November 4, 2015

What is the Role of ET in Agriculture?

Forrest Melton, a well-respected senior research scientist, recently said: “When people hear that NASA (the National Aeronautics and Space Administration) is interested in understanding E.T., the last thing that may come to mind is agriculture.”

Though extraterrestrials may come to mind first when confronted with the ET acronym, the reality is Melton is talking about evapotranspiration (ET), which is an abstract term for many, though current research efforts are starting to change this perception.  Of particular interest is the work that the NASA Applied Sciences Program is conducting on operational testing of satellite data for ET applications.  But what do we know about ET and how does it relate to agriculture?

I have found a lot of interesting information recently related to the use of ET in irrigation management from interactions with producers, academia, researchers, technology firms, NASA and the California Department of Water Resources (CDWR).

Evapotranspiration can be complicated to many people, but the term simply refers to the sum of water evaporated from the land surface and transpired from plants.  ET provides a measurement of vegetation water use, and weather stations data enable calculations of reference evapotranspiration (ETo).  However, the actual ET for a particular crop is often calculated from ETo and crop coefficients (Kc) that account for the crop type, stage of growth and plant health.  Kc’s have been developed for many crops and are available through UC Cooperative Extension publications.  The crop-specific ET has the potential to be a great tool for irrigation scheduling because it accounts for ET losses by crop.  This means ET provides the amount or depth of water that needs to be replaced by an irrigation system to meet water needs for that particular crop.

Many producers in the Central Coast rely on different tools to make irrigation decisions.  These tools include web-based information provided through soil monitoring devices, the use of ETo data provided by weather station systems, crop coefficients provided by UC Extension, the stage of the crop, weather forecasts, the use of soil probes, actual observation and the “feel and appearance” method.  In general, California, Arizona and Colorado producers utilize a combination of irrigation systems/tools to manage irrigation.  The use of ET is greater among producers of longer cycle crops such as wine grapes, avocados, nuts, wheat, corn and alfalfa than other shorter cycle crops like leafy greens.  The use of ET is still uncommon among producers of vegetable crops.

Two recent research projects highlight interesting developments in the use of ET in irrigation management.  The first project, Satellite Mapping of Agricultural Water Requirements in California, was sponsored by NASA and resulted in the development of a web interface (the Satellite Information Management System – SIMS) that provides visualization and data access through mobile platforms and irrigation management tools.  SIMS provides information on crop development and water demands for approximately eight million irrigated acres in California and is updated every eight days.  Field testing is currently ongoing with several producers, and to date, field trials have documented good agreement with ground-based ET measurements.

The second project, Testing Decision Tools for Irrigation Management in California Specialty Crops, was sponsored by USDA and the California Dept. Food and Agriculture.  It compared crop yields and quality from ET-based irrigation scheduling with standard irrigation practices.  Two decision tools were tested: SIMS and the UC Cooperative Extension’s CropManage.  Senior Research Scientist Lee F. Johnson, one of the project researchers, found that both tools provided similar guidance on crop water requirements.  Using these tools, commercial yields of head lettuce and broccoli were realized under full ET-replacement irrigation regimes with applied water 20 to 30 percent below standard irrigation practices.  A second set of trials is currently ongoing to examine ET-based irrigation in leaf lettuce and cabbage.

These two projects suggest that ET can offer potential benefits for irrigation management.  Evapotranspiration has not been completely explored among many vegetable crops for different reasons, including: 1) lack of knowledge about how to use it, 2) lack of confidence about the reliability of the data, 3) current irrigation delivery systems constraints (some producers must place water orders 2 to 7 days in advance and cannot adjust quickly to meet real-time water needs) and, 4) lack of technical assistance.

However, many ongoing efforts are addressing these challenges.  For example, current research efforts are aimed at providing data reliability and integrating ET data into online irrigation scheduling calculators.  Outreach efforts and technical assistance, business logistics and infrastructure are also playing a key role.

WG’s Science and Technology Department will be hosting a webinar in November on this topic to provide practical information about the challenges and benefits associated with the use of ET in irrigation management.  WG members are invited to join this webinar.  Additional information and registration information can be found on our website, under the events section (  Join this conversation by providing your comments and thoughts.