When water comes down as precipitation, some of the water will seep down into the ground where it stay for some time until plants take it up. Som seeps down below ( infiltrates) to reach the underground water, some moves horizontally to reach nearby water while some makes its way back to the surface as surface water .
The process by which water from the ground’s surface enters the soil is called water infiltration. That is, the water moves from the soil surface into the soil layers. It is a key part or fundamental component of the natural water cycle, and is commonly studied in hydrology and soil sciences.
The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meters per day but can also be measured in other units of distance over time if necessary. The infiltration capacity decreases as the soil moisture content of soils surface layers increases. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier.
INFILTRATION PROCESS
Whrn rain water or irrigation watet get to the ground, the water move through spaces in the soil to sustain life and the ecosystems within the soil. This is how surface water enters the soil ( moving downwards through pores). This fundamental process replenishes ground water and supports plants growth directly.
FORCES THAT DRIVES INFILTRATION
Two forces are responsible for driving infiltration of water through the soil;
1. Gravity force
2. Capillary force
1. GRAVITY FORCE: Gravity pulls the water down from the surface of the soil down the horizons.
2. CAPILLARY FORCE: This force drive the water into tiny soil pores through the process called capillary action.
Capillary action is the attraction between water molecules and soil particles, drawing water into the smaller pores against gravity.
The rate at which both forces takes place is based on soil properties like soil texture, structure , porosity etc
HOW INFILTRATION OCCUR
Water can get to the soil through various means. It can get to the soil through precipitation, irrigation water, rise of water table and also from surface water bodies. The water cycle is an important aspect of water that infiltrates into the soil.
The water cycle starts when water vapor condenses in the clouds and falls to Earth as rain, snow, sleet, or hail. This water can move in several different ways. It can be collected in one spot, runoff, infiltrate through the ground or be taken up by plants or animals. This water returns to vapor by evaporation or transpiration until it condenses again and starts the cycle over. This water from rain or irrigation or other sources enters the soil through porous areas, where the soil grains have enough space for water to pass between them. The water then percolates through the soil and rock until it reaches a saturated zone, where the water table is located.
CAUSES OF INFILTRATION
When water reaches the soil surface, it seeps through the soil pore spaces between soil particles . The rate at which this happens depend on the inflence by several key factors.
Infiltration is caused by factors such as; gravity, capillary forces, adsorption, and osmosis. Many soil characteristics can also play a role in determining the rate at which infiltration occurs. Such soil characteristics include soil texture, soil porosity, compaction, bulk density, Precipitation, Soil moisture content; Organic materials in soils; Land cover; soil structure, vegetation types, and soil temperature. etc.
For example; sand soils contain larger pore spaces that generally allow water to drain off or infiltrate faster than clay soils which has more smaller pores that restrict water movement. Therefore, sand soils have higher infiltration rate than clay soils
IMPORTANCE OF SOIL INFILTRATION
Understanding infiltration helps helps explain how much rainwater will turn to runoff, cause erosion and how much water will replenish ground water reserves.
Knowing water infiltration capacity of different land area helps predict irrigation management and flood control. Apart from these, water infiltration also replenishes aquifers.
It enables farmers and soil scientist etc tomanage water in the soil ecosystem. It also allow more efficient agricultural practices to take place.
It allows smarter urban devwlopment and more efficient environment.
FACTORS THAT AFFECT INFILTRATION
1. PRECIPITATION:
Rainfall leads to faster infiltration rates than any other precipitation event, such as snow or sleet. At the beginning of a rainfall, infiltration rates are usually higher preceded by a dry period, and decrease as the rain continues.
When precipitation occurs, infiltration increases until the ground reaches saturation, at which point the infiltration capacity is reached. After which infiltration slows down.
This precipitation – infiltration relationships depend on factors such as the rainfall intensity ,amount, type, duration and the vegetation properties of the local area. For example, the duration of rainfall can impacts infiltration capacity when at the initially state of rainfall, infiltration will occur rapidly as the soil is unsaturated, but as the rain continues, the soil will reach a state of saturation at which infiltration rate slows down. If the rain continues, the soil continues to becomes more saturated, leading to runoff. Also, if rainfall occurs on wet soils, runoff will occur.
2 SOIL CHARACTERISTICS
a. SOIL POROSITY:
Soil porosity has a direct effect on infiltration. The higher the soil pore space, the higher the infiltration rate and vise versa. When soils have
higher porosity meaning more pore space then the rate of infiltration becomes higher. for example, coarse soils like sandy soils.
