In hydrogeology, groundwater flow is defined as the "...part of streamflow that has infiltrated the ground, has entered the phreatic zone, and has been discharged into a stream channel, via springs or seepage water". It is governed by the groundwater flow equation. Groundwater is water that is found underground in cracks and spaces in the soil, sand and rocks. An area where water fills these spaces is called a phreatic zone or saturated zone. Groundwater is stored in and moves slowly through the layers of soil, sand and rocks called aquifers. The rate of groundwater flow depends on the permeability (the size of the spaces in the soil or rocks and how well the spaces are connected) and the hydraulic head (water pressure).