No nationwide, systematic ground water level monitoring program exists. At present, there are approximately 42,000 long-term observation wells in the United States that have five or more years of water-level records. The density of existing monitoring wells, however, varies considerably from state to state. The extent of water level monitoring varies even more among major aquifers, with very limited monitoring in many aquifers. Thus, an inventory of existing water-level networks for major aquifer systems would be made early in the assessment to identify data gaps and opportunities for collaboration across the nation.
To have national or even regional significance, indices of ground water levels have to be based on repeated observations at relatively large numbers of observation wells located in a wide range of representative hydrogeological environments. Ground water systems are dynamic and continually adjust to short-term and long-term changes in climate, ground water withdrawals and land uses. Water levels in wells change in response to a number of types of local and regional stresses, some of which are natural and some of which are human-induced. Because sub-surface hydraulic properties are highly variable, water level responses to stresses vary considerably with location and depth. Stresses take time to propagate through ground water systems, so water level changes are transient phenomena that are strongly affected by distances from the monitoring wells to imposed stresses.