Tools & Calculators

Evapotranspiration Resources

Evapotranspiration (ET) represents the loss of water from the earth’s surface through the combined processes of evaporation from soil and plant surfaces, and plant transpiration.

ET information is critical for irrigation system design and water management. However, dramatically different approaches to calculating ET and challenges in finding local data have negatively impacted the industry’s ability to improve irrigation water-use efficiency.

To address these concerns, the Irrigation Association asked the American Society of Civil Engineers to develop a standardized approach to calculating reference ET using accepted, science-based methods and existing climate data. The IA-endorsed standardized equation:
  • Is applicable to both agricultural and landscape irrigation.
  • Facilitates the use and comparison of crop and landscape coefficients for a wide range of climates and locations.
  • Provides guidelines for calculating ET in regions with limited climatic data.

Available ET resources include:

  • Detailed ASCE report on the standardized equation, including recommendations for use, sample calculations and more.
  • ET Connection, a regularly updated list of state resources for ET data.