MECAG 309 INFILTRATION
1.
2.
3.
Rainfall simulators
Flooding infiltrometers
Basin
Ring
Watershed
4.
Infiltration
1
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 2
1 Definitions
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Infiltration refers to the downward movement of water into soils and may be defined for rain or ponded conditions (Richards 1952) as:
"The maximum rate at which a soil, in a given condition at a given time, can absorb rain." or
"The rate at which a soil will absorb water ponded on the surface at a shallow depth when the ponded area is infinitely large or when adequate precautions are taken to minimize the effect of divergent flow at the borders."
Infiltration " is the volume of water passing into the soil per unit of area per unit of time and has the dimensions of velocity ." (L 3 L -2 T -1 ). The term infiltrability refers to the infiltration rate resulting when water at atmospheric pressure is made freely available at the soil surface
(Hillel 1982). Infiltrability is thus dependent upon soil conditions ( surface controlled ), such as moisture, matric potential, pore size distribution, whereas the infiltration rate is supply controlled . The infiltration rate can be greater than the infiltrability if water is ponded sufficiently deep on the soil surface or the infiltration rate can be smaller if water is supplied at a lower rate than the infiltrability. If a shallow layer of water is instantaneously applied and maintained at the surface of an initially unsaturated soil, the full range of soil infiltrability may be measured (Fig. 1). The infiltration rate decreases with time to a constant steady rate while the cumulative infiltration (the time integral of the infiltration rate) increases with time but with a decreasing slope (Hillel 1982). steady
infiltr ability
Decreasing infiltrability steady infiltrability
Time (s) Time (s)
Fig. 1 Time dependence of infiltrability and of cumulative infiltration ( adapted from Hillel 1982).
The determination of the infiltratibility of a soil is an important parameter for irrigation, erosion studies, and many engineering projects.
Infiltration can be considered as a three step sequence: surface entry, transmission through the soil, and depletion of storage capacity in the soil (Chou 1964). Surface entry is influenced by such properties as surface vegetation, soil texture, porosity, structure, and the water stability of the soil aggregates. Transmission rates depend upon texture, porosity, pore size distribution, soil stratification, antecedent soil moisture content, salinity, and biotic
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 3 activity. Storage capacity depends upon porosity and changes in pore size distribution in the soil profile.
2 FACTORS AFFECTING INFILTRATION
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The infiltration rate of a soil profile is the result of the interaction of numerous soil, biotic, and hydrologic properties.
Porosity and pore size distribution are the main determinations of infiltration. The surface area, size, and shape of soil particles influence pore size, shape and continuity with other pores. Although particle size and particle distribution may be a major determinate of infiltration rates (Table. 1) the pore size distribution is modified by organic matter content, aggregation, tillage, and compaction. Compaction loads as small as a person walking can significantly reduce infiltration rates. Compaction from trucks being driven over a sandy loam just after a rain, reduced infiltration rates from 15 to 0.3 cm /h (Akram and Kemper
1979).
Table 1. Infiltration rates for certain particle-size classes (FAO 1979).
Textural
Class
Clay
Silty Clay
Clay Loam
Loam
Sandy Loam
Sand
Infiltration rate (cm/h)
Minimum
0.01
0.03
0.25
0.8
1.3
2.5
Mean
0.05
0.25
0.8
1.3
2.5
5.0
Maximum
0.1
0.50
1.5
2.0
7.6
25.0
In general any profile discontinuity, such as a change in texture, that affects pore size distribution, will result in differing soil porosity size distribution and thus decreased water movement between horizons. Where a coarse textured material such as sand overlies a finer textured material such as a loam, the infiltration rate will be dependent upon the loam layer and water will accumulate in the sand layer. Where a finer textured material overlies a coarse material the initial infiltration rate will be governed by the surface layer, then will reduce when the wetting front encounters the larger pores in the coarse layer. Water will not enter the larger pores until it has accumulated in the fine layer to a point where it can overcome the adhesive and cohesive forces of the finer pores in the fine layer and flow can then take place into the larger pores of the underlying coarse layer (Miller 1973, Gardner 1979).
Vegetation and land use can influence infiltration rates through modification of the soil porosity and pore size distribution (aggregation, organic matter, compaction, tillage) and through interception of the raindrops by the plant canopy (Fig. 2). Generally for agricultural soils the greater the vegetative cover and the greater the time since disturbance the higher the cumulative infiltration. Where soil has been disturbed by plowing and cover does not protect the soil from direct raindrop impact, the pores tend to become clogged by silt and clay
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 4 particles as the aggregates break down. Organic matter in the soil not only serves to create larger pore spaces due to increased aggregation it also provides a stronger 'glue' to bind the aggregates together.
