HOW TO USE THIS WEB SITE FOR DETERMINING THE IRRIGATION WATER NEEDS OF PLANTS AND CROPS

METHOD 1: Select your weather station. Choose the number of days to be included: 3-day summary, 5-day summary, 7-day summary or 2-week summary (by default, the 7-day summary is shown). Choose the calculator: Landscape or Crop. The calculator is automatically populated with the ETo total for the number of days listed and with rainfall. If your rainfall is different, enter it. Fill in the remaining variables and click "Compute" to determine the water needs for your landscape or crop.

METHOD 2: Track the daily and weekly ETo values and use this information subjectively to estimate water needs.

METHOD 3: Use the average historic ETo values to determine seasonal irrigation water needs and establish monthly irrigation schedules.

NOTE:
If you utilize this site often, you may want to create an account where you can set up sites, quickly adjust crop coefficients during the growing season, and view or receive calculated water requirements by email.

Click the link below to obtain a copy of the Website User's Manual:

Texas ET Network and Website User's Manual

Evapotranspiration (ET) is a measurement of the total amount of water needed to grow plants and crops. This term comes from the words evaporation (i.e., evaporation of water from the soil) and transpiration (i.e., transpiration of water by plants). Different plants have different water requirements, so they have different ET rates.

In 1948, a British scientist name Howard Penman presented a theory (i.e., mathematical model) for calculating ET using local climate data. The resulting Penman equation has been modified since, but forms of this equation are still used today for this purpose.

Since there are thousands of cultivated plants, scientists simplified matters by establishing a standard ET rate for general reference and use. The standard is referred to as the potential evapotranspiration ETo (pet) . This is the "potential" ET since we are assuming the crop is in a deep soil and under well watered conditions. A cool-season grass is widely used for this standard crop. The technical term for this is the "Potential Evapotranspiration of a Grass Reference Crop" or "ETo" for short.

ETo depends on the climate and varies from location to location. Special weather stations are used to collect the climatic data for calculating ETo, including temperature, dew point temperature (relative humidity), wind, and solar radiation. The American Society of Biological and Agricultural Engineers has established a standard for weather stations used for calculation of ET which can be found on this website.

The water requirements of specific crops and turf grasses can be calculated as a fraction of the ETo. This "fraction" is the called the crop coefficient (Kc) or turf coefficient (Tc). Crop coefficients vary depending on the type of plant and its stage of growth. Detailed information on crop and turf coefficients and how to use them is presented at other locations on this website.

We are using the standardized Penman-Monteith method to calculate ETo from the weather station data. This method is also referred to as the ASCE standard method (for more info, see: The ASCE Standardized Reference Evapotranspiration Equation | Books (ascelibrary.org) . This is one of several methods that can be used to determine ETo and ET. Organizations, such as the International Committee on Irrigation and Drainage and the Water Requirements Committee of the American Society of Civil Engineers, have proposed establishing the Penman-Monteith method as a world-wide standard. Such a standard would help facilitate the sharing of ETo data and development of crop coefficients.

For more information on the Penman-Monteith equation see the FAO website . For information on the standardized equation and other methods for determining ET, see the book:

Evapotranspiration and Irrigation Water Requirements, edited by M.E. Jensen, R.D. Burman, and R.G. Allen. Published by the American Society of Civil Engineers, New York, NY. 2016.
https://doi.org/10.1061/9780784414057