Lal R., Shukla M.K. Principles of Soil Physics

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Water flow in unsaturated soils 375

Water Infiltration in Soil

Water is one of the most precious natural resources for the survival and advancement of

civilization. Depending upon the intensity, a part of the rainfall enters the soil surface and

the remaining runs off as an overland flow. An important soil/ecosystem function is to

enhance infiltration of precipitation water into the soil with less runoff and erosion. This

process is controlled by biological and physical interactions, which create a stable soil

structure with enough macropores to rapidly transmit water. Soil that is continually

disturbed with tillage and other anthropogenic activities often develops poor structure,

leading to surface sealing of pores and crusting, and consequently less infiltration and

high runoff. This chapter describes the concept of steady and unsteady state infiltration

and different methods of measurement or prediction of infiltration rate and parameters

affecting it.

14.1 DEFINITIONS AND BASIC CONCEPT

When water is applied on the soil surface either as irrigation or as rainfall, depending

upon the hydraulic conductivity, soil’s antecedent moisture content, and soil aggregation,

a part or whole of the applied water enters the soil matrix. At the same time the remaining

water flows over the soil surface as overland flow. If the soil surface has depressions then

some of that water is initially ponded in those depressions and subsequently enters the

soil profile (Fig. 14.1). The entry of water into the soil matrix through air-soil interface is

called infiltration. The water entry is generally referred to as vertical downward

infiltration. The rate of infiltration of water into soil matrix governs the amount of water

storage in soil, which is available for plants. It also influences the amount of runoff and

erosion. Therefore, knowledge of water infiltration into soil is essential for soil and water

conservation, and minimizing the risk of nonpoint source pollution.

How much water can infiltrate the soil? An answer to this question can be obtained by

applying water using a sprinkler system. When a sprinkler is run initially at a low

discharge, all the water enters or infiltrates into the soil profile. With an increase in

discharge of a sprinkler, a stage comes when water either begins to pond on the soil

surface or begins to move down a slope as overland flow, also called the surface runoff.

The start of runoff or ponding means the rate of water application has exceeded the rate

of water infiltration into the soil. This limiting rate of water entry into the profile is

known as the soil infiltration capacity, and is defined as maximum rate of infiltration into

soil. The infiltration rate is the volume flux of water entering through a unit soil surface

area. According to Richards (1952), the term infiltration rate is to be preferred over

infiltration capacity because capacity refers to the volume rather than to the flow rate.

The infiltration rate of a soil depends on texture, structure, antecedent soil moisture

content (i.e., the moisture content of soil profile before rainfall or irrigation begins),

continuity and stability of pores, and soil matric potential (Lal and Vandoren, 1990). Soil

management including tillage, rotations, residue management, etc., influences these

factors. With low

FIGURE 14.1 A schematic of

processes during a rainfall or irrigation

event along with the components of

hydrological cycle.

antecedent moisture content, (e.g., initially dry soil) applied water rapidly enters into the

soil matrix. With a continuous supply of water by rainfall or irrigation, the rate of entry of

water or infiltration rate decreases over time until it reaches a steady state or a constant

rate. The constant rate is also termed as steady state or equilibrium infiltration rate.

Figure 14.2 presents a typical time dependent infiltration rate and cumulative infiltration

curve.

The high initial infiltration rate in Fig. 14.2a can be explained by large suction

gradients. When water is supplied to an initially dry soil, the suction gradients across the

soil surface become very high, which results in a high infiltration rate. As the wetting

front moves downward, the suction gradient across the soil profile decreases, which

limits the rate of water infiltration into the soil surface. Eventually, after a long time, the

infiltration rate approaches zero. However, in actual practice, if ponding on soil surface

continues for a long time, the infiltration rate gradually becomes steady gravity driven

flow and is equal to the saturated hydraulic conductivity (K

) of the homogeneous rigid

soil. The decrease in infiltration rate may also be caused by dispersion of aggregates or

slaking, soil compaction and surface sealing, or clogging of soil pores (Shukla et al.,

2003a). Slaking is a term used to describe the initial fragmentation of soil aggregates

Water infiltration in soil 377

several millimetres in diameter, which may disintegrate further to become micro-

aggregates. For details on soil aggregates, readers are referred to Chapter 4.

