Estimating Evapotranspiration ET
Estimating Evapotranspiration (ET)
California has developed a statewide system of computer operated driven weather stations to generate ET values on a daily basis through the California Irrigation Management Information System (CIMIS). Since it would be too cumbersome and complex to generate ET values for the thousands of different kinds of plants grown in California, each weather station generates only one ET value per day. This one daily value is called real-time reference evapotranspiration (ETo) because it and represents the water use of a standard pasture (the reference plant) and is derived from current weather date (real-time data). The standard pasture grass is tall fescue, mowed at four to six inches in height, and maintained in optimal, non-stressed condition.
CIMIS Data
ETo data from CIMIS is calculated from hourly average data from weather station sensors using a modified Penman equation. ETo provides a good estimate of the daily water use of the pasture grass in inches and indicates the impact of local climate on plant water use. The CIMS stations and their data output are managed by the California Department of Water Resources. CIMIS data is available on the Web at http://www.cimis.water.ca.gov.
Historical ETo
Historical ETo, also known as average values of ETo, have been determined for most areas of California from evaporation, solar radiation, and temperature records. Historical ETo values are available on the Web at http://www.cimis.water.ca.gov.
The historical ETo information is of great value in predicting plant water use and for determining generalized irrigation schedules. One can irrigate fairly accurately using historic ETo data, but remember that these numbers are averages and actual daily data can vary significantly from a 30 or 40-year average.
For example, historic ETo for a day in February in Oceanside is 0.09 inches. However, on February 14, 1992 the real-time ETo was 0.04 inches and on February 23, 1992 the real-time ETo soared to 0.20 inches.
Thus, ETo information is available as historical data, based on 30 to 40 year averages or as real-time data from CIMIS or on-site weather stations. Real-time ETo data are useful for weekly updating of generalized irrigation schedules.