Daily Reference Evapotranspiration (ETref) Calculator
User's Guide for PMday.xls


Copyright (2000) Regents of the University of California

Created July 2000          

Revised February 2006 (Note: The column with ETr in MJ m-2 d-1 was corrected)


R. L. Snyder, Biometeorology Specialist
Department of Land, Air and Water Resources
University of California
Davis, CA 95616, USA

S. Eching, Senior Land and Water Use Scientist
California Department of Water Resources
Office of Water Use Efficiency

P.O. Box 942836
Sacramento, CA 94236, USA



The Excel application program PMday.xls is used to calculate daily reference evapotranspiration (ETref) rates using the Penman-Monteith equation (Monteith, 1965) as presented by the Environmental Water Resources Institute (EWRI) - American Society of Civil Engineers Committee (ASCE) on Reference Evapotranspiration (ASCE-EWRI, 2004). The equations used in the hourly EWRI Standardized Reference Evapotranspiration equation are provided on the WEB page: http://www.kimberly.uidaho.edu/water/asceewri/. The symbols ETo and ETr are commonly used for equations that estimate ETref for 0.12 m tall, cool-season grass and 0.5 m tall alfalfa. The subscript ‘s’ was added to indicate that the ASCE-EWRI (2004) standardized equations were used. Strictly speaking, calculated ETos and ETrs provide estimates of a hypothetical 0.12 m and 0.5 m tall surface with specific values or equations for estimation of soil heat flux density, canopy resistance and aerodynamic resistance. However, in reality, ETos and ETrs give ETref rates that are similar to ETo and ETr, respectively. The PMday.xls application program calculates ETos and ETrs from the same daily weather data and then calculates the monthly means. Comparison graphs are also provided.

To obtain a copy of the program, click on PMday.xls. The equations used to develop PMday.xls are presented in the downloadable file PMdaydoc.pdf. A sample CIMIS data set in comma delimited “csv” format containing all of the daily weather data from Davis, California for the year 2003 can be downloaded by clicking on DavisDay2003.csv. The data file can be opened using MS Excel software. Then edit the data to create a file with the required daily weather data. For analysis of the data, there should be columns of total solar radiation (MJ m-2d-1), maximum and minimum temperature (oC), wind speed (m s-1), and mean dew point temperature (oC). From CIMIS, the solar radiation data are expressed in mean power units (Watts m-2d-1). To convert solar radiation to MJ m-2d-1, multiply the Watts m-2d-1 by 0.0864. After modifying the solar radiation units, the proper columns of climate data can be copied and pasted into the PMday.xls application program. Be sure to input the latitude and elevation for the site at the top of the input worksheet. To access other California hourly weather data, go to http://wwwcimis.water.ca.gov/cimis/welcome.jsp to register and download files from CIMIS weather stations.



The station latitude and elevation must be input to make calculations. If the cells are left blank, then latitude 0o and elevation 0 meters above sea level are used. Weather data are entered into the cells corresponding to the correct month. Data must be entered for solar radiation, air temperature, wind speed and humidity to obtain an ETos or ETrs estimate from the Penman-Monteith equation. For the humidity data, either the maximum and minimum relative humidity or the dew point temperature must be input. Either the daily mean dew point temperature or the maximum and minimum relative humidity can be input for calculations. If input, the dew point temperature is selected over relative humidity, so leave the dew point temperature cells blank if relative humidity data are to be used. Only temperature data are needed for the Hargreaves-Samani equation.


Daily ETos, ETrs and HS ETo rates from the Penman-Monteith and Hargreaves-Samani equations are displayed in the DYETo worksheet. Monthly means are also calculated and displayed in the METo worksheet. The results are compared in plots of HS ETo and ETos versus date and ETos and ETrs versus time. The mean daily ETo , ETrs and HS ETo data by month are plotted in the chart “monthly”.




Multiply by

W m-2

MJ m-2d-1


Cal cm-2d-1

MJ m-2d-1


Miles per hour

m s-1


Miles per day

m s-1


To convert from oF to oC use:  


Allen, R.G., Pereira, L.S., Raes, D., and Smith, M. 1998. Crop evapotranspiration Guidelines for computing crop water requirements. FAO Irrig. and Drain. Paper 56. Food and Agriculture Organization of the United Nations, Rome. pp. 300.


Allen, R.G., Walter, I.A., Elliott, R., Mecham, B., Jensen, M.E., Itenfisu, D., Howell, T.A., Snyder, R., Brown, P., Eching, S., Spofford, T., Hattendorf, M., Cuenca, R.H., Wright, J.L., and Martin, D. 2000. Issues, requirements and challenges in selecting and specifying a standardized ET equation. ASCE.


ASCE-EWRI. 2004. The ASCE Standardized Reference Evapotranspiration Equation. Technical Committee report to the Environmental and Water Resources Institute of the American Society of Civil Engineers from the Task Committee on Standardization of Reference Evapotranspiration. 173 p. 


Hargreaves, G.H., and Samani, Z.A. (1982) Estimating potential evapotranspiration. Tech. Note, J. Irrig. and Drain. Engrg., ASCE, 108(3):225-230.


Hargreaves, G.H., and Samani, Z.A. (1982) Reference crop evapotranspiration from temperature. Appl. Engrg. in Agric., 1(2):96-99.


Monteith, J.L. 1965. Evaporation and the environment. 205-234. In the movement of water in living organisms, XIXth Symposium. Soc. of Exp. Biol., Swansea, Cambridge University Press.


Walter, I.A., Allen, R.G., Elliott, R., Jensen, M.E., Itenfisu, D., Mecham, B., Howell, T.A., Snyder, R., Brown, P., Eching, S., Spofford, T., Hattendorf, M., Cuenca, R.H., Wright, J.L., and Martin, D. 2000. ASCE’s Standardized Reference Evapotranspiration Equation. ASCE.