14.2 MECHANISM OF WATER MOVEMENT AT THE WETTING

FRONT

As long as the rate of water application to the soil surface is less than the instantaneous

infiltration rate of soil, all the water infiltrates into the soil

FIGURE 14.2 Typical (a) time

dependent (dI/dt) or rate and (b)

cumulative infiltration (I) curves.

profile. Under this circumstance, supply rate determines the infiltration rate and the

process of infiltration is called “flux controlled.” On the other hand, if water is applied at

a rate higher than the instantaneous infiltration rate of soil, the soil water transmission

properties determine the rate of actual infiltration and cumulative infiltration. Under this

circumstance, the infiltration is called “profile-controlled.” According to Childs (1969),

the infiltration process is dependent upon both hydraulic conductivity (K

) and the

hydraulic gradient (∆H/L) of the soil profile. Therefore, the entire soil profile rather than

just the surface layer governs the infiltration rate in a profile-controlled process. During

infiltration, a clear water divide is often seen during the wetted region overlying the drier

region. This distinct sharp boundary between wet and dry regions is known as the

“wetting front.”

A soil-water profile of a homogeneous soil column under ponded infiltration can be

divided into three distinct zones: saturated zone (θ=s=1), transmission zone (θ<θs;

∆H/L=1), and wetting front (Fig. 14.3). The wetting front is a visible wet/dry soil

boundary and may be smooth in a clayey soil and a diffused/fingered in a coarse-textured

or non-homogeneous soil. The water movement at the wetting front occurs through a

condensation-evaporation process. With the advance of the

Principles of soil physics 378

FIGURE 14.3 Schematic of saturated

zone, transmission zone, and wetting

front advance through a soil profile.

(Adapted from Collis-George and Lal,

1971, 1972.)

liquid wetting front, the water vapor moves ahead of the liquid front and condenses. The

condensation releases heat of wetting and increases soil temperature in the zone

immediately ahead of the liquid wetting front. According to Collis-George and Lal

(1972), the average thickness of the zone just ahead of wetting front where condensation

occurs varies with soil type. This zone can be identified by the color change due to vapor

condensation and the exponential temperature rise from the initial ambient to a maximum

(Fig. 14.4). The magnitude of temperature rise depends on soil texture, clay minerals, and

the degree of wetness. The rise in temperature is due to the heat of wetting and latent heat

of condensation. After attaining a maximum, the temperature declines prior to the arrival

of the liquid wetting front. The decline in temperature is due to evaporation of water and

the attendent loss of heat representing the latent heat of evaporation (Fig 14.4). Thus, the

vapor-wetting front comprises evaporation zone and a condensation zone. There is a

critical soil matric potential (Φ

) at which soil begins to lose water vapors through

evaporation. The water movement at the liquid wetting front is not regular in that it

remains at one point for a few seconds before it colonizes the granules and pores below

(Collis-George and Lal, 1970). The wetting front actually behaves in a quantum-like

fashion (Fig. 14.5). The length of each step is time dependent, the life of each step being

smaller in the initial phases of infiltration. The discontinuous movement of wetting front

is more pronounced in soils with macro- than microaggregates.

Water infiltration in soil 379

FIGURE 14.4 Zone of vapor

condensation and temperature rise (a),

zone of temperature decline between

temperature maximum and wetting

front (b), and zone of decreasing

temperature of high to near saturated

moisture content (c) for a chernozem

soil at 176 cm. The wetting front

reached 40 cm at 176s with velocity of

0.17 cm s

−1

. (Redrawn from Collis-

George and Lal, 1972.)

Principles of soil physics 380

FIGURE 14.5 Schematic of depth of

wetting front varying with time for

1.0–2.0 mm size fraction. These

measurements were taken from still

prints with a magnification of 13 years.

(Adapted from Collis-George and Lal,

1970.)

FIGURE 14.6 Schematic of wetting

front movement through soils when (a)

soil is uniform and coarse textured; (b)

soil is uniform and fine textured; (c)

Water infiltration in soil 381

soil is layered with coarse textured

overlying fine textured; and (d) soil is

layered with fine textured overlying

coarse textured soil.

