Ground water is one of the nation's most important natural resources. It is the principal source of drinking water for about 50 percent of the United States population, providing approximately 96 percent of the water used for rural domestic supplies and 40 percent of the water used for public supplies. In addition, more than 30 percent of the water used for agricultural purposes is withdrawn from wells. Ground water also is a significant, but often unrecognized, component of the nation's surface water resources. Much of the flow in streams and the water in lakes and wetlands is sustained by the discharge of ground water, particularly during periods of dry weather.
The U.S. Geological Survey (USGS) has collected water-level data for more than 100 years, and many state and other agencies have a long history of water level monitoring. However, water level monitoring in the United States is fragmented and largely subject to the vagaries of existing local projects. A stable, base network of water level monitoring wells exists only in some locations. Moreover, agency planning and coordination vary greatly throughout the United States with regard to construction and operation of water level observation networks and the sharing of collected data.