To answer the question “How much water has to be applied to the plant material?” and “How often and how long does the system need to run?”, a number of factors need to be examined. The local climate is one of the main things that influences how much water will need to be applied to maintain good plant growth. The plant water requirement includes the water lost by transpiration and evaporation. Transpiration and evaporation are the two principle ways in which water in the soil is depleted. Transpiration is the process by which plants consume water. Evaporation is the direct loss of water from the soil surface. Water loss can also occur with runoff and percolation below the level of root absorption. Evapotranspiration rate is often used to indicate the total loss due to evaporation and transpiration. Evapotranspiration is sometimes referred to as “consumptive use”.


  • Plant Species and Related Variables
  • Cultural Practice
  • Water Quality
  • The Environment Above and Below the Ground


  • Temperature
  • Solar radiation
  • Humidity
  • Wind Velocity

Evapotranspiration rates available for a regional area are, in most cases, based on a formula using the average of long term weather data. While this does not include all of the variables, it is a practical solution for design and is considered to be reliable for estimating water requirements. ETO rates, commonly used in landscape irrigation, include general use tables with monthly rates for a city, region or type of climate. Listed rates for a regional area are based on average weather conditions using historical data. While useful to estimate water loss, rates do not indicate the actual loss for a given day, week or month in the future.

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.


Monthly Avg. Rainfall
Evapotranspiration Rate
January 3.75 2.93 -.82
February 3.25 3.64 .39
March 2.75 5.37 2.62
April 3.50 6.26 2.76
May 4.25 7.57 3.32
June 5.00 8.27 3.27
July 4.75 8.42 3.67
August 3.75 7.69 3.94
September 5.25 6.26 1.01
October 3.75 5.13 1.38
November 3.75 3.43 -.32
December 3.75 2.69 -1.06


Texas A&M University provides daily weather summaries and PET data for over 15 locations in Texas. Interactive calculators are provided to allow users to determine the irrigation water needs (or ET) of landscape plants and agricultural crops.

Texas ET contains weather information, current and average evapotranspiration data, and irrigation watering recommendations.


The Complete Guide to Turf and Landscape Irrigation Systems
By Richard B. Choate