The process of water storage in soil.
Rain water can also be stored in the ground. Soils consist of particles and pores. Those pores can be filled with air but also with water. The amount of pores is a soil is different for different types of soil. The pores in a clay soil account for 40% to 60% of the volume. In fine sand this can be 20%–45%
The soil particles have small pores in them where water can enter (soil water) and between the particles are larger pores that can be filled. The soil is filled with water up a certain level. This level goes up and down with changing weather conditions. This water level is the ground water level.
The process of water entering the soil is called infiltration. When the soil has taken up all the water it can, we say that it is saturated. If you walk over a saturated soil, you feel that it is wet and soggy, like biscuits dipped in tea.
Part of the water that infiltrates, will move on. It will go to underground storage reservoirs or to underground rivers and may, through ground water flows, eventually reach a river or a lake. Another part will be used by plants or will evaporate.