Evapotranspiration (ETo) Calculator
User's Guide for HRPM.EXE
ET007 Quick Answer
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 Analyst
California Department of Water Resources
Office of Water Use Efficiency
The HRPM.EXE is an executable program that calculates reference evapotranspiration (ETo) for a grass reference using hourly weather data and the Penman-Monteith equation. To obtain a copy, click on HRPM. It is an executable program requiring no additional software. Inputs include the hourly (1) solar radiation (W m-2 d-1), air temperature (oC), wind speed (m s-1), and humidity [dew point temperature (oC)]. The program calculates ETo using the Penman-Monteith equation (Monteith, 1965) following the guidelines recommended by the ASCE (Walter et al., 2000; Allen et al., 2000).
Data are read from a comma
with the extension ".csv". To obtain a sample data set, click
on DavisHR.csv. A sample of the first
of DavisHR.csv is shown below. Any csv file with the same format can be
analyzed. Note that the last three values in the first row are
current year, the previous year, and the Microsoft number identifying
December of the previous year. In this
case, 37986 corresponds to
Station, date, Time, DOY, Rs (W), T (oC), U2 (m/s), Td (oC)
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, elevation and longitude of the local time meridian). Note that the local time meridian for
evapotranspiration is calculated and the data are output to a file with
same name, but with the extension ".hly" in the following format.
davishr, 38.5, 122.5, 18.5, 120
Sta, CalDate, Hr, DOY, Rs, T, U, Td, Rn, G, H, LE, ETo
The first row again contains site information and the second row contains the column headings. In addition to the input variables, the variables net radiation (Rn in W/m*m), soil heat flux density (G in W/m*m), sensible heat flux density (H in W/m*m), latent heat flux density (LE in W/m*m), and reference evapotranspiration (ETo in mm/h) are saved. The flux density data and ETo are calculated using the approach of the ASCE hourly ETo method using the Penman Monteith equation.
The program also adds up the hourly data and outputs the daily sums to a file with the
same name and an extension ".dly" in the following format.
davishr, 38.5, 122.5, 18.5, 120
CalDate, DOY, Rs, Tx, Tn, U2, Td, Rn, G, H, LE, ETo
The first two rows contain site information and column headings. The Rs data are in MJ m-2, Tx and Tn are in oC, U2 in m s-1 average for the day, Td is in oC, Rn, G, H, and LE are in MJ m-2 and ETo is in mm d-1.
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., 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.
J.L. 1965. Evaporation and the environment. 205-234. In the movement of
in living organisms, XIXth Symposium. Soc. of Exp. Biol.,
I.A., Allen, R.G., Elliott, R., Jensen, M.E., Itenfisu, D., Mecham, B.,
T.A., Snyder, R., Brown, P., Eching, S., Spofford, T., Hattendorf, M.,
R.H., Wright, J.L., and Martin, D. 2000. ASCE’s Standardized Reference
Evapotranspiration Equation. ASCE.