Description:
This field trip focuses on subsidence features caused by removal of groundwater at
rates greater than natural recharge in the Picacho basin. Land subsidence is common in
Arizona's alluvial basins, where extensive groundwater withdrawal has lowered water
tables by as much as six hundred feet. The water pressure (weight) of groundwater
produces a subsurface buoyancy force. Removal of groundwater and the associated
buoyancy force results in compaction of basin sediments, reduction of pore space, and
subsidence at the Earth's surface. Subsidence has occurred in every basin in Arizona
where use of groundwater has significantly lowered water levels. Earth fissures typically
appear at the margins of subsiding basins and accommodate subsidence of the basin
interior relative to basin margins and flanking bedrock.
Multiple, extensive earth fissures have developed around the Picacho basin, an
agricultural area where groundwater is being depleted. This field trip includes a visit to an
area where an earth fissure system has developed that is over one mile long. The fissure
system displays most of the features typical of earth fissures around the state.