7
6
Old pasture, not grazed
P asture, lightly grazed
5
4
P asture heavily grazed
3
2
Stripped cropped or mixed cover
Grain
1
Recent tilled
Bare ground, crusted
0
0 10 20 30
Time (minutes)
40 50 60
Figure 9.2. Cumulative infiltration curves for different land uses for a silt loam soil
( adapted from Holtan and Kirkpatrick 1950)
3 METHODS OF MEASUREMENT
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Introduction
Numerous methods have been developed for measuring infiltration rates of soils in the field.
These methods may be classified into three groups:
1. Rainfall simulators in which infiltration is determined as the difference between water applied and runoff.
2. Instruments which impound water in a confined area, maintaining a constant head of water;
3. Watershed methods which allow determination of infiltration from rainfall and runoff data.
Rainfall Infiltrometers
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Basically a rainfall infiltrometer simulates rainfall with the use of special spray nozzles set a certain distance (usually 2 to 3 m) above the soil surface. The soil surface tested is usually enclosed so that once runoff commences it can be collected at an opening and the volume measured with time. The difference between the application rate and the runoff rate is taken to be the infiltration. Bernard (1965) lists four conditions that should be met to produce
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 5 accurate and representative measurements of the soil infiltration rate using rainfall infiltrometers:
1. The distribution of drop sizes must be uniform over the plot area;
2. The artificial rainfall must be similar to the natural rainfall being simulated in respect of drop size, drop velocity, intensity range, and total energy value.
3. The plot area must be large enough to sample the population and give reproducible results
(approximately 1 m2); and
4. The artificial rainfall must be applied not only to the plot but also to an adequate buffer area around the plot.
Many of these instruments, originally designed to measure the erosivity of a soil, have been redesigned to apply water at lower rates so that runoff is minimized. Simulators range from simple telephone booth sized installations to ones that require a semi-trailer truck to move.
Cost can be expensive especially if rain characteristics such as intensity, drop size, drop distribution, and velocity are to be accurately simulated. The normal length of an infiltration test employing a rainfall infiltrometer is 30 to 120 minutes with the infiltration rate becoming constant after 20 to 60 minutes.
The main advantage with rainfall infiltrometers is that they simulate the action of rain upon the soil surface. Unprotected soil surfaces will thus reflect surface sealing effects, while those with vegetation will reflect the interception of the rain by the canopy (Parr and Bertrand
1960).
Flooding Infiltrometers
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Introduction. Flooding infiltrometers enclose an area and pond water to a specified depth.
The infiltration rate is calculated from the drop in water level per unit time or the amount of water required to maintain the specified depth or head of water per unit time. Flooding infiltrometers measure the maximum rate of entry of water into the soil. They do not simulate raindrop activity; they measure water penetration rather than rainfall infiltration.
Usually there is a buffer zone of water around an inner compartment of water to correct for lateral movement of water (due to matric potential); thus the inner compartment will be a measurement of the true vertical infiltration rate. Basically there are two types of flooding infiltrometers; the basin infiltrometer which uses earth retaining walls; and the ring infiltrometer which uses metal rings inserted into the ground to retain the water.
Basin Infiltrometer. The basin infiltrometer uses soil from the outside of the basin to construct the paired dykes, thus not disturbing the soil within the dyked areas. The size of the plots, 1m 2 up to 0.2 ha usually accounts for any local soil variation thus reducing the need for replication (Bertrand and Parr 1960). These sizes also reduce errors due to lateral flow especially when a buffer compartment is built. The disadvantages to this type of infiltrometer is the site disturbance and the necessary power equipment and labour to construct the basins and to supply the water.
Ring-Type Infiltrometer. The ring infiltrometer is perhaps the most common type of infiltrometer used. It is inexpensive to construct and operate, it requires relatively little water compared with the basin infiltrometer, and only one person can set up and run several tests simultaneously. The simplicity of its design allows for ease in replication and operation.
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 6
Two concentric rings of stainless steel are commonly employed, the larger ring forms a buffer compartment around the inner to account for lateral flow (Fig. 3). The rings are jacked or hammered into the ground 5 to 10 cm. Care is taken to minimize disturbance of the soil surface and the soil structure during installation. A specific and constant head of water (less than 5 cm of depth) is maintained in both rings, while the rate of water usuage from the inner ring is measured. The length of time required to achieve steady infiltration rate ranges from 2 to 6 hours depending upon soil type, texture, and antecedent soil moisture conditions.
