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Hydrologic Cycle: Subsurface

The subsurface part of Earth's water cycle relates to processes that occur below Earth's surface.

Groundwater

Groundwater - or all the water stored in Earth's subsurface - is the largest potential freshwater source on Earth. It is estimated to be larger than all of Earth's surface lakes and streams combined. Groundwater provides about 80% of the world's irrigation water for agriculture, and 50% of its drinking water.

Groundwater is stored in the spaces between the materials in Earth's subsurface (e.g. rock, gravel, sand). Groundwater actually flows underground as it does on Earth's surface!
Groundwater is stored in the spaces between the materials in Earth's subsurface (e.g. rock, gravel, sand). Groundwater actually flows underground as it does on Earth's surface!

Groundwater is replenished primarily through precipitation percolating downward after it reaches Earth's surface. While infiltration is the process of precipitation breaking through Earth's surface, percolation is the slow passage of water downwards through soil or rock. When soil becomes saturated, any water surplus gradually percolates downward into groundwater.

 

Water Table

The top zone where soil is completely saturated is the water table. The water table is the upper part of the zone of saturation where Earth's crust is completely saturated with groundwater.

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After first infiltrating Earth's surface, water percolates slowly downward through soil and/or rock until it reaches the water table.

Whenever the water table intersects with the land surface, water naturally discharges. This can often be seen with stream beds, as in the illustration below:

When the water table intersects with Earth's surface, the water discharges from Earth's crust to the surface. Seasonal fluctuations in precipitation can cause some streamflows to be seasonal, with streams that only flow during wet seasons and dry up during dry seasons.
When the water table intersects with Earth's surface, the water discharges from Earth's crust to the surface. Seasonal fluctuations in precipitation can cause some streamflows to be seasonal, with streams that only flow during wet seasons and dry up during dry seasons.

 

Aquifers

An aquifer is a subsurface layer of permeable rock or material (e.g. sand, silt, gravel) through which groundwater can flow. Aquifers are recharged through percolation of precipitation through Earth's crust. As a result, recharge rates can vary widely across different regions - for example, the Gobi Desert in Mongolia only receives an average of 40 mm (1.6 inches) of precipitation annually, while the Republic of Congo's tropical forests can exceed 2000 mm (78.7 inches) of annual precipitation.

Different aquifers across the world recharge at different rates - some recharge quickly, while others can take thousands or even millions of years.
Different aquifers across the world recharge at different rates - some recharge quickly, while others can take thousands or even millions of years. 

 

Humans typically extract groundwater using wells, which are drilled downward until they meet the water table or aquifer. If an aquifer is utilized beyond its flow and recharge capacity, this is referred to as groundwater mining.

The removal of groundwater can cause land subsidence, or the settling and sinking of surface Earth materials from Earth's crust. This can also result in soil compaction and can require infrastructural construction and repairs.

 

A photograph showing land subsidence near Lucerne Lake in the Mojave Desert (California, U.S.) due to groundwater withdrawal.
A photograph showing land subsidence near Lucerne Lake in the Mojave Desert (California, U.S.) due to groundwater withdrawal.