Ground water, which is in aquifers below the surface of the Earth, is one of the Nation's most important natural resources. Ground water is the source of about 37 percent of the water that county and city water departments supply to households and businesses (public supply). It provides drinking water for more than 90 percent of the rural population who do not get their water delivered to them from a county/city water department or private water company. Even some major cities, such as San Antonio, Texas, rely solely on ground water for all their needs. About 42 percent of the water used for irrigation comes from ground water. Withdrawals of ground water are expected to rise as the population increases and available sites for surface reservoirs become more limited.