The water table is the upper surface of the zone of saturation, which is where the pores and fractures of the ground are saturated with water. It is the boundary between the unsaturated zone and the saturated zone underground. The water table is the surface where the water pressure head is equal to the atmospheric pressure, and it may be visualized as the "surface" of the subsurface materials that are saturated with groundwater in a given vicinity. The water table may vary due to seasonal changes such as precipitation and evapotranspiration. In undeveloped regions with permeable soils that receive sufficient amounts of precipitation, the water table typically slopes toward rivers that act to drain the groundwater away and release the pressure in the aquifer. Springs, rivers, lakes, and oases occur when the water table reaches the surface. Groundwater entering rivers and lakes accounts for the base-flow water levels in water bodies.
To find the depth to the water table in a specific location, the USGS recommends using a groundwater technical procedures document (GWPD) that provides standardized technical procedures for many aspects of groundwater science, including site and measuring-point establishment, measurement of water levels, and measurement of water quality.
