This background of this approach is described in detail here . For some locations, resources exist to help the researcher understand and estimate groundwater flows. Probably the best resource for the student and researcher is the United States Geological Survey, which describes most of the principle aquifers of the US in great detail in its Groundwater Atlas of the United States . Complications Real aquifers do not have uniform composition. Another complication is that, in reality, aquifers are not in steady state.
Inflows vary seasonally with rainfall and evaporation.
Longer-term trends may result from water demands associated with population growth or changes in land uses. On the daily time scale, the interface between the aquifer and the sea is usually influenced by tides, which imparts a periodic variation to the exchange of water between the aquifer and the sea. For purposes of estimating average rates of flow, these effects can be ignored.