Evapotranspiration 101

Evapotranspiration is the combination of evaporation and transpiration.  The two forces together define how much water your soil and plants lose into the atmosphere on a particular day.

• Evaporation is the process by which water absorbs energy, most commonly in the form of solar radiation, and changes from a liquid to a gas (water vapor).
• Transpiration is essentially the same process as evaporation, but occurs inside plants. The vapor is released through tiny gaps in the plant’s leafs, called stomata.

(Drawing courtesy of CIMIS, the California Irrigation Management Information Systems .)

Why does WaterSmart Software care? In order to give households advice on how to water outdoors efficiently, we need to know how much water is being lost, and, therefore, how much water needs to be replaced.

Evapotranspiration is not a constant. According to the U.S. Geological Survey, the amount of water that plants transpire varies greatly geographically and over time, as well as based on plant type. There are a number of factors that determine transpiration rates:

• Temperature: Transpiration rates go up as the temperature goes up, especially during the growing season, when the air is warmer due to stronger sunlight and warmer air masses. Higher temperatures cause the plant cells, which control the openings (stoma) where water is released to the atmosphere to open, whereas colder temperatures cause the openings to close.
• Relative humidity: As the relative humidity of the air surrounding the plant rises the transpiration rate falls. It is easier for water to evaporate into dryer air than into more saturated air.
• Wind and air movement: Increased movement of the air around a plant will result in a higher transpiration rate. This is somewhat related to the relative humidity of the air, in that as water transpires from a leaf, the water saturates the air surrounding the leaf. If there is no wind, the air around the leaf may not move very much, raising the humidity of the air around the leaf. Wind will move the air around, with the result that the more saturated air close to the leaf is replaced by drier air.
• Soil-moisture availability: When moisture is lacking, plants can begin to senesce (prematurely  age), which can result in leaf loss and transpiration of less water.
• Type of plant: Plants transpire water at different rates. Some plants, which grow in arid regions, such as cacti and succulents, conserve precious water by transpiring less water than other plants.

So, how much should you water over the course of the year?  How much should you water in a given week?  You’ll need to know the evapotranspiration rate for your area for that week, and the type of plant you are watering.  For instance, for your lawn:

Irrigated area (sf) x ET rate (inches) x 0.6233 = Water requirement in gallons

This calculation will give you a rough estimate of the amount of water your landscape needs over the course of the entire irrigation season.

You can search for your local evapotranspiration rate online.  Many government agencies and universities calculate these rates.  In California, you can visit the site of the (California Irrigation Management Information Systems )for a zone map with detailed monthly information.

Ready for Evapotranspiration 102?  Send us a note and let us know what additional questions you have.