Actual weather conditions are rarely equal to the variations that have occurred in the past. In most cases, ET rates in standard tables indicate the consumptive use of a high water requirement plant under well watered conditions. Listed values must be adjusted to estimate the rate for a particular type of plant. Reference evapotranspiration (ETO) rates are used in most applications in landscape irrigation. A plant factor (crop coefficient) is used as a multiplier to adjust the ETO for a particular plant species. The system design should be based on realistic ETO rates for the region in which the project is located. Standard rates for design purposes are normally based on average ETO values for the maximum temperature month. Examples of ETO per day for Houston are shown below in Table 1. These values do not represent the absolute maximum ETO rate that may occur on a given day. Listed rates are the “average” per day for the maximum temperature month (July, in most cases).

Calculated rates are based on historical weather data. The actual rate for any day or month may be more or less, depending on the actual weather conditions. In most cases, an extra allowance (safety factor) should be added to the average ETO rate per day for design purposes. The ETO rate should be obtained from a local agriculture extension service whenever possible. In addition to the maximum ETO rate for design, the average monthly rate for the normal irrigation season will be useful for irrigation scheduling. The best information for actual schedules (to cover normal variation) is ETO data based on current weather conditions.