Ground water in the Southwestern United States is derived chiefly from unconsolidated to semiconsolidated alluvial deposits. Where these deposits contain confined water, they may be susceptible to compaction and related land- surface subsidence, if artesian pressures are reduced.
Compaction of artesian-aquifer systems can be estimated from core tests if the artesian-pressure decline is known. Compaction occurs chiefly in the finer grained deposits ; porosity decrease is greater near the top of the confined aquifer than near the bottom.
Because most of the compaction of these aquifer systems is permanent, the storage coefficient during the initial decline of artesian pressure greatly exceeds the storage coefficient during a subsequent pressure decline through the same depth range, after an intervening period of pressure recovery.