PLAYAS ARE SHALLOW basins that periodically fill with rainwater. They have been described as among the most dynamic geomorphic features, reacting to seasonal and sometimes daily changes in the environment. The term playa owes its origin to the Spanish word for “beach” or “shore.” A large number of playas began as lakes, drying later as a result of climate change or a reduction in stream inflow. Playas captured the imagination of geologists such as Israel Cook Russell during early (1890) investigations of the western U.S. states. The ubiquitous occurrence of playas throughout the world has led to a variety of regional terms. For example, in CHILE and other Spanish-speaking countries they are called salar, in North Africa sabkha, and in IRAN kavir. It is estimated that there are approximately 50,000 playas on Earth; the majority are less than 1 square mi (2.6 square km) in size.
Playas frequently have annual evaporation to precipitation ratios exceeding 10 to 1. Unlike perennial lakes that are fed by rivers and streams, the water in playas is supplied through rainfall. A nearly impermeable layer of clay on the surface of playas prevents water from percolating downward. In the absence of streamflow or frequent precipitation, the water evaporates quickly, leaving basins dry until rain renews the cycle. Winds remove loose surface particles through deflation.
Climatic conditions favorable to playa formation can be found in tropical and subtropical areas of high pressure with latitudes from 35 to 15 degrees. Playa basins originate in a variety of ways, including downwarping, faulting, and deflation. Subsidence and solution processes can also result in playa formation in areas with underlying limestone or gypsum. Many playas represent the remnants of immense lakes that formed from melting Pleistocene glaciers. The former Lake Manly, the site of present-day Death Valley, extended more than 100 mi (160 km) in length and 600 ft (182 m) in depth and was sustained by three rivers carrying meltwater from Sierra Nevada glaciers. As the climate became drier, the lake disappeared through seepage and evaporation.
The arid appearance of playas may create an impression that they are hydrologically inactive. In reality, playas play an important role in arid-zone hydrology. Where groundwater is found close to the surface, playas discharge large amounts of capillary water into the atmosphere. Playas also play a role in the circulation of groundwater. Occasional flooding of playa surfaces removes surface evaporite accumulations.
Early human use of playas has focused on the harvesting of salt. Playas with large salt accumulations, known as saline playas or salinas, are sources of borax, nitrates, and potash. Playas are also important resources for agricultural operations in arid regions. Although playas themselves are not suitable for growing crops, they are often used to store runoff water for later use. They may also serve as surface focal points for recharging aquifers. In the UNITED STATES, playas have been used for automobile speed trials or as landing sites for experimental air and spacecraft.
The Bonneville Salt Flats in UTAH is the site of land speed records, while the vast expanse of Edwards Air Force Base in CALIFORNIA has been an ideal location for testing military aircraft. In the western United States, playas have been impacted by the widespread lowering of the water table and by the migration of vegetation.