Drylands, places with an arid climate or arid environment, are defined in technical terminology as a region where the mean annual ratio of precipitation (P) to potential evapotranspiration (PET) is signficantly less than one. Which is to say, a place that is kept dried out by natural processes, not enough rain and what rain there is soon evaporates or is lost by transpiration by plants. PET, potential evapotranspiration, is the sum of evaporation and transpiration. A desert is a type of dryland characterized by very little available water.

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