Fig. 12.3. Double Ring Infiltrometer.
Lateral flow is the most serious limitation to the use of ring infiltrometers (Hills 1971). The proportion of lateral flow to vertical flow is dependent upon rings sizes, antecedent soil moisture, texture, stratification, soil structure, and time. Higher antecedent soil moisture contents result in the gravitational potential having a larger relative effect on flow (vertical) than matric potential (vertical and lateral). Finer textures and the presence of stratification results in increases of lateral flow relative to vertical flow. The double ring system providing the most accurate results (while correcting for lateral flow), while at the same time permitting easy portability and installation is one with an outside ring diameter of 0.60 m and an inside ring diameter between 0.20 m and 0.40 m (Swartzendruber and Olson 1961). Larger rings do provide greater accuracy but are too cumbersome for one person to move and install.
A serious limitation to the use of ring infiltrometers is the method of placement. Hammering or jacking the rings into the ground can result in shattering of the soil structure (for dry soils) or compression (for moist soils). Shattered soils, more commonly caused by hammering, can disturb the interface between the soil and the metal ring resulting in leakage and abnormally high and variable infiltration rates. Jacking the rings (insertion of a truck jack under the back bumper) is the preferred method.
Field research has shown that the double ring method usually has higher infiltration rates than that of the basin infiltrometer and that of the rainfall simulator. A series of tests done on similar soils by Bedwany and Schumacher (1979) found that infiltration rates for the double ring was up to 10 times greater than that of the basin, the difference being due to lateral
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 7 seepage and soil cracking. Chou (1964) reports data in which the double ring infiltrometer gave rates 8 times larger than a rainfall infiltrometer for overgrazed pasture and 2 times larger for lightly grazed. Apparently the raindrop impact resulted in aggregate disruption and blockage of conducting pores.
Watershed Methods
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Watershed hydrography ( i.e., measurement of rainfall and runoff from a given area) is often used to calculate infiltration rates. The watershed area is basically the drainage basin as defined by topographic boundaries, all runoff waters collects and flows out of one stream.
Precipitation is measured using rainfall collectors and snow measurements. Runoff is measured from a weir. The main disadvantage is the difficulty of conducting simultaneous comparative studies on such factors as soil type, densities of vegetative cover and tillage practices since few watersheds are similar enough to compare. Watershed hydrography is the common unit of measurement in forested environments.
4. Infiltration Equations (Jury et al., 1991)
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Horton Equation i
H
= i f
+ (i o
- i f
) e  t where i H is the Horton infiltration rate, i o
is the initial infiltration rate, i f
is the final constant infiltration rate that is achieved at large times, and B is a soil parameter that describes the rate of decrease of infiltration.
Philip Equation i
P
= 1/2 St -1/2 + K p where i
P
is the Philip infiltration rate, S = S(  o
,  i
) is called the sorptivity and is a function of the boundary and initial water contents,  o
, and  i
. It is constant during a given experiment in which the inflow end of the uniform horizontal soil column is held at a constant water content. Sorptivity may be measured simply by determining the slope of I versus t 1/2 at initial values of t. The parameter K p , with certain assumptions, may be taken to be saturated hydraulic conductivity or that of steady infiltrability.
5. Field Examples
Frozen soils
Frozen moisture plugs pores; the higher the frozen moisture content the lower the infiltration rate upon melt as the large pores are plugged.
Prairie soils stay frozen until all the snow has melted. The soil is frozen too deep to thaw first before melt (as occurs in the east).
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 8
Mid winter melts can especially reduce infiltration during spring melt. The midwinter melt will freeze into an icy layer on the soil or just in the soil surface and form an impentrable layer to future melts.
The dryer the soil going into the winter, the rougher the surface (increase surface storage), the more 'cracked' the greater will be spring melt infiltration.
Rough rule of thumb; half the snowpack will runoff, less if the soil is dry and cracked, more if there was a wet fall.
Direct seeding might increase runoff due to higher surface densities or it might decrease surface runoff through more larger continuous pores from undisturbed root and insect channels
75
50
Wate r in snowpack be fore me lt: SWE = 75 mm
INF = 5(1 -
 p)SWE 0.584
Runoff
(mm)
25
0
Infiltration
(mm)
25
50
75
0.60
0.40
Relative saturation of soil (
 p)
0.20
cracked
Gray, 1991
2. Sodic soils
Solonetizic soils are notorious for poor infiltrability. This is due to their ability to swell when wet. Infiltration can be improved through deep ripping (below 40 cm), deep plowing and layer inversion, gypsum applications, proper N additions and selection of appropriate grasses or crops. Should consider that deep ripping is a temporary effect (several years). Definite results from chemical and plant management might not show for 50 years.
