INFILTRATION

Introduction

Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes saturated. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier. It is related to the saturated hydraulic conductivity of the near-surface soil. The rate of infiltration can be measured using an infiltrometer.

Infiltration is governed by two forces: gravity and capillary action. While smaller pores offer greater resistance to gravity, very small pores pull water through capillary action in addition to and even against the force of gravity.

The rate of infiltration is determined by soil characteristics including ease of entry, storage capacity, and transmission rate through the soil. The soil texture and structure, vegetation types and cover, water content of the soil, soil temperature, and rainfall intensity all play a role in controlling infiltration rate and capacity. For example, coarse-grained sandy soils have large spaces between each grain and allow water to infiltrate quickly. Vegetation creates more porous soils by both protecting the soil from pounding rainfall, which can close natural gaps between soil particles, and loosening soil through root action. This is why forested areas have the highest infiltration rates of any vegetative types.

The top layer of leaf litter that is not decomposed protects the soil from the pounding action of rain; without this the soil can become far less permeable. In chaparral vegetated areas, the hydrophobic oils in the succulent leaves can be spread over the soil surface with fire, creating large areas of hydrophobic soil. Other conditions that can lower infiltration rates or block them include dry plant litter that resists re-wetting, or frost. If soil is saturated at the time of an intense freezing period, the soil can become a concrete frost on which almost no infiltration would occur. Over an entire watershed, there are likely to be gaps in the concrete frost or hygroscopic soil where water can infiltrate.

Once water has infiltrated the soil it remains in the soil, percolates down to the ground water table, or becomes part of the subsurface runoff process.

The process of infiltration can continue only if there is room available for additional water at the soil surface. The available volume for additional water in the soil depends on the porosity of the soil and the rate at which previously infiltrated water can move away from the surface through the soil. The maximum rate that water can enter a soil in a given condition is the infiltration capacity. If the arrival of the water at the soil surface is less than the infiltration capacity, is sometimes analyzed using hydrology transport models,mathematical models that consider infiltration, runoff and channel flow to predict river flow rates and stream water quality.

5.1.2 Infiltration characteristics
The infiltration capacity is the maximum rate at which water can be absorbed by a given soil per unit area under given conditions.

Infiltration regime i(t) depends on the supply regime (irrigation, rain), but also on soil properties. The cumulative infiltration I(t), is the total amount of water infiltrated during a given period.

where:
	I(t)	the cumulative infiltration during the t period (mm)
	i(t)	the infiltration regime during the t period (mm/h)

Hydraulic conductivity at saturation k_s, is an essential parameter of infiltration. It represents the limiting value of infiltration if the soil is saturated and homogenous. Percolation is the vertical water flow in soils (porous unsaturated environment) on the groundwater layer under the influence of gravity. This process follows infiltration and has a major influence on the underground layer water supply.

Net rain is the amount of rain that falls to the ground surface during a shower. The clear rain is deduced from the total rain, diminished by the intercepted fraction of vegetation and that which is stored in ground depressions. The difference between the infiltrated rain and the drained rain on the ground surface is called production function.

5.1.3 Factors which influence infiltration

The main factors that influence the infiltration are:

• the soil type (texture, structure, hydrodynamic characteristics). The soil characteristics influence capillary forces and adsorption;
• the soil coverage. Vegetation has positive influence on infiltration by increasing the time of water penetration in soil;
• the topography and morphology of slopes;
• the flow supply (rain intensity, irrigation flow);
• the initial condition of soil humidity. Soil humidity is an important factor of infiltration regime. The infiltration regime evolves differently in time for dry or wet soils;
• soil compaction due to rain drop impact and other effects. The use of hard agricultural equipment can have consequences on the surface layer of soil.

Figure 5.2 The infiltration regime depending on time for different types of soil [Musy,2001]

5.2 Models Used to Estimate Infiltration Rates

Infiltration processes can be estimated by means of different models:

• models based on empirical relations involving 2, 3 or 4 parameters;
• physically based models

Infiltration Capacity

A soil under given condition has an upper limit on its absorbing capacity. The infiltration capacity of a soil under given condition is defined as the maximum rate at which it is capable of absorbing water and is denoted by f. The actual infiltration observed in a given soil f_a , will be equal to or less than its infiltration capacity f  depending on whether or not the rate of source supply is more or less than the infiltration capacity. Specifically if i denotes the rate of rainfall, then

f_a = f , i  f   and   f_a = i , if  i  f

The information regarding the infiltration rates is required in the estimation of surface runoff and groundwater recharge. Unfortunately f  is not a constant but varies within the duration of storm as well as seasonally. The factors affecting the infiltration capacity are now discussed.

FACTORS AFFECTING INFILTRATION CAPACITY

The variations in the infiltration capacity are large. The infiltration capacity is influenced by many factors. Some factor contribute to special variation while the others to temporal variation.

Depth of Surface Detention and Thickness of Saturated layer. Infiltration takes place due to combined influences of gravity and capillary forces. The infiltration of water through a soil surface may be visualized as a flow through a large number of tiny pipes as in Figure below.

As the infiltration continues, the wet front will be travelling downwards. At any instant of time the resistance to flow is proportional to the thickness of the saturated layer up to wet front L. while the driving head is proportional to (L + d), d being the depth of detention. At the beginning of the process when L is either less than d or of comparable magnitude with d, the resistance to flow is rather less and hence water enters rapidly. This is one of the reasons why f  is large at the beginning of the process. When the thickness of saturated layer L becomes large and hence d becomes negligible compared to L any increase in L produces same effect both on resistance and driving head and so f  remains constant. This suggest that the infiltration capacity should decrease with time in a continuous rain and become a constant, f_c, ultimately as shown figure below.

