Infiltration is the process where water on the ground surface moves into the soil. Scientists in hydrology and soil science often study this process. The infiltration capacity is the fastest rate at which water can enter the soil. This is usually measured in meters per day, but other units that show distance over time may also be used. When the top layers of soil already contain more water, the infiltration rate slows down. If rain falls faster than water can enter the soil, water will flow over the surface unless there is a physical barrier blocking it.
Tools such as infiltrometers, parameters, and rainfall simulators help measure how quickly water enters the soil.
Infiltration happens because of several factors, including gravity, capillary forces, absorption, and osmosis. Soil features, such as type, texture, and structure, also influence how fast infiltration occurs. When it rains, extra water can move through large spaces in the soil called macropores.
Factors that affect infiltration
Precipitation affects how much water can enter the ground, called infiltration. The amount, type, and length of precipitation all influence this process. Rainfall causes water to enter the ground faster than other types of precipitation, like snow or sleet. More precipitation generally increases infiltration until the ground becomes completely full of water, known as saturation. At this point, the ground can no longer absorb more water. The length of rainfall also matters. At the start of a rain event, infiltration happens quickly because the soil is not full, but as the soil becomes more saturated, infiltration slows down. This relationship between rainfall and infiltration determines how much water flows over the ground as runoff. If rainfall happens faster than the ground can absorb water, runoff occurs.
The structure of soil, called porosity, plays a key role in infiltration. Soils with small pores, like clay, allow water to enter more slowly than soils with large pores, like sand. However, if clay is dry, it can form cracks that increase infiltration. Soil compaction reduces porosity, which decreases infiltration.
After wildfires, some soils may become hydrophobic, meaning they repel water. This can stop or greatly reduce infiltration. When soil is fully saturated, it cannot absorb more water, so infiltration stops, and more runoff happens. If soil is only partially saturated, infiltration occurs at a moderate rate. Fully unsaturated soil allows the most infiltration.
Organic materials, such as plants and animals, help increase infiltration. Plant roots create spaces in the soil, allowing water to move more easily. Vegetation also reduces soil compaction, which improves infiltration. Without plants, infiltration rates can be very low, leading to more runoff and erosion. Animals that dig in the soil also create spaces that help water enter.
If land is covered by impermeable surfaces, like pavement, water cannot soak into the ground. This increases runoff. These areas often have drains that carry water directly to rivers or lakes, so no infiltration occurs.
Vegetation on the land affects infiltration. Plants can catch rain before it hits the ground, reducing runoff. More vegetation also increases evapotranspiration, which can lower infiltration rates. However, plant debris, like leaves, can protect soil from heavy rain, increasing infiltration.
In semi-arid grasslands, the amount of water entering the soil depends on how much of the ground is covered by plant litter and the roots of grasses. On sandy loam soil, infiltration can be nine times faster under litter cover than on bare soil. Bare areas often have a hard, sealed surface that limits infiltration. Water moves quickly through the roots of grasses and is directed toward their roots.
Steeper slopes cause more runoff and less infiltration.
Process
Infiltration can only happen if there is space in the soil for more water. The amount of space available depends on how porous the soil is and how quickly water already in the soil moves downward. The fastest rate water can enter the soil under certain conditions is called infiltration capacity. If water reaches the soil surface slower than this maximum rate, scientists sometimes use hydrology transport models. These models are mathematical tools that study how water moves into the soil, flows over the surface, and moves through channels. They help predict how much water flows in rivers and the quality of stream water.
Research findings
Robert E. Horton proposed that the ability of soil to absorb water decreases quickly at the start of a storm but becomes more stable after a few hours during the rest of the event. Water that has already soaked into the ground fills the spaces in the soil, which makes it harder for new water to be pulled into the soil pores. Clay in the soil can expand when it gets wet, which makes the soil pores smaller. In places without a layer of forest litter, raindrops can knock soil particles loose. These particles can then be washed into the soil's surface pores, which can slow down how much water soaks in.
Infiltration in wastewater collection
Wastewater collection systems include pipes, junctions, and lift stations that carry sewage to a wastewater treatment plant. If these pipes are broken, cracked, or damaged by tree roots, stormwater can flow into the system. This can cause sewage to overflow from the sewer system and be released into the environment without being treated.
Infiltration calculation methods
Infiltration is a part of the overall water balance in the environment. Scientists use different methods to estimate how much water soaks into the ground and how fast it moves. The most accurate method that connects groundwater and surface water through soil with different layers is solving Richards' equation, a complex mathematical equation. A newer method, called the Finite Water-Content Vadose Zone Flow method, connects groundwater and surface water in soil layers that are uniform and is related to Richards' equation. When soil is uniform and well-drained, simpler methods can estimate infiltration during a single rain event. Examples include the Green and Ampt method (1911) and Parlange et al. (1982). Other methods, like the SCS method and Horton's method, are based on fitting curves to data but are less precise.
The overall water balance includes infiltration (F). If all other variables are known, simple math can solve for infiltration. However, care must be taken to avoid using the same variable twice, such as including both evaporation (E), transpiration (T), and evapotranspiration (ET) in the equation, since ET already includes T and part of E. Interception, the water caught by plants or surfaces before reaching the ground, must also be considered in addition to raw rainfall.
The most accurate way to calculate infiltration is using Richards' equation, a complex equation with nonlinear terms. However, solving this equation is computationally demanding, may not always work, and can have issues with keeping track of water mass. A simpler method approximates Richards' equation by focusing less on water diffusion. This method is based on comparing solutions of the Soil Moisture Velocity Equation with exact solutions using specific soil properties. It works well because diffusion effects are small and the method guarantees accurate results. It assumes water flows only vertically and that soil layers are uniform.
The Green-Ampt method, named after Green and Ampt, considers factors like soil suction (how tightly soil holds water), porosity (space between soil particles), hydraulic conductivity (how easily water moves through soil), and time. This method accounts for more variables than simpler approaches like Darcy's law.
To calculate infiltration volume or rate, the Green-Ampt method uses equations that solve for F(t), the total infiltration over time. Solving requires guessing a starting value for F, which can be based on calculations involving hydraulic conductivity (K), time (t), and soil suction (ψ). The method assumes the depth of water above the ground (h₀) is negligible. Once F is found, it can be used to calculate the infiltration rate at a specific time.
Horton's equation, named after Robert E. Horton, describes how infiltration starts at a high rate (f₀) and decreases over time until it reaches a steady rate (f_c). This method can also calculate total infiltration volume over time.
Kostiakov's equation, named after its creator, assumes infiltration decreases over time following a power function. It uses two parameters (a and k) to describe this decline. However, it assumes infiltration eventually stops, which is not always true. The modified version, called the Kostiakov-Lewis equation, adds a term to account for a steady infiltration rate.
A simplified version of Darcy's law is sometimes used to estimate infiltration. While this method is easy to apply, it is less accurate than the Green-Ampt method because it makes assumptions about soil layers and the depth of water above the ground. It also assumes the soil suction head below the wetting front is a fixed value.