Declines in groundwater storage are an indication that more groundwater is leaving the system through groundwater pumping, subsurface flows, evapotranspiration or other factors, than is entering the system via natural or artificial recharge, subsurface flows, precipitation, or other sources.
Groundwater storage is a term used to refer to the amount of usable water that remains in an aquifer. Significant and reasonable declines in the amount of groundwater storage may threaten the aquifer’s ability to “produce” or provide water long-term.
Changes in groundwater levels in different aquifers can serve as good proxy for changes in groundwater storage over time. Thus, as discussed in groundwater level declines, it is important that GSAs develop groundwater monitoring networks to sufficiently track changes in groundwater levels and understand how their system responds to changes conditions.
Well-developed and calibrated groundwater models can help water managers understand how changes in pumping, land use or other variables may impact groundwater storage long-term. These models rely on long-term groundwater level data for calibration.
NASA's Gravity Recovery and Climate Experiment (GRACE) satellites can provide estimates of changes in groundwater storage. However, satellite estimates should be used cautiously, as they are more regional in nature and do not provide the level of accuracy necessary to fully determine the conditions at the basin-scale. Land-based gravity methods can measure changes in groundwater storage locally and are more accurate than satellite estimates; however, these methods must be calibrated using local groundwater monitoring networks.