Also, Soils that have smaller pore sizes, such as clay, have lower infiltration capacity and slower infiltration rates than soils that have large pore sizes, such as sands. One exception to this rule is when the clay is present in dry conditions, in this case, the soil can develop large cracks which lead to higher infiltration capacity.
Soil porosity may be determined by Pore size distribution, soil texture and the subsoil horizons.
Dense subsoil horizons with low permeability can restrict rainwater infiltration. Deep tillage can improve subsoil permeability and allow more rainwater to infiltrate.
b. SOIL COMPACTION: Soil compaction also impacts infiltration capacity. When soils are compacted due to for example heavy machine movement on soils, this results in decreased porosity within the soils, which decreases infiltration capacity.
It should be noted that large pores are more effective at moving water through the soil than smaller pores. When these pores are sealed up as a result of compaction, infiltration rates will be reduced. This can significantly lead to increased runoff and flooding and poor water quality.
c. HYDROPHOBIC SOILS: Hydrophobic soil is soil that repels water instead of absorbing it. It is caused by a waxy residue that coat the soil particles, making it difficult for water to penetrate.
Hydrophobic soil can be common in sandy soils, dried-out potting mixes, and soils with unrotted organic matter.
Hydrophobic soils can develop after wildfires have happened, which can greatly diminish or completely prevent infiltration from occurring. Such soils have a lower rate of water infiltration than non-hydrophobic soils. This is because water molecules are attracted to each other by cohensive force and have a weak ability to bond with the waxy soil particles. Therefore, droplets of water are formed with a high contact angle. This high surface tension of the water prevents the water droplets from spreading out over a large area.
d. SOIL MOISTURE CONTENT:
Saturated soils have no capacity to hold more water. This means that infiltration capacity has been reached and the rate of infiltration cannot increase beyond this point. This leads to much more surface runoff. When soil is partially saturated then infiltration can occur at a moderate rate and fully unsaturated soils have the highest infiltration capacity.
e. ORGANIC MATERIALS IN SOILS:
Organic materials like manure and plant residue which decompose to form organic matter and humus can improve soil infiltration by improving soil structure. Organic materials bind soil particles together into aggregates, which improves soil structure. When soil structure is improved, water flow downward through the soil easily and also improve the ability of the soil to absorb and hold water. This makes organic matter to improve infiltration rate.
Vegetation contains roots that extend into the soil which create cracks and fissures in the soil. This create rapid infiltration and increased infiltration capacity. Vegetation can also reduce the surface compaction of the soil which again allows for increased infiltration. When no vegetation is present, infiltration rates can be very low, which can lead to excessive runoff and increased erosion levels. Similarly to vegetation, animals that burrow in the soil also create cracks in the soil structure. Thus, increasing infiltration rate.
f. IMPERVIOUS SURFACES:
Some land areas are usually covered with impervious surface or impermeable surfaces especially during construction. Example is pavement, tired areas etc. infiltration cannot occur at such land areas because water cannot infiltrate through an impermeable surface. This can leads to increased runoff. Areas that are impermeable often have drainages that drain directly into water bodies, which means no infiltration occurs.
g. LAND COVER:
Vegetative cover over the land also have great impact on the infiltration capacity. Vegetative cover can lead to more interception of precipitation, which can decrease intensity of rain droplets leading to less runoff, more interception and increased infiltration rate. Increased abundance of vegetation also leads to higher levels of evapotranspiration which can decrease the amount of infiltration rate. Debris from vegetation such as leaf cover can also increase the infiltration rate by protecting the soils from intense precipitation events. Trees have strong roots that can penetrate deep into the soil, which promotes infiltration.
In semi-arid savannas and grasslands, the infiltration rate of a particular soil depends on the percentage of the ground covered by litter, and the basal cover of perennial grass tufts. On sandy loam soils, the infiltration rate under a litter cover can be nine times higher than on bare surfaces. The low rate of infiltration in bare areas is due mostly to the presence of a soil crust or surface seal. Infiltration through the base of a tuft is rapid and the tufts funnel water toward their own roots.
h. SLOPE
Infiltration increases with increasing slope gradients and runoff decreases.
When the slope of the land is steeper, runoff occurs more readily which leads to lower infiltration rates. Gentle slopes on the other hand, lead to decreased runoff, which favours infiltration.
