 evapotranspiration evapotranspiration understanding the way your plants function helps provide the right amount of water to ensure a healthy landscape and maximizes the efficiency of water applied to your landscape one way to do this is by using local weather conditions to help determine how much water your lawn and plants need to maintain healthy growing conditions the basics of how plants function just as you release water vapor when you breathe plants do too it s called transpiration  when plants transpire faster than water is being replaced they can wilt or die similarly soils also lose moisture through evaporation  combine the two and you ve got evapotranspiration  or et which is the combination of water lost from the soil through evaporation and from plants through transpiration the amount of water lost through et is equal to the amount of water that must be replaced to maintain healthy growing conditions just remember et values are calculated from local weather conditions to help you determine how much water is needed for your landscape et changes with local weather conditions et is affected by amount of sun wind and air temperatures et varies for different types of plants so why are we talking about it  here in the portland metro area water use can more than double in the summer months unfortunately our usage increases at a time when there is less water available to us by making a few simple changes to your watering routine  you can make a big difference for your plants and your pocketbook how are evapotranspiration and the weekly watering number related  the consortium contracts with a weather forecasting service to provide a free weather forecast and weekly watering number each thursday the weekly watering number is derived that week s evapotranspiration number  evapotranspiration previous   outdoor  