The primary source of fresh water in the Earth is not surface water (in streams, lakes and oceans), but that in ice and in the subsurface.
The flow of water in the Earth system is driven by insolation. The sun provides the necessary energy input.

Water Cycle

Water moves in the Earth via the Water Cycle.
Water heated by the sun evaporates and moves to the atmosphere as water vapor.
In cold, dry, and windy climates, water can also undergo sublimation going from ice to vapor.
As water vapor cools, it condenses into a liquid and forms clouds.
When the water in clouds becomes less energetic, it falls back to earth as precipitation.
- The precipitation need not reach the ground as it may be re-integrated into water vapor.
- Alternatively, it may be absorbed by plants via transpiration. The water will eventually be released back to the atmosphere as vapor.
Precipitation may seep into the ground in a process called infiltration. This water remains in the soil or rises back to the surface. This process can also happen near bodies of water like lakes and streams.
Water that percolates far enough into the ground becomes Groundwater.
Water may also undergo surface runoff. This occurs due to rainfall or snowmelt or via seeping groundwater. Here, water flows along the surface, pools into streams and lakes and eventually finds its way back to the ocean to start the cycle again.

Water Stores

The following are the stores of water on earth:
- The oceans and seas. Water here is primarily salt water and it is the largest source of water on earth.
- The Atmosphere in the form of water vapor.
- Glaciers which account for the majority of fresh water. This also includes snow, ice caps and permafrost.
- Soil Water in about the same amount as the atmosphere.
- Lakes which account for a majority of surface fresh water but this water may also be salty or brackish.
- Streams, Swamps and other surface fresh water sources. Swamps account for more fresh water than Streams.
- Ground Water which occurs in between sediment or within rock fissures. It accounts for a good chunk of fresh water although it may also be saline or brackish.
- Deep Interior Water which is water bound to minerals in the crust and mantle.
- Biological Water found in organisms.
We can consider these stores as part of a global system. The dynamics of which are governed by inflows and outflows of water between water stores (as dictated by the water cycle).

Stores which hold water only for a short time are more responsive to changes in energy.

The table below shows how long water tends to stay in a particular store

Store	Residence Time	Explanation
Atmosphere	Very short	Small store with High Inflow and Outflow.
Oceans	Very Long	Large store with Low inflow and outflow (only affecting the surface water)
Continents	Medium (between Atmosphere and Ocean)	Large store with Low inflow and outflow

A particular area’s water balance is calculated based on inflows and outflows.
- Water is primarily gained through precipitation or surface runoff (i.e., via snowmelt).
- Water is primarily lost through evaporation or transpiration.