There are many analytical and empirical equations that are currently used to calculate reference evapotranspiration (ETo) using one or more climatic factors. The California Irrigation Management Information System (CIMIS) uses the Modified Penman, also known as the CIMIS Penman, equation to estimate ETo (see the link below for details). Throughout this web site, ETo that is calculated from the CIMIS Penman equation is referred to as CIMIS ETo.
The CIMIS Penman equation employs the modified Penman equation (Pruitt and Doorenbos. 1977. Proceedings of the International Round Table Conference on Evapotranspiration, Budapest, Hungary) with a wind function that was developed at the University of California, Davis. The method uses hourly average weather data as an input to calculate hourly ETo. The 24 hourly ETo values for the day (midnight-to-midnight) are then summed to produce estimates of daily ETo.
In addition to the CIMIS ETo, CIMIS also provides ETo values estimated using the Penman-Monteith (PM) equation for interested users (see the link below for details). ETo estimated using the PM equation is referred to as the Penman-Monteith ETo or simply PM ETo. Studies have shown that there are no significant differences between the PM ETo and the CIMIS ETo.
Although PM has a version that directly calculates daily ETo from daily average weather parameters, CIMIS calculates hourly PM ETo and sums them up to obtain daily PM ETo for consistency with the daily CIMIS ETo. The hourly PM equation that CIMIS uses to estimate hourly PM ETo is the Food and Agricultural Organization's version that is described in Irrigation and Drainage Paper No. 56 (Allen, R.K., L.S. Pereira, D. Raes, and M. Smith. 1998. Crop Evapotranspiration: Guide lines for computing crop water requirements, FAO, Rome). However, the bulk surface resistance is adopted from the ASCE Task Committee on Standardization of Reference Evapotranspiration.
Climatic factors such as solar radiation, air temperature, wind speed, and relative humidity are all measured at the CIMIS weather stations. Both methods (CIMIS Penman and PM) involve calculations of intermediate values from the measured parameters using analytical and/or empirical relationships (see the links below). The measured and estimated intermediate values are then used to calculate ETo.
The following links provide detailed information on ETo equations used by CIMIS: