INFILTRATION

DEFINITION

 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 infiltrate.

          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-grainedsandy 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. 

INFILTRATION CAPACITY

• Defined as the maximum rate in absorb water at a certain type of soil and in a given time.
• It is designated as f_c and is expressed in units of cm/h or mm/h.The actual rate of infiltration f can be expressed as
  
  
  

Where ,

  i = intensity of rainfall

 fc = constant infiltration rate

A graph showing the time-variation of infiltration capacity if the supply were continually in excess of infiltration capacity.

• The infiltration capacity of a soil is assumed highly at the beginning of a storm and has an exponential decay as the time elapses.It is continues in decreasing until it is reach at the constant level (saturated)

FACTORS AFFECTING INFILTRATION

• Precipitation

 The greatest factor controlling infiltration is the amount and characteristics (intensity, duration, etc.) of precipitation that falls as rain or snow. Precipitation that infiltrates into the ground often seeps into streambeds over an extended period of time, thus a stream will often continue to flow when it hasn't rained for a long time and where there is no direct runoff from recent precipitation.

• Soil characteristics

Some soils, such as clays, absorb less water at a slower rate than sandy soils. Soils absorbing less water result in more runoff overland into streams. 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. 

• Soil saturation

 Like a wet sponge, soil already saturated from previous rainfall can't absorb much more,thus more rainfall will become surface runoff.

• Land cover

Some land covers have a great impact on infiltration and rainfall runoff. Vegetation can slow the movement of runoff, allowing more time for it to seep into the ground. Impervious surfaces, such as parking lots, roads, and developments, act as a "fast lane" for rainfall - right into storm drains that drain directly into streams. Agriculture and the tillage of land also changes the infiltration patterns of a landscape. Water that, in natural conditions, infiltrated directly into soil now runs off into streams.

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 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.

• Slope of the land

Water falling on steeply-sloped land runs off more quickly and infiltrates less than water falling on flat land.

·      Evapotranspiration

 Some infiltration stays near the land surface, which is where plants put down their roots. Plants need this shallow groundwater to grow, and, by the process of evapotranspiration, water is moved back into the atmosphere.

Subsurface water

        As precipitation infiltrates into the subsurface soil, it generally forms an unsaturated zone and a saturated zone. In the unsaturated zone, the voids—that is, the spaces between grains of gravel, sand, silt, clay, and cracks within rocks—contain both air and water. Although a lot of water can be present in the unsaturated zone, this water cannot be pumped by wells because it is held too tightly by capillary forces. The upper part of the unsaturated zone is the soil-water zone. The soil zone is crisscrossed by roots, openings left by decayed roots, and animal and worm burrows, which allow the precipitation to infiltrate into the soil zone. Water in the soil is used by plants in life functions and leaf transpiration, but it also can evaporate directly to the atmosphere. Below the unsaturated zone is a saturated zone where water completely fills the voids between rock and soil particles.

INFILTRATION MEASUREMENT

1)        Flooding Infiltrometers                                     

         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, 1m2 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.

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.

            Double Ring Infiltrometer

          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. 

2)        Rainfall Infiltrometers            

  

     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 accurate and representative measurements of the soil infiltration rate using rainfall infiltrometers:

ü  The distribution of drop sizes must be uniform over the plot area;

ü  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.

ü  The plot area must be large enough to sample the population and give reproducible results (approximately 1 m2); and

ü  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

3)        Watershed Methods        

          

         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.