Evapotranspiration is the sum of
evaporation and
plant transpiration. Potential evapotranspiration is the amount of water that could be evaporated or transpired at a given
temperature and
humidity, if there was plenty of
water available. Actual evapotranspiration can not be any greater than
precipitation, and will usually be less because some water will run off in
rivers and flow to the
oceans. If potential evapotranspiration is greater than actual precipitation, then soils are extremely dry during at least a major part of the year.
