HR: 14:55h
AN: H33G-06    [Abstracts]
TI: Repeat Microgravity Surveys for Estimating Ground-Water Storage Change, Recharge, and Specific Yield
AU: * Pool, D R
EM: drpool@usgs.gov
AF: US Geological Survey, Arizona Water Science Center, 520 N. Park Ave, Tucson, AZ 85719
AB: Repeat microgravity surveys are being used in arid and semiarid regions to better define ground-water budgets and estimate specific yield. Repeated measurements at single stations or networks of stations are differenced to determine gravity change. Gravity change across a network of stations is integrated to estimate change in total mass and ground-water storage. Calculations are based on the assumption that there are no significant non-ground-water sources of mass change, such as movement of magma. Specific yield values can be estimated at observation-well sites where water-level and gravity changes are correlated. Ground-water budget components of inflow (recharge), outflow, and storage change are commonly uncertain. Rarely is any one component completely defined by measurement. Only outflow at discrete locations, such as wells and streams, can readily be measured. Inflow is difficult to measure because it is normally dispersed across large areas and occurs episodically. Storage change is normally calculated as a residual of outflow and inflow, and includes all of the uncertainties the other components. Ground-water budgets in heavily developed arid and semiarid basins are commonly dominated by storage change. Gravity methods can be used to estimate change, often leaving a single unknown, recharge, that can be estimated as a residual of measured or estimated outflow and storage change. Four types of gravity instruments are currently being used to measure changes in the distribution of mass on the Earth including: absolute meters, relative meters, super-conducting meters, and satellites. Modern absolute meters use lasers and precise clocks to measure the rate of fall of a mass in a vacuum. Relative meters use a very sensitive spring to measure differences in the force of gravity among sites. Super-conducting meters monitor the strength of a magnetic field necessary to levitate a mass against the changing gravity field. Variations in satellite orbits are also a measure of regional variations in gravity and mass. Accuracy of meters ranges from submicrogal (10-6cm/sec2 , which is equivalent to the acceleration of gravity produced by an extensive 1-inch thick layer of water) for satellites and super-conducting meters, to 1-2 microgal for absolute meters, and several microgal for relative meters. Each type of meter has different applications in water-resource investigations. Super-conducting and relative meters are most applicable to small, local-area investigations. Absolute meters are needed to establish control for small- to large-scale investigations. Satellite methods are most applicable to regional and subcontinent-scale studies. Gravity methods have been used to improve water budgets and estimate specific yield in several areas. Absolute and relative gravity surveys have been used to estimate specific yield and improve water budgets on subbasin scales for areas in Arizona, California, Nevada, New Mexico, Nebraska, and Germany. Local recharge has been estimated near ephemeral streams and artificial-recharge facilities in Arizona. Space-based measurements of the Gravity Recovery and Climate Experiment (GRACE) constrain the global water budget and improve water budgets for large watersheds, such as the Amazon River and Mississippi River Basins, and intermediate sized basins, such as the Colombia River and Colorado River Basins.
DE: 1217 Time variable gravity (7223, 7230)
DE: 1835 Hydrogeophysics
DE: 1836 Hydrological cycles and budgets (1218, 1655)
DE: 1876 Water budgets
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: Fall Meeting 2005