SOIL INFILTRATION IN RELATION WITH VARIOUS TERMINOLOGIES
a. INFILTRATION RATE:
Most people wander why moisture pools up in certain areas and not in others on thesame land. The answer is duw to the infiltration rate.
The rate at which water enters the soil is called the infiltration rate, and is usually measured in inches per hour. It depends on initial soil moisture. ( Thus, difference between infiltration rate and saturated hydraulic conductivity).
For example, Initially, when a soil is dry, it has an initial high infiltration rate as water will seep through it faster. But as the dry soil begins to get wet, the rate approaches its saturated hydraulic conductivity.
Infiltration rate is controlled by several factors such as porosity, rain intensity or accumulation and soil moisture.
–POROSITY: Soils with fewer pore spaces or pore sizes like clay soils will have low infiltration rate compared to sand soils with larger and many pore spacea
– RAIN INTENSITY/ACCUMULATION: When rain falls and water accumulates on soil surfaces, this means water infiltration rate is low. The soil pores had become saturated due to fewer pores or the soil may be compacted.
–SOIL MOISTURE: When a soil is dry and water applied to it, the soil will seep the water quickly. This is common during dry season or summer periods. Therefore, such dry soils have high infiltration rate. But moist soils can no longer hold water, instead, the water applied becomes accumulated on the soil surface. Thus,such soils have low infiltration rate.
Infiltration rate is calculated by the volume of water entering the soil per unit area per unit time.
b. INFILTRATION IN RELATION TO SOIL SATURATION: When a soil is saturated, its infiltration rate decreases significantly, thus, increasing surface runoff.
Predicting infiltration helps distinguish between water runoff and ground water recharge.
c. INFLITRATION IN RELATION TO SATURATED HYDRAULIC CONDUCTIVITY: Saturated hydraulic conductivity measures the ease with which water can move through a pore medium once it is saturated. This term describe soil saturation in relation to infiltration rate.
d. INFILTRATION RATE IN RELATION TO METRIC POTENTIAL:
Metric potential is the suction or tension with which water is held within the soil pores actively, drawing water downwards along side gravity. It influences water flow.
e. INFILTRATION CAPACITY: Infiltration capacity is the maximum rate at which infiltration can occur. Or it is the maximum rate at which a specific soil can absorb water at a given time. It is the maximum absorption rate, and the rate actual water enters the soil. It does not only describe water disappearance or water rise, but also provide the foundation and understanding of sustainable agricultural practices. It is the cornerstone of water science.
Note: Both infiltration rate and infiltration capacity help hydrologist to quantify and predict water movement, considering factors like antecedent moisture movement.
Grasping infiltration enables scientists to sustainably manage water resources and manage the soil ecosystem.
ANTECEDENT MOISTURE MOVEMENT: This is the amount of water already in the soil.
EFFECTS OF INFILTRATION
Sand soils have higher infiltration rates than clay soils due to their large pore space. Clay soils have very low infiltration rates. Therefore, a balance must be maintained in infiltration rate. It must not be too high or too low but medium.
Too high infiltration rate can lead to leaching of nutrients. While too poor infiltration rate can lead to pnding of water on soil surface, cause erosion and also flooding.
MEASURING INFILTRATION
Infiltration rates can be measured using devices like infiltrometers, rainfall simulators, and infiltration tests.
1. WATER INFILTRATION TEST
The water infiltration test is a simple test whereby two fields of different agricultural practice are being carried out are used for the test.
For example:
Materials needed:
-Two separate fields ( first field is a conventional field- with practices including; heavy use of chemicals, lots of tillage operation, lack of diversity and non rotation practices.
Second field is a corn/ hay rotation field- fewer chemicals usage, animal integration, and diverse corn and hay are rotated).
-2 by 6 inch diameter ring,
-plastic wraps,
-500ml of water.
-Stop watch
-Hammer and flat metal bar
PROCEDURE
-On each field, place the 6 inch diameter ring on the field with the end of each ring sharpened for ease of insertion into the soil.
-Place a metal bar on top of the rings and hammer the metal bar to insert the rings into the soil. Never use hammer on the edge of the ring.
-Leave about 2 or 3 inches of the rings above the soil.
-Place the plastic wrap on top of the ring and pour the water on the wrap without spilling out, off the ring.
