Case Study: Land Subsistence in the American Southwest

a) Describe the causes of land subsistence in the American Southwest and explain why its getting worse over the years?

Overdrafting of aquifers is a major cause of subsidence in Southwestern United States, and as ground-water pumping increases, land subsidence also will increase. In many aquifers, ground water is pumped from pore spaces between grains of sand and gravel. It's getting worse and worse each year because a large amount of groundwater has been excessively withdrawn from aquifers. For instance the lowered water pressure causes slow drainage from clay and silt beds, it also results in a loss of support for the clay and silt beds, and it can also lower the lands surface.

b) Describe the potential impacts of land subsistence. 

Land subsidence causes many problems including: 

(1) changes in elevation and slope of streams, canals, and drains

(2) damage to bridges, roads, rail roads, storm drains, sanitary sewers, and canals
(3) damage to private and public buildings

(4) failure of well casings from forces generated by compaction of fine-grained materials in aquifer systems. In some coastal areas, subsidence has resulted in tides moving into low-lying areas that were previously above high-tide levels. 

c) Outline ways to prevent further land subsistence.  

In some areas where ground-water pumping has caused subsidence, the subsidence has been stopped by switching from ground-water to surface-water supplies. If surface water is not available, then other means must be taken to reduce subsidence. Possible measures include reducing water use and determining locations for pumping and artificial recharge that will minimize subsidence. Optimization models coupled with ground-water flow models can be used to develop such strategies.