deserts are often defined as areas that receive less than 10 inches of average annual rainfall, but a more accurate defining factor is aridity. In addition to low rainfall, deserts are characterized by a high rate of water loss from the ground (evaporation) and through plants (transpiration). Together this is called evapotranspiration. Potential evapotranspiration is the amount of water that would be lost through evaporation and transpiration if it were available. Scientists measure this using a large pan of water under controlled conditions. In deserts the rate of potntial evapotranspiration exceeds the average annual rainfall by factors ranging from 2:1 to 33:1 or more. In other words, a desert is arid.