– Remove the plastic wrap gently for the water to stay within the ring.
-Time when the water will infiltrate into the ground completely.
-Perform the task as many times as possible and record the time when the water infiltrate completely.
(This will indicate how many inches of soil the water can infiltrate).
RESULT FROM THE EXPERIMENT
The corn/hay rotation field infiltrates 5inches of water within 5 minutes, while the convectional field infiltrates only 2 inches within 4 minutes. This is half the infiltration rate on corn/hay rotation field.
(This will be used to explain infiltration in relation to soil health below)
2. RING INFILTROMETER TEST
The ring ingiltrometer test of of two kinds. The first os the single ring infiltrometer test and the second is the double ring infiltrometer test.
PRINCIPLES OF INFILTROMETER
An infiltrometer is defined as a device used to measure the rate of water infiltration into soil, often involving methods such as the double ring infiltrometer, which utilizes two concentric rings to minimize lateral flow and determine the field saturated hydraulic conductivity of the surface soil.
DIFFERENCE BETWEEN SINGLE RING AND DOUBLE RING INFILTROMETER METHOD
The single ring infiltrometer involves driving a single metal ring partially into the soil and filling it with water. Double ring infiltrometers in contrast require two rings (inner and outer), which creates a one dimensional flow of water from the inner ring.
. DOUBLE RING INFILTROMETER
A double-ring infiltrometer is a standard field device with two concentric metal cylinders (rings) driven into the soil to measure the rate water infiltrates into the ground, crucial for irrigation, drainage, and hydrology. Water is added to the inner ring, while the outer ring acts as a buffer to prevent lateral water flow, ensuring the measured water loss from the inner ring reflects true vertical infiltration, often measured over time and expressed in cm/hr or inches/hr.
MATERIALS NEEDED
-Galvanized steel rings /cylinders (( a small sized inner and a large outer cylinders in various diameters)
-Mariotte tubes (for constant water level)
-Driving cap and splash guards
-Support stands and tubing
-Measuring ruler, water and the field.
PROCEDURE
-On the field area, remove all surface stones, pebnles and rocks.
–Setup: The two cylinders, one smaller (inner) and one larger (outer), are placed on the cleared area of land – -Sharpen the edge of the cylinders for ease of penwtration into the soil.
-Drive the two cylinders using hammer into the soil. The cylinders should be hammered to about 15-16cm into the soil, while the remaining lenght (cm) project on the soil surface. ( both rings are installed to the same depth).
-To drive the cylinders into the soil, a cross designed flat metal bar can be placed on the rings and the hammer is used to drive them into the soil at even lenght as required.
-Measure the lenght of the cylinders again.
-To the edges of both cylinders, seal up the edges with soil so that water do not easily seep off through edge openings of the cylinders.
Watering: Both rings are partially filled with water (or another liquid) by;
-Place a foil wrap into the inner cylinder and pour water to about 7.5cm lenght of the cylinder
-In the outer cylinder, pour water to about 12 cm lenght of the cylinder. Note: Take the volume of the water in both cylinders as top up will de done to the inner cylinder.)
-Remove the foil wraps in the inner cylinder gently to release the water into the inner cylinder. Then top up with water to make thesame volume with the outer cylinder. (A constant water level in the inner ring should be maintained using a Mariotte tube, which measures the volume of water needed to replenish it).
-Immediately, time the seepage/infiltration.
Calculation: The volume of water added over time gives the infiltration rate (water per unit area per unit time).
FEATURES AND BENEFITS OF INFILTROMETER METHODS
i. Accurate measurement: Minimizes lateral flow, providing a more accurate field measurement of hydraulic conductivity.
ii. Applications: Essential for designing irrigation systems, drainage projects, and assessing seepage losses.
iii. Automation: Can be automated with IoT for more efficient, precise, and remote data collection, reducing labor.
PRECAUTIONS WHEN USING RING INFILTROMETER
i. Sharpen the edges of the cylinder rings to drive them easily into the soil.
ii. Do not drive the rings into the soil separately but together at thesame time.
iii. Do not hammer to top of the rings inorder to drive them into the soil but place a cross designed metal bar on them. Hammering the edges will destroy the rings.
iv. The cross metal bar shoild be hammered at the middle of the cross to ensure even penetration.
v. After the rings are driven into the soil, slightly move soil to the edges of the rings so that gaps around the inner parts are sealed up and no seepage can occur.
vi. Do not pourwater dirwctly on the soil but place an handle on it with water poured on the handle than directly onthe soil.