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 9
3. Tillage
Direct seeding and leaving straw on the surface improves infiltration for rain events so that surface runoff and associated erosion becomes almost zero. Direct seeding (zero tillage) disturbs only about 30% of the surface each year thus large channels caused by roots and burrowing insects/animals remain open from the surface to deep within the soil. Thus waters can infiltrate faster. Straw protects the soil surface from raindrop impact which can cause crusting and resulting runoff
10
No-till 5 yr
No-till 10 yr
No-till 13 yr
Conservation tillage
Summerfallow
8
6
4
2
0
0 20 40 60 80 100
Time from commencement of rain (minutes)
120
Infiltration rates measured with a rainfall simulator on 5 fields near Indian Head, Sk
4. Bare soil and crusting
Bare soil especially on construction sites, road sides, and agricultural soils that have lost their topsoil are especially prone to runoff and erosion. Organic matter is essential to gluing together soil particles into water stable aggregates that are resistant to raindrop impact. A plant canopy also intercepts raindrops thus protecting the surface from crusting. Any exposed soil without organic matter and vegetation needs some form of protection for runoff control.
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 10
7
6
5
No-till for 5 yrs
No-till for 10 yrs
No-till for 13 yrs
Conservation tillage
Conventional Summerfallow
4
3
2
1
20 30 40 50 60 70
Cumulative infiltration (mm) after 60 minutes
80
Cumulative infiltration as related to soil organic matter in 5 agricultural fields near Indian Head, Sk
5. Rangeland
Infiltration capacity is reduced with increased grazing intensity and reduced range condition, mainly through vegetation and litter removal, soil structure deterioration, and compaction.
...Living plant material and associated litter are more significantly correlated with infiltration than any other measured variable. Plant material increases infiltration rates by decreasing the impact of raindrops, improving soil structure through formation of larger soil aggregates, and creating a rougher microtopography that increases infiltration opportunity (Naeth et al., 1990)
REFERENCES
Akram, M. and W.D. Kemper. 1979. Infiltration of soils as affected by the pressure and water content at the time of compaction. Soil Sci. Am. J. 43: 1080-1086.
Bedwany, A.L. and A.E.S. Schumacher. 1976. A comparitve study of two methods of infiltration rate measurements on some soils in Lesotho. Montreal Engineering Company
Limited, Calgary, Alberta. 11pp.
Bertrand, A.R. 1965. Rate of water intake in the field. In: Methods of Soil Analysis, Part 1;
Physical and Mineralogical Properties, Including Statistics of Measurement and Sampling.
Black, C.A. (ed.). No 9 in the series Agronomy. Amer. Soc. of Agron. pp 197-209.
Chou, V. 1964. Handbook of Applied Hydrology: A Compendium of Water Reservoir
Technology. McGraw Hill Book Company. New York.
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan

MECAG 309 INFILTRATION 11
Food and Agricultural Organization of the United Nations. 1979. Soil Survey Investigations for Irrigation. Prepared by Soil Resources, Man. and Cons. Serv. Land and Water Rev.
Div. Rome, Italy.
Gardner, W.H. 1979. How water moves in the soil. Crops and Soils. 32:13-18.
Hillel, D. 1982. Introduction to Soil Physics. Academic Press Inc.
Hills, R.C. 1971. Lateral flow under cylinder infiltrometers: A graphical correction procedure. J. Hydrology 13:153-162.
Holtan, H.N. and M.H. Kirkpatrick. 1950. Rainfall, infiltration, and hydraulics of flow in runoff computation. Trans. Am. Geophys. Union 31:771-779.
Miller, D.E. 1973. Water retention and flow in layered soil profiles. In: Bruce, R.R.,
K.W. Flack, and H.M. Taylor (eds.) Field Soil Water Regime.Soil. Sci.Soc. Special
Publications: 107-117.
Parr, J.F. and A.R. Bertrand. 1960. Water infiltration into soils. Adv. Agron. 12:311-363.
Richards, L.A. 1952. Report of the subcommittee on permeability and infiltration,
Committee on terminology. Soil Sci. Soc. Am. Proc. 16:85-88.
Swartzendruber, D. and T.C. Olson. 1961. Model study of the double-ring infiltrometer as affected by depth of wetting and particle size. Soil Sci. 92:219-225.
M05infiltration.doc Dept of Agricultural & Bioresource Engineering, Univ. of Saskatchewan