without withdrawing the bit 

for clearing and cooling. The initial drilling 

constructs a hole the same size or slightly 

smaller than the wire size to be used for 

pins. The holes are then reamed to the 

proper size in two or three steps using a 

hand drill and successively larger drill bits.

Drill diameters do not increase in uniform increments, which causes some difficulty in hole construction. No. 18 gage chromel A wire is used for the flow-control pins 

in the capillaries, because this is the size 

that Adams used. The required hole diameter, to provide the desired range of application rates with 18-gage pins, is 0.044 inch; 

this is between the No. 57 and No. 56 drill 

sizes. No. 19 gage wire (0.036 in.) is recommended for use because it permits closer 

control of the hole size. For No. 19 gage 

wire the initial drilling should be with a No. 

65 drill.

Control of drop size requires a surface to 

which the drops can cling until they grow to 

the required volume before falling off. A 

flat face one-fourth inch in diameter similar to the end of a glass capillary tube will 

support a drop 5.5 mm in diameter. This 

type of surface has been formed by machining a plastic block with a modified spot-face 

tool or by casting in molds.

As an alternate drop-forming surface, we 

found that if the capillary holes drilled in a 

plastic block are countersunk, the drops will 

cling to the conical countersink surface and 

their size will be influenced by the diameter 

of the countersink. This type of control surface seems to be more effective than the flat 

faces, and it is easier to construct.

The plastic material used as the rainulator 

should have certain physical characteristics. 

It should be wettable so that waterdrops will 

cling to it readily. If waterdrops do not 

cling to the size-control surfaces, their size 

will be uncontrolled. The dimensions of the 

plastic should be stable in contact with water. The material should not absorb water 

and swell, and it should not soften. Any

change in the dimensions of the capillary 

holes due to swelling or abrasion will result 

in a changing flow rate. Plexiglass, polystyrene, and metallic-filled epoxy resin have 

been used; of these, the plexiglass is the 

most satisfactory.

EVALUATION OF THE SYSTEM

The infiltrometer system was tested and 

used during three field seasons. It produced 

results that comply with the requirements 

set for it. Its extreme portability, low water 

requirements, simplicity of operation, and 

versatility make it a useful instrument for 

hydrologic studies.

Table 2 shows a comparison between the 

USGS microrainulator infiltrometer and three 

other infiltrometers that are being used for 

research on western United States lands. 

The U.S. Forest Service "Rocky Mountain" 

infiltrometer, described by Dortignac a (1951), 

is one of a long series of sprinkler-type infiltrometers pioneered by the Flood Control 

Group of the Soil Conservation Service and 

adopted by the U.S. Forest Service.

The infiltrometer described by Osborn 

(1950) was developed for the U.S. Soil Conservation Service in Texas and Oklahoma. It 

uses a drip screen to simulate controlled 

precipitation, and it has gone through several 

stages of development.

The infiltrometer described by Barnes and 

Costel (1957) is an adaptation of the drip 

screen type described by Osborn. It has been 

redesigned so that it can be mounted on a 

pickup, truck, and water requirements have 

been reduced considerably. However, the 50 

to 60 gallons per hour required is still too 

much to permit use of distilled water or 

chemical solutions.

When working on nonuniform soils, the 

USGS microrainulator infiltrometer may be 

expected to show some scatter in the results 

because the smaller plot size permits more 

precise definition of point values and less integration of larger areas. As the cost of operation is lesg than one-fourth of the operating cost of the older systems, more measurements can be made to provide a statistical integration.

Lateral movement of moisture past the 

plot boundary is a source of inaccuracy with
14 DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR INFILTROMETER

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16 DEVELOPMENT OF A HAND PORTABLE RAINFALL-SIMULATOR INFILTROMETER

this system. This should be corrected for by 

the method suggested by Marshall and Stirk 

(1950). Disturbance of the soil surface and 

lateral movement of infiltrating water past 

the plot boundary both cause the indicated 

infiltration rates to be high. With no correction for lateral movement, the system indicated lower infiltration rates when compared 

with the results from the "Rocky Mountain" 

infiltrometer. When corrections are made 

for lateral movement, results with this system can be considered very good.

Calibration of the system in terms of depth 

on the plot has simplified measurements and 

the computation of results. This helps to 

eliminate recording errors and detects anomalous behavior, such as sudden changes in the 

infiltration rate during a measurement.

The infiltrometer system is convenient, 

versatile, and economical. Results obtained 

with it are comparable with other systems in 

use in the United States.

REFERENCES CITED

Adams, J. E., Kirkham, D., and Nielson, D. R., 

1957, A portable rainfall-simulator infiltrometer and physical measurements of

soil in place: Soil Sci. Soc. Amer. Proc., 

v. 21, p. 473-477.

Barnes, O. K., and Costel, Gerald, 1957, A 

mobile infiltrometer: Agron. Jour., v. 49, 

p. 105-107.

Bodman, G. B., and Colman, E. A., 1943, 

Moisture and energy conditions during 

downward entry of water into soils: Soil 

Sci. Soc. Amer. Proc., v. 8, p. 116-122.

Dortignac, E. J., 1951, Design and operation 

of Rocky Mountain Infiltrometer: U.S. forest Service, Rocky Mountain Forest and 

Range Expt. Sta., Paper no. 5.

Laws, J. O., 1941, Measurements of the fallvelocity of waterdrops and raindrops: Am. 

Geophys. Union Trans., v. 22, p. 709-721.

Laws, J. O., and Parsons, D. A., 1943, The 

relation of raindrop size to intensity: Am. 

Geophys. Union Trans., v. 24, p. 452-460.

Marshall, T. J., and Stirk, G. B., 1950, The 

effect of lateral movement of water in soil 

on infiltration measurements: Australian 

Jour. Agr. Research, v. 1, p. 253 265.

Osborn, Ben, 1950, Measuring soil splash and 

protective value of cover on rangeland: 

Soil Conserv. Service (Mimeo. rept.).

Swartzendruber, D., and Huberty, M. R., 1958, 

Use of infiltration equation parameters to 

evaluate infiltration differences in the 

field: Am. Geophys. Union Trans., v. 39, 

p. 84-93.

INT.DUP..D.C.63- 38001