The depth to the water table varies. In other settings, the depth to the water table can be hundreds of feet below land surface. Ground water commonly is an important source of surface water. The contribution of ground water to total streamflow varies widely among streams, but hydrologists estimate the average contribution is somewhere between 40 and 50 percent in small and medium-sized streams. Ground water also is a major source of water to lakes and wetlands.
Ground water serves as a large subsurface water reservoir.
Of all the freshwater that exists, about 75 percent is estimated to be stored in polar ice and glaciers and about 25 percent is estimated to be stored as ground water. Freshwater stored in rivers, lakes, and as soil moisture amounts to less than 1 percent of the world's freshwater. The reservoir aspect of some large ground-water systems can be a key factor in the development of these systems. Velocities of ground-water flow generally are low and are orders of magnitude less than velocities of streamflow. The movement of ground water normally occurs as slow seepage through the pore spaces between particles of unconsolidated earth materials or through networks of fractures and solution openings in consolidated rocks.