Infiltration is the process by which rainwater becomes what?

A. Ocean water.

B. Groundwater.

C. Saltwater.

D. River water.

The correct answer is B: Groundwater.

Infiltration is the movement of water from the top of the ground to below the surface of the ground. Rainwater that falls on soil can either move through the soil or run off the soil surface. The rainwater at first moves into and through the soil until it reaches the water table where it becomes groundwater.

Soil zones showing water infiltration
Soil zones showing water infiltration

How quickly the water can infiltrate is called the rate of infiltration and is often measured as millimeters per hour. There will be runoff from the soil when the rate of rainfall is greater than the rate of infiltration.

Several factors influence infiltration rate including soil type, which includes soil structure and texture. How the soil is structured is important including how many spaces there are between the soil particles.

This is known as the porosity of the soil and is very important because how much space there is between soil particles will determine how much water the soil can hold.

Of course, the presence of vegetation, amount of water already in the soil and temperature of the soil also play a role in determining the infiltration rate.

Water moves through soil by capillary action and because of gravity. Capillary action is the movement of water through the pores of the soil by means of the forces of surface tension, cohesion and adhesion. Gravity acts as a force to pull the water deeper through the soil.

Zones of the soil

The zone of the soil through which the water infiltrates is known as the vadose zone.  The vadose zone is also referred to as the unsaturated zone of soil and it extends from the top of the soil, the surface, down to the water table. This zone is not saturated with water so more water can move through it.

The water table is in the top layer of the saturated zone of the soil where all the pores of the soil are fully saturated with water. Furthermore, the water table is also known as the groundwater table and is where the water that infiltrates ends up.

The groundwater table is the boundary between the saturated and unsaturated zones in the soil. It is important to realize that the exact position of the water table changes from one year to the next and depending on changes in weather and climate over time.

Groundwater can form aquifers if it reaches a level where the soil is so saturated with water that it becomes a good source of water. An aquifer can be closed off by solid rock, and this would thus be called a confined aquifer.

However, these are rare, and most often an aquifer is called an unconfined aquifer since the water table which sits directly above it is open to the atmosphere and not overlain by solid rock. People often drill into such aquifers when they are constructing a well.

Movement of water

The water seeps down into the soil from the surface of the soil because of gravity and capillary action. Capillary action occurs because of the forces that occur between water molecules. In fact, water molecules have the property of cohesion, which means they like to stick together.

A water molecule consists of an oxygen atom and two hydrogen atoms. Furthermore, the hydrogen atoms have a slightly positive charge, while the oxygen atom has a slightly negative charge.

A consequence of this is that there are unequal charges on the actual water molecule itself, so the hydrogen of one water molecule is attracted to the oxygen of another water molecule.

Adhesion is another property of water in which the molecules are attracted to molecules of other substances. Capillary action is largely due to cohesion and adhesion properties of water.

Furthermore, gravity acts as a force to pull water downwards through the soil. The result is that water molecules move steadily downwards through the soil towards the groundwater table.

Infiltration rate

Factors that influence the rate of infiltration include the soil structure, which is really how soil particles are grouped to produce porous compounds.

These groups of soil particles are called aggregates and can occur in different shapes with different amounts of space between particles, which all influence how easily and fast infiltration happens.

These shapes can be prismatic, granular, blocky, and massive. The granular soil has the fastest infiltration rate by water while massive has the slowest infiltration rate.

This is because the granular soil has spaces between while massive slabs of tightly bound soil particles will not have many or any spaces between them.

Soil texture is the soil particle size, which is also important in influencing the infiltration rate of water. In general, water moving through soils like clay which have very fine sized particles will have a slower infiltration rate than sand with larger particles.

This is because there is less space between fine sized particles than between larger sized particles. Water thus can more easily move through a soil like sand compared with a soil-like clay.

Infiltration rate varies from low at less than 15mm per hour, to high at 50mm per hour. A moderate infiltration rate is between 15 and 50 mm/hr.

These rates can be measured with instruments known as infiltrometers. There are different types of infiltrometers that can be used, but in all cases, the investigator records how quickly water moves into the soil.


Vegetation will impact the infiltration rate of water because plant roots function to break up the soil. In addition, the vegetation will attract other animals, including animals that live in the soil.

For instance, earthworms and moles will move through the soil and loosen soil particles in the process. This all creates more space for water to move into and through.

The presence of vegetation growing in soil leads to increased organic matter as plants drop leaves and as plants die and decompose. Increased rates of infiltration are evident in soils that have more organic matter compared with soils that have less organic matter.

This is because the organic matter increases the porosity of the soil, which makes it easier for water to seep into and through the soil.


  1. AJ Franzluebbers (2002). Water infiltration and soil structure related to organic matter and its stratification with depth. Soil and tillage research.
  2. Editors of Encyclopedia Britannica. (2018). Groundwater. Retrieved from
  3. A Klute (1986). Methods of soil analysis, 2nd ed. Madison, Wisconsin: Soil Science Society of America.
  4. T.P.A. Ferré, A.W. Warrick (2005). Infiltration. Encyclopedia of soils in the environment. Retrieved from
  5. JR Nimmo (2009). Vadose water. Encyclopedia of inland waters. Retrieved from


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