Evapotranspiration (ET)

Water exits the soil-water-plant relationship by run-off, deep percolation, and evapotranspiration (ET). Run-off and deep percolation losses are a result of poor irrigation practices during the irrigation season. ET is the loss of water to the atmosphere by the combined processes of evaporation (from the soil and plant surfaces) and transpiration (from the plant tissue). ET is an indicator of how much water the plant is losing on a daily basis.

The two primary sources of water that a plant uses during the growing season come from precipitation and irrigation. Part of this water is temporarily stored on the plant and soil surface. This water is the part that is readily available for evaporation. The remaining water infiltrates the soil. Plants can easily use the top 25% of the water in the soil. The plants can use an additional 25% of this soil moisture but this extraction can cause the plant a great deal of stress. The remaining 50% can not be extracted by the plant.

Photosynthesis is the process by which plants use sunlight to change water and carbon dioxide into sugars and oxygen. A plant extracts water from the soil with its roots and transports this water to the leaves. Stomata, specialized cells on the lower surface of the leaves, extract carbon dioxide from the atmosphere and exchange it with oxygen generated during photosynthesis in a process called transpiration. During this gas exchange water vapor is lost to the atmosphere. The amount of photosynthesis and transpiration that occurs during the day is directly related to specific weather conditions around the plant.

Many factors affect ET, including: weather parameters such as solar radiation, air temperature, relative humidity, and wind speed; soil factors such as soil texture (Soil Triangle), structure, density, and chemistry; and plant factors such as plant type, root depth and foliar density, height, and stage of growth. ET can be physically measured using a number of expensive devices, but these require daily maintenance and are time consuming. Therefore, a number of empirical equations have been developed to estimate ET based on measured weather parameters.

Most ET equations were developed to correlate measured weather parameters that directly or indirectly affect ET. Since there are so many factors affecting ET, it is extremely difficult to formulate an equation that can produce estimates of ET under different sets of conditions. Therefore, the idea of reference evapotranspiration was developed by researchers. Reference ET is the ET rate of a reference crop expressed in inches or millimeters of water lost. The two reference crops used are grass (normally a cold weather fescue), and know as the short reference crop or ETo, and alfalfa which is designated the tall reference crop or ETr. Most of the weather stations in California use grass as the reference crop.

The historical daily ETo average during the growing season for the Camino weather station is seen in above graph. The calculated averages are based on 25 years of measured weather information. The y-axis of this chart is the calculated ETo and the units are inches of water lost per day. The greatest factor that affects ETo is the amount of solar radiation hitting the leaf surface. The highest rates of ET occur at and just after the summer solstice when the sun reaches its highest aspect and the greatest duration of daylight hours. The ET rate during this time of year is 0.3 inches/day. The lowest rates of ETo occur at and just after the winter solstice when the sun reaches its lowest aspect and the lowest amount of daylight hours.