3. TABLE TOP INFILTROMETER METHOD
This type of infiltrometer method is carried out indoor rather than on the field. Soil samples are collected from different spots on the field and brought indoor for testing.
The soils samples should be of thesame texture but managed differently. For example, Soil cropped with tillage operation and soil cropped without tillage operation but of thesame texture.
MATERIALS NEEDED:
-2 small transparent buckets, perforated underneath.
-2 small calibrated bowls.
-2 bottles of water. The water must be of thesame volume.
-Flat perforated plates
PROCEDURE
-Place the 2 calibrated bowls on the table.
-Place the buckets on top of the bowls, making sure that the buckets base are of thesame circumference with the mouth of the bowls.
-Place thesame amount of the different soils with thesame texture in the buckets
-Place the perforated plastic plates on the buckets to simulate rain drops into the buckets of soils.
-Pour thesame amount of water into the plates, to drop water into the buckets.
-Check how the water drops into the small calibrated bowls at the base of the buckets
RESULTS
From the experiments, Poor infiltrated soil will show water ponding on the soil rather than draining into the calibrated bowl below. This may be due to soil compaction, collaped pore spaces and the colour of the water is dirty with more sediments ontop of the soil.
Good infiltrated soil will show water infiltrating into the calibrated bowls under, the water is clear and transparent and not dirty both on top of the soil and those dropping into the calibrated bowls.
From the result, the heavily tilled soil will show poor infiltration.
RUNOFF
If the rate of precipitation is faster than the rate of infiltration, runoff will occur unless there is a physical barrier. Also, when water is supplied to the soil at a rate that exceed infiltration capacity (saturation), such water result in runoff especially on slopy areas or pond up on flat surfaces. When runoff occur on bare land or poorly vegetated soils, erosion will occur. The runoff will carry along with it nutrients, applied agrochemicals and soils to deposited them elsewhere. Thus decreasing soil health and productivity.
Ponded water on the soil surface means the soil is oversaturated and pore spaces are overfilled with water. Therefore, poor aeration of soil occurs which leads to anaerobic condition within that soil. This therefore leads to poor root function and plant growth, reduced nutrient availability, reduced cycling by soil microorganisms, and soil strength decreases. Also, soil structure is destroyed, high rate of soil particle detachment occurs, and the soil becomes more erodible.
The ponded water are also exposed to increased evaporation, hence reduced water availability to plants.
IMPACT OF INFILTRATION ON AGRICULTURE
i. Understanding infiltration optimizes irrigation schedule and prevent nutriwnt runoff on the field
ii. Predicting infiltration helps manage storm water and reduce flood risks
iii. It is used for tracking pollutant movement and safeguarding groundwater quality.
As water inflitrates through the soil, it forms a wetting front which is a boundary btween wetted soil and dry soil ( that is, boundary where water meet dry soils as the water infiltrate).
HOW TO IMPROVE SOIL INFILTRATION.
Several conservation practices result to reduced infiltration. By proper consideration and application of these practices, infiltration will be improved. Some of the conservation practices that reduce infiltration include: burning, harvesting of crop residues, tillage methods and soil disturbance activities, prevention of soil organic matter decomposition and accumulation, equipment and Livestock trafficking, and reduced soil porosity.
To maintain and improve soil infiltration, conservation measures should be properly applied. such measures include: Proper managing of crop residues, increasing vegetative covers, increasing soil organic matter, practicing zero tillage or non disturbance of soil, protect soil from erosion, encourage development of good soil structures, and breaking of compacted areas in soil etc.
SOIL INFILTRATION AND SOIL HEALTH.
From the experiments above, soil health principles are described as measures that improve infiltration rates. Such principes include; keep soil covered to prevent high impact of rainfall and the cover crops residue can rot to add organic matter to soil, thus, improve soil structure and texture, minimize soil disturbance which may cause compaction and collaps of pore spaces, increase crop diversity. Crops with different roots-shallow tooted and deep rooted crops can be rotated to break and tap nutrients from different horizons within the soils. Keep living roots in the soils, and incorporate livestock to use their manure as soil amendments and organic fertilizers. Also as biocontrol agents etc.
In conclusion, infiltration must not be too high or too low but medium for sustainable production of crops and to keep the soil healthy.
