Groundwater is part of the water cycle. It is the water located beneath the earth's surface in soil pore spaces and in the fractures of rock formations. It is stored in and moves slowly through geologic formations of soil, sand and rocks called aquifers. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table and this water is called groundwater. The study of the distribution and movement of groundwater is hydrogeology, also called groundwater hydrology. Groundwater makes up about twenty percent of the world's fresh water supply, which is about 0.61% of the entire world's water, including oceans and permanent ice. Global groundwater storage is roughly equal to the total amount of freshwater stored in the snow and ice pack, including the north and south poles. This makes it an important resource that can act as a natural storage that can buffer against shortages of surface water, as in during times of drought. Hydrogeologists use Groundwater models which are computer models of groundwater flow systems. A groundwater model may be a scale model or an electric model of a groundwater situation or aquifer. These Groundwater models are used to simulate and predict aquifer conditions. Groundwater models are used to represent the natural groundwater flow in the environment. Some groundwater models include (chemical) quality aspects of the groundwater. Such groundwater models try to predict the fate and movement of the chemical in natural, urban or hypothetical scenario.