Groundwater is subsurface water, but not all subsurface water is groundwater. Having an appreciation of the types of subsurface water and knowledge of local geology can explain why some water table levels may vary by tens of feet and others in the same area may only change by inches and some hardly at all. The upper surface of groundwater is the water table. Below this surface, all the pore spaces and cracks in sediments and rocks are completely filled (saturated) with water. These saturated layers, known as the saturated zone (or the phreatic zone), are where groundwater occurs. Strictly speaking, only water found in the saturated zone is groundwater.
In the top layers of soil, unconsolidated sediments or bedrock, pore spaces may not be completely filled with water. Some may contain water, some air and some may only be partly filled with water. This is known as the unsaturated zone (also called the zone of aeration or the vadose zone). After heavy rainfall, this zone may be almost saturated, while during a long dry spell, it may become almost dry. Precipitation infiltrates downwards through the unsaturated zone. This infiltrating water is known as soil water when it is still shallow enough to be used by plants, and as vadose water when it is below root level, but still unsaturated. With further infiltration, however, excess water will eventually reach the water table. (Just above the water table in sedimentary rocks there is a often a short vertical zone known as the capillary fringe, but further discussion of this is beyond the scope of this article.)