Soil Moisture. If a soil is completel dry at the beginning of rain here is a strong capillary attraction for moisture in the subsurface layers that acts in the same direction as gravity and gives high initial value of infiltration. As water percolates down, the surface layers become semi-saturated and the capillary forces diminish and hence f also reduces. If the soil is moist to begin with, the infiltration starts with lesser capacity compared to that of a dry soil. These two trends are shown in figure above. Another aspect of soil moisture is that when the soil is subjected to wetting, very wet soil particles called colloids wil swell slightly and reduce the size of voids. This is another reason why f reduces with time.

Compaction. The clay-surfaced soils are compacted even by the impact of rain drops which reduce f. This effect is negligible on sandy soils. Compaction not only reduces the porosity but also the pore sizes. When the compaction is artificial due to man-made effect the initial infiltration capacity is very low and it is further reduced during the storm. Overgrazed pastures, playgrounds, and areas subjected to heavy vehicular traffic will have less infiltration capacity. Clay soils when exposed dry to the storm, apart from being compacted by the rain drop impact may become highly impermeable because of the in wash of fine particles along with rain water into the pores. This factor also reduces f during the storm. High infiltration capacities produced by ploughing and cultivation, burrowing animals and insects, and decay of vegetable matter such as root are often reduced very rapidly by compaction due to rain.

Surface Cover Conditions. The nature of surface cover has also an important influence on the infiltration. The presence of a dense cover of vegetation on the surface increase f. The vegetative cover retards the movement of overland flow and causes high depths of detention. It reduces the effect of rain drop compaction. It also provides a layer of decaying organic matter which promotes the activity of burrowing insects and animals. Transpiration by vegetation tends to keep the soil moisture at low levels. All these factors tend to increase f.

Temperature. The effect of temperature on filtration is explained through viscosity. The flow through soil pores is by and large laminar for which the resistance is direct proportional to viscosity. At high  temperatures since viscosity of water is low, high infiltration capacities are expected. In winter months when low temperature prevails, the infiltration capacity is also low. This is one of the factors responsible for seasonal variations in f.

Other. The other factors which may marginally affect the infiltration capacity include the entrapped air in the pores, the quality of water, freezing, etc. The presence of entrapped air increases the resistance to flow and therefore reduces infiltration. The quality water, particularly its turbidity in terms of clay and colloids it contains, and the salts it contains such as fertilizer residues from alkaline soils or other sources, may have diminishing influence on infiltration capacity.

MEASUREMENT OF INFILTRATION

            Infiltration rates are required in many hydrologic problems such as runoff estimation, soil moisture budgeting and in irrigation. There are two general approaches to the determination of infiltration rate. One of these approaches analyses the observed rainfall hyetograph and runoff hydrograph from a small plot or a natural watershed to estimate the infiltration rates. The other method uses the infiltrometers as shown the figure below.

The infiltrometer always gives the infiltration capacity rate and the information from such infiltrometer tests at various locations in the basin may give a satisfactory estimate of the average infiltration capacity rate for the entire basin as a whole. In the hydrograph analysis, method the actual infiltration rate curve is obtained. However, the derived estimate of infiltration is as accurate as the precision of the measurement of rainfall and runoff from the basin.

PHI INDEX

Infiltration indexes generally assume that infiltration occurs at some constant or average rate throughout a storm. Consequently, initial rates are underestimated and final rates are overstated if an entire storm sequence with little antecedent moisture is considered. The best application is to large storms on wet soils or storms where infiltration rates may be assumed to be relatively uniform.

The most common index is termed the phi ( ) index for which the total volume of the storm period loss is estimated and distributed uniformly across the storm pattern. Then the volume of precipitation above the index line is equivalent to the runoff (figure below). A variation is the W index, which excludes surface storage and retention. Initial abstractions are often deducted from the early storm period to exclude initial depression storage and wetting.

            To determine the  index for a given storm, the amount of observed runoff is determined from the hydrograph, and the difference between this quantity and the total gauged precipitation is then calculated. The volume of loss (including the effects of interception, depression storage, and infiltration) is distributed uniformly across the storm pattern as shown in that figure.

            Use of the  index for determining the amount of direct runoff from a given storm pattern is essentially the reverse of this procedure. Unfortunately, the  index determined from a single storm is not generally applicable to other storms, and unless it is correlated with basin parameters other than runoff, it is of little value.

Where,             P = total rainfall / precipitation (cm)

R = Total runoff (cm)

t_e = time of rainfall

Example :

A storm with 10cm rainfall produced a direct runoff of 5.8 cm. Table below show the time distribution of the storm, estimate the  index.

Time (hour)	1	2	3	4	5	6	7	8
Rainfall (cm/h)	0.4	0.9	1.5	2.3	1.8	1.6	1.0	0.5

Solution :

Total rainfall, P           =          0.4(1) + 0.9(1) + 1.5(1) + 2.3(1) + 1.8(1) + 1.6(1) + 1.0(1) + 0.5(1)

                                    =          10 cm

Total runoff, R            =          5.8 cm

Then ,

But this value of  makes the rainfall of the first hour and eight hour ineffective as their magnitude is less than 0.525 cm/h. The value of t_e is need to modified.

Then assume t_e is 6 hours.

Total rainfall, P = 10 – 0.4 – 0.5 = 9.1 cm

Then ,

This value of  is satisfactory and by calculating the rainfall excess.

Time (hour)	1	2	3	4	5	6	7	8
Rainfall (cm/h)	0	0.35	0.95	1.75	1.25	1.05	0.45	0

 Total rainfall excess = 5.8 cm = total runoff