Water Conservation at the City of San Bruno

ET Controllers for Maximum Water Efficiency
ET is the rate of plant water use - evapotranspiration - soil evaporation and plant transpiration is affected by temperature, humidity, sunshine and wind.

Evapotranspiration and Irrigation
Evapotranspiration (ET) is the combined process of plant transpiration and soil evaporation (Figure 1). Plant transpiration is the movement of moisture from the plant to the air through tiny pores in the leaves known as stomates. The water enters the plants through the roots in a liquid form and leaves the plants through the stomates in a gaseous form. Soil evaporation is the direct evaporation of water from the surface of the soil into the atmosphere.

ET irrigation scheduling has been used for many years in the irrigation of large landscape areas, golf courses, and agricultural fields. In recent years, ET irrigation scheduling has also been used for irrigating residential landscapes.

Why is ET important in irrigation scheduling? The ET rate is closely related to a plant's water requirement. Therefore, if we know the ET rate for a specific site, we can use this ET information to determine an irrigation schedule for the site. By scheduling the irrigation based on ET rates, the plants will receive the required amount of water with very little waste of water.

ET rates are expressed in amount of water removed over a period of time. The common units for ET rates are inches per day or millimeters per day.

There are four primary factors that affect ET rates. They are solar radiation, temperature, relative humidity and wind (Figure 1). The ET rate generally increases with each increase in solar radiation, temperature and/or wind. However, the opposite is true with relative humidity. An increase in relative humidity, will generally result in a decrease in the ET rate. During the summer, in southern areas of the United States, ET rates are quite high and will generally average 0.25 inches or higher per day. During the winter, in these same southern areas, the ET rates will generally be 0.1 inches or less per day.

Soil moisture, plant type and stage of plant development are other factors that affect the ET rate. If there is no soil moisture, there cannot be any ET. Also, some types of plants may have low transpiration rates whereas other plants may have high transpiration rates. Furthermore, there will generally be less transpiration from young, immature plants than fully mature plants.

There has been over 50 years of research on ET. During this period several different methods have been used to determine ET rates, including energy balance, soil water balance, and ET rates computed from meteorological data. Today, ET rates are frequently determined by ET mathematical formulas involving one or more weather factors. The weather factors are solar radiation, temperature, relative humidity and wind, as listed above. The most commonly used ET mathematical formula is the Penman-Monteith Equation, which uses these four weather factors. However, there are other mathematical formulas, such as Hargraeves' formula that is based on temperature and Jensen-Haise's formula that is based on solar radiation.

The ETo curves in Figure 2 were derived by the Penman-Monteith Equation. Based on the ETo curve for Riverside, CA, a daily irrigation application of 0.25 inches of water would be required to maintain plants in a healthy condition during the month of July, 1999 in Riverside, CA. As mentioned earlier, there are other factors that have to be considered in addition to the ET rate. Therefore, the recommended irrigation application rate may be slightly less than or more than 0.25 inches per day.

Water removed from the soil and plants by ET may be replaced by natural rainfall rather than irrigation. This is especially true during the winter in the southern part of the United States. Generally, there is more rainfall during the winter than during any other time of the year. Also, the ET rate is generally lower during the winter than during the spring, summer or fall. Therefore, in many places in the south, there may not be a need for irrigating the landscape during the winter. However, during the spring, summer and fall, there generally will not be adequate rainfall to replace the water removed by evapotranspiration. Therefore, irrigation will likely be required during the spring, summer and fall seasons in most areas of the southern part of the United States as well as many other areas of the United States.

ET information can be obtained from various sources, including weather stations, newspapers, radio stations, WEB sites, etc. A homeowner, using this ET information, can manually change the settings on their irrigation controller to efficiently irrigate the landscape. Alternatively, if they have an ET irrigation controller, such as, the Aqua Conserve ET-6, ET-9, ET-14, ET-16, ET-24 or ET-32 controller, the controller will automatically vary the irrigation applications to meet the water requirements of the plants based on ET data and other factors.