GRAVITY METHODS FOR MONITORING GROUND-WATER STORAGE CHANGE

Estimation of ground-water storage change and status of ground-water budgets in overdrafted aquifers is often highly uncertain. The primary components of the budget-inflow, outflow, and storage change-are commonly estimated using highly uncertain indirect methods. Improved measures of any of the components would greatly improve understanding of the water budget and resource evaluation. Direct measure of change in mass of ground water using repeated precise-gravity surveys is one means of improving estimates of ground-water budgets.

Storage change is often estimated as the residual of outflow and inflow, both of which are estimated with variable uncertainty. Another common method applies an estimated storage coefficient to an estimated water-level change derived from measurements in wells. Poor definition of the hydrogeologic system and storage property distributions can result in large uncertainty in estimates of ground-water storage change derived from water-level change. Both methods can result in unacceptably large errors.

Gravity methods directly measure ground-water storage change through measurement of change in the mass of water beneath the gravity instrument. Gravity can be measured on the land surface or from space. Land-based instrumentation includes relative, absolute, and cryogenic gravity meters. Relative meters have the lowest accuracy, typically better than a layer of water that is 15 centimeters in thickness, but can be used to quickly measure many stations in a single survey with an accuracy of several centimeters of water relative to a base station. Absolute meters provide high-accuracy control, about 5 centimeters of water or better, for relative meter surveys. Cryogenic meters continuously record a highly accurate time-series of gravity, better than a centimeter of water, at a single site over periods of days to years. Space-based methods use small perturbations in satellite orbits to measure large-scale changes in the distribution of the Earth's mass. The space-based accuracy of the current NASA GRACE Program is expected to be about a centimeter of water over 1 year for an area of about 220 kilometer radius. Future missions are expected to provide better resolution for smaller areas.