The water flow through a homogeneous soil profile, which consists of either coarse-

textured soils or fine-textured soils (Fig. 14.6) is very uniform and closely resembles a

piston flow. If the coarse-textured soil overlies a fine-textured soil the flow through this

layered soil system is again relatively uniform. However, if fine-textured soil overlies a

coarse-textured soil, the flow through coarse-textured soil occurs as pipe flow or fingers

within the layers. Under these circumstances a large portion of the soil is bypassed.

According to Hill and Parlange (1972), the finger formation occurs due to instability at

the gravity driven air-water interface. The size of the fingers depends on pore size and

therefore, increases with an increase in coarseness of soil texture.

The theory of moisture movement in soil under temperature and potential gradients,

which incorporates the interaction of vapor flow with liquid and solid phases, was

developed by Philips and De Vries (1957) and DeVries (1958). In chemical reactions the

change in enthalpy (a thermodynamic state function) is related to the changes in the free

energy and entropy by the Gibbs equation [see Eq. (17.4)]. Entropy is a thermodynamic

quantity and is a measure of the degree of disorder within any system. The greater the

degree of disorder, the higher is the entropy.

The movement of vapor ahead of the liquid wetting front is demonstrated by observing

the temperature fluctuations at the wetting front (Anderson and Linville, 1962; Anderson

et al., 1963; Collis-George and Lal, 1970; 1971). The vaporization curves of most liquids

have similar shape, and the vapor pressure steadily increases as the temperature increases.

The process can be understood by developing a mathematical model for the pressure

increase as a function of temperature. Laboratory experiments under controlled

conditions have showed that the pressure P, enthalpy of vaporization, ∆H

vap

, and

temperature T are related, and can be represented by the Clausius–Clapeyron equation as

follows:

(14.1)

where R (=8.3145 J/mol/K) is the gas constant and A is an unknown constant. The

Clausius–Clapeyron equation can be used to estimate the vapor pressure at another

temperature, if the vapor pressure is known at some temperature, and if the ∆H

vap

known. Collis-George and Lal (1970; 1971; 1972) suggested that for unstable slaking

soils the heat of wetting and its rate of production controls the infiltration rate. The water

potential at known moisture content, in appropriate units relative to a free water surface,

is the partial specific free energy

of the soil water at that moisture content.

Therefore, using the Clausius–Clapeyron equation a plot of

versus 1/T, where T

is the absolute temperature, produces a straight line of slope, ∆H

, the partial specific

enthalpy at that moisture content, and with intercept equal to the partial specific entropy

(∆S

Principles of soil physics 382

TABLE 14.1 Thermodynamic Description of an

Oven-Dried (o.d.) Chernozemic Soil

(cal g −

O) g−

(o.d. soil))

w(g g

−1

) 20°C 25°C 30°C 35°C ∆H

∆S

∆H(cal g

−1

soil)

0.02 −95.2 −91.3 −87.0 −82.4 −324 −0.8 −25.3

0.10 −13.7 −12.3 −11.1 −11.1 −92 −0.27 −3.32

0.20 −0.52 −0.45 −0.34 −0.34 −6 −0.019 −0.12

Source: Adapted from Collis-George and Lal, 1972.

The thermodynamic description of a chernozemic soil used by Collis-George and Lal

(1972) in laboratory infiltration experiments is given in Table 14.1. It shows that as the

moisture content of soil increases the attendant

(at 20°, 25°, 30°, and 35° C), ∆H

∆S

, and integral enthalpy or heat of wetting (∆H) also increase.

14.3 CONCEPTUAL INFILTRATION MODELS

There are several models to predict the infiltration rate into soil. Some of these models

are conceptual and based on basic processes, while others are empirical or statistical

(Shukla et al., 2003b).

14.3.1 Conceptual Models

There are two principal conceptual models:

Green and Ampt Model

Green and Ampt (1911) made several simplifying physical assumptions to develop a

mathematical relationship for predicting the infiltration rate (Kutilek and Nielsen, 1994).

The soil-water profile during infiltration was assumed to be a steplike function and

infiltration into the soil was assumed to be piston flow going progressively deeper with

time. The wetted soil profile at time t was replaced with a uniformly wetted region of

thickness L. The soil profile is considered homogeneous and isotropic; therefore,

saturated hydraulic conductivity (K

) is a unique parameter. Darcy’s law can be used to

predict the infiltration rate as follows:

(14.2)

Water infiltration in soil 383