In the Great Plains states west of the Mississippi river, annual rainfall is less than annual water usage for irrigation. The level of groundwater in the Ogallala aquifer that underlies the region is dropping precipitously. On its western edges the aquifer has already run dry and ghost towns exist. Does this depletion of the groundwater represent an efficient use of resources? If not, why aren't unregulated markets efficiently allocating the scarce resource of groundwater in this area? What policy remedy would you suggest?