Daily Reference Evapotranspiration ETos, ETrs and HS ETo Calculator using the ASCE-EWRI (2004) method
User's Guide for DYPM.EXE
ET006 Quick Answer                                             

Copyright (2001) Regents of the University of California
Created on October 25, 2001

Revised November 2004

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

S. Eching, Senior Land and Water Use Analyst
California Department of Water Resources
Office of Water Use Efficiency
P.O. Box 942836
Sacramento, CA 94236, USA


The DYPM.exe is an executable program that calculates reference evapotranspiration for a short (0.12 m) canopy (ETos), for a tall (-0.5 m) canopy (ETrs) and for the Hargreaves Samani equation (HS ETo) for a grass reference using daily weather data.  To obtain a copy of “DYPM.exe” and a sample data file “Lindday.csv”, click on DYPM.zip.  After downloading the zip file, you must extract the DYPM.exe and the Lindday.csv files to the same folder on your computer. Then click on DYPM.exe to run the program. To run the program with other data sets, you must create a “.csv” file with the same format as the Lindday.csv sample file. The format is explained below. Inputs include the daily mean (1) solar radiation (MJ m-2 d-1), air temperature (oC), wind speed (m s-1), and humidity [e.g., dew point temperature (oC) or relative humidity (%)]. The program calculates ETos and ETrs using the Penman-Monteith equation (ASCE-EWRI, 2004) and HS ETo using the Hargreaves-Samani equation (1982,1985). Documentation of the equations used in the DYPM.exe application is available by clicking on PMdayDoc.pdf.



Data are read from a comma delimited file with the extension ".csv".  A sample data set is included in the zip file DYPM.zip. The data filename is Lindday.csv. A sample of the first few rows of Lindday.csv is shown below. Any csv file with the same format can be analyzed. 

CalDate,DOY, Rs, Tx, Tn, U2, Td
26-Apr-01,116, 25, 31, 13.3, 1.2, 14.8
27-Apr-01,117, 27.8, 29.3, 12.6, 1.1, 12.9
28-Apr-01,118, 17.5, 20.8, 11.5, 1.2, 10.9
29-Apr-01,119, 27.7, 24.3, 8.9, 1.2, 10.5
30-Apr-01,120, 28, 28.8, 11,1. 1, 12.7
1-May-01,121, 27.7, 29, 12.8, 1.3, 12.8
2-May-01,122, 28.5, 23.1, 12.8, 1.4, 5.6
3-May-01,123, 28.3, 26.2, 10.7, 1.2, 6.3

It is not important to have the correct spacing between variables, but it is important to have commas between the variable names. The first row contains the site information (Name, latitude, longitude, and elevation). The second row contains the column headings. Data with each row includeds solar radiation (Rs in MJ m-2d-1), max temperature (Tx in oC), min temperature (Tn in oC), mean daily wind speed (U2 in m s-1), and mean daily dew point temperature (Td in oC). 


Reference evapotranspiration is calculated and the data are output to a file with the same name, but with the extension ".day" in the following format.

Lindday, 36.49, 119.01, 146.3, 120
CalDate, DOY, Rs, Tx, Tn, U2, Td, ETo, HS
26-Apr-01, 116, 25.0, 31.0, 13.3, 1.2, 14.8, 5.0, 5.9
27-Apr-01, 117, 27.8, 29.3, 12.6, 1.1, 12.9, 5.2, 5.6
28-Apr-01, 118, 17.5, 20.8, 11.5, 1.2, 10.9, 3.1, 3.7
29-Apr-01, 119, 27.7, 24.3, 8.9, 1.2, 10.5, 4.6, 4.8
30-Apr-01, 120, 28.0, 28.8, 11.0, 1.1, 12.7, 5.1, 5.7
1-May-01, 121, 27.7, 29.0, 12.8, 1.3, 12.8, 5.3, 5.6
2-May-01, 122, 28.5, 23.1, 12.8, 1.4, 5.6, 5.0, 4.1
3-May-01, 123, 28.3, 26.2, 10.7, 1.2, 6.3, 5.1, 5.2
4-May-01, 124, 28.8, 28.7, 9.3, 1.1, 6.5, 5.3, 5.9

The first row again contains site information and the second row contains the column headings. In addition to the input variables, the variable Penman Monteith ETo in mm/d is saved.


From W m-2 to  MJ m-2 d-1 multiply by  0.0864

From Cal cm-2 d-1(ly d-1)  to  MJ m-2 d-1 multiply by  0.04187

From Miles per hour (mph) to  m s-1  multiply by 0.447

From Miles per day (mpd)  to  m s-1 multiply by 0.018625

From degrees Fahrenheit to degrees Celsius


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.