A better way of defining deserts is specifically linked to the water balance and was used by Charles Thornwaite to define an aridity index (AI). This approach is based on the relationship between inputs of water as precipitation (P) and the output of moisture from evaporation and transpiration (evapotranspiration: E) and changes in the amount of water stored in the ground. In desert areas, the actual evapotranspiration (AE) is compared with potential evapotranspiration (PE: the amount of water loss that would occur if it was always available).