Every day an actively growing plant transpires 5 to 10 times as much water as it can hold at once. Precipitation in the form of rain, snow and hail comes from clouds. Clouds move around the world, propelled by air currents. For instance, when they rise over mountain ranges, they cool, becoming so saturated with water that water begins to fall as rain, snow or hail, depending on the temperature of the surrounding air. Runoff is the visible flow of water in rivers, creeks and lakes as the water stored in the basin drains out.
Subterranean water is held in cracks and pore spaces.
Depending on the geology, the groundwater can flow to support streams. It can also be tapped by wells. Some groundwater is very old and may have been there for thousands of years. When water suply is limited actual ET will be less than potential.