When water hits the ground, one of two things can happen: 1 some of it evaporates back into the atmosphere, or 2 it penetrates the surface and becomes groundwater. Groundwater is water that has infiltrated as deep as 200 feet or more beneath the earth's surface.
Groundwater flows in underground channels called "trenches" or "voids." It may be fresh (derived from rainfall or melted snow) or saltwater (from the ocean). The salty water sinks down through the soil toward the Earth's core where it picks up additional salt and then rises back to the surface at a place where there are no tectonic plates present (i.e., where there is an active plate boundary). These places are often along divergent boundaries between rigid lithospheric plates (such as those that make up the continents) where there are gaps in the rock formation that allow water to flow underneath them.
The movement of these plates creates large areas of crustal extension (where one piece of crust stretches while another recedes) which create suction effects that draw groundwater toward them. This is why you will often find large bodies of freshwater near convergent plate borders; there isn't enough pressure against the walls of the trench to cause groundwater flow, but there is enough tension to cause uplift and erosion.
Because of evaporation, almost all of the precipitated water evaporates back into the atmosphere. The remainder either drains off the land or soaks into the earth, becoming ground water.
When rain falls on the Earth's surface or runoff runs across it, part of the water soaks into the earth and seeps below to form groundwater. Most groundwater is saltwater that has infiltrated porous rock such as sandstone or shale. As the salty water moves through the rocks, it leaves behind void spaces called pores. In this way, the salt slowly leaches out of the rocks and into the surrounding soil.
The amount of freshwater in the world is estimated to be about 1 billion cubic kilometers (0.5 billion cu mi), but only about 10% of that is available for human use. The rest is either frozen in ice caps and glaciers or otherwise inaccessible due to being undersea. That means there is barely enough freshwater to meet our needs once we have taken into account any overuse or pollution.
Even if you live in a region with abundant rainfall, you will still need to understand how groundwater is used by humans and livestock, because these activities can deplete your supply quickly. For example, farmers irrigate their fields with water from canals or wells. Each time they do this they are using up the groundwater that will not be replenished until it is extracted.
Infiltration is the process of water seeping into the earth. This mechanism so recharges the groundwater. Groundwater is sometimes held beneath the water table between layers of hard rock. Sometimes it pours in through natural fissures in rocks. Sometimes water flows under land due to unevenness of the soil or underground rivers.
Infiltrating water reaches the soil by passing through the plant root system and into small pores or cracks between grains of sand or silt. Here it can evaporate or be absorbed by the soil. Infiltrating water enters the ground near the average rainfall level for your area. It then moves downward until it reaches an aquifer, which is saturated material that allows water to flow through it. From there, the water may move upward again toward the surface or spread out over a large area under the influence of gravity.
The amount of infiltration depends on many factors such as the moisture content of the soil, the frequency and intensity of rainstorms, and the type of vegetation around your home. Vegetation has two effects on infiltration: It provides more surface area for water to soak into and also reduces evaporation from the soil surface. A thick layer of grass, for example, can reduce precipitation runoff by as much as 30 percent!
You can help increase infiltration at home.
Water evaporates off the earth directly from lakes, puddles, and other bodies of surface water. Water also enters the atmosphere through a process known as transpiration, in which plants release water into the air from their leaves that has been drawn up from the soil by roots. This occurs especially during the growing season when plants use the gas carbon dioxide to produce food cells for growth and reproduction.
Evaporation and transpiration are two ways that water is lost from land surfaces. Losses due to evaporation and transpiration account for about 95% of the total water loss from all land surfaces. The remaining 5% is lost through other processes such as runoff and groundwater discharge.
Runoff is the rapid flow of water from a slope or channelized area. It can be either rain-driven or snow-melt driven. Groundwater discharge is the flow of water from beneath the ground's surface into adjacent soils or oceans. It can be caused by natural processes such as rainfall and percolation or it can be due to human activity such as irrigation or drainage. Runoff and groundwater discharge are important to maintain ecological flows within river systems. An ecological flow is the amount of water that will not cause flooding but still allows for the preservation of aquatic ecosystems.
Flooding occurs when the volume of runoff exceeds the ability of the landscape to absorb it.