To monitor progress in complying with the legislation, ground-water withdrawals are measured and estimated, and annual recharge is estimated. Late in 1997, the USGS established a network of about 60 microgravity stations within the TAMA to measure basinwide changes in ground-water storage. Microgravity methods are based on the principles of Newton's Law of Gravitation that states that the acceleration due to gravity within an object's gravitational field is directly related to the mass of the object and inversely related to the distance to the center of the object. In simple terms, the greater an object's mass, the stronger its gravitational field. Differences in measured gravitational fields over the earth's surface have been used by geophysicists for years to map variations in crustal thickness, the presence of magma bodies, and the subsurface distributions of different rock types.
Ground water is stored within the pore spaces of aquifers.
As an aquifer is drained by pumpage or filled by recharge, its mass changes, which results in changes in the strength of its gravitational field. Recent technological advances in geophysical techniques have made measurement of the extremely small gravitational changes caused by fluctuations of water volume practical. The USGS microgravity network is based on the University of Arizona network and is the first basinwide application of microgravity methods to the measurement of changes in ground-water storage. Rillito Creek, which is normally dry, had substantial streamflow from December 1992 to March 1993. Infiltration of streamflow through channel sediments recharged the underlying aquifer, causing water levels and microgravity readings to rise.