Hourly Reference Evapotranspiration (ETo) Calculator
User's Guide for HRPM.EXE

ET007 Quick Answer                                                   


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

Revised August 12, 2004
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

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

INTRODUCTION                                                                                                                        

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 ENTRY

Data are read from a comma delimited file with the extension ".csv".  To obtain a sample data set, click on DavisHR.csv.  A sample of the first few rows 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 the current year, the previous year, and the Microsoft number identifying 31 December of the previous year.  In this case, 37986 corresponds to 31 December 2003.  When editing the data file in Excel, enter the equation ‘=date(2003,12,31)’, where the first four digits are the year number for the previous year, and Excel will automatically determine the date number. 

Davis, 38.5, 122.5, 18.5, 120, 2004, 2003, 37986

Station, date, Time, DOY, Rs (W), T (oC), U2 (m/s), Td (oC)

6, 1-Jan-04, 1, 1, 0, 4.4, 1.3, 3.8

6, 1-Jan-04, 2, 1, 0, 4.2, 1.5, 3.7

6, 1-Jan-04, 3, 1, 0, 4.4, 2.3, 3.9

6, 1-Jan-04, 4, 1, 0, 5.8, 2.6, 5.4

6, 1-Jan-04, 5, 1, 0, 5.7, 1.5, 5.3

6, 1-Jan-04, 6, 1, 0, 4.5, 1.3, 4.2

6, 1-Jan-04, 7, 1, 0, 4.2, 1.4, 3.9

6, 1-Jan-04, 8, 1, 26, 4.4, 1.3, 4.1

6, 1-Jan-04, 9, 1, 77, 5.5, 1.1, 5.2

6, 1-Jan-04, 10, 1, 224, 7.1, 1.7, 6.2


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
California “Pacific Time” is 120 degrees west of Grenwich, England. For longitudes east of Grenwich, use a negative number for the longitude and the time meridian. The second row contains the column headings. Each row contains the hourly mean data for solar radiation (Rs in W m-2), temperature (T in oC), wind speed (U2 in m s-1), and dew point (Td in oC). 

OUTPUT

Reference evapotranspiration is calculated and the data are output to a file with the 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

   6,   1-Jan-04,   1,   1,    0,   4.4,   1.3,   3.8, -72.2, -36.1, -27.2,  -8.9, -0.01

   6,   1-Jan-04,   2,   1,    0,   4.2,   1.5,   3.7, -72.1, -36.1, -27.7,  -8.3, -0.01

   6,   1-Jan-04,   3,   1,    0,   4.4,   2.3,   3.9, -72.0, -36.0, -29.7,  -6.3, -0.01

   6,   1-Jan-04,   4,   1,    0,   5.8,   2.6,   5.4, -71.2, -35.6, -29.2,  -6.3, -0.01

   6,   1-Jan-04,   5,   1,    0,   5.7,   1.5,   5.3, -71.2, -35.6, -26.6,  -9.0, -0.01

   6,   1-Jan-04,   6,   1,    0,   4.5,   1.3,   4.2, -71.7, -35.8, -26.3,  -9.5, -0.01

   6,   1-Jan-04,   7,   1,    0,   4.2,   1.4,   3.9, -71.8, -35.9, -26.8,  -9.1, -0.01

   6,   1-Jan-04,   8,   1,   26,   4.4,   1.3,   4.1, -51.7, -25.9, -19.2,  -6.6, -0.01

   6,   1-Jan-04,   9,   1,   77,   5.5,   1.1,   5.2, -11.9,  -5.9,  -4.7,  -1.2, -0.00

   6,   1-Jan-04,  10,   1,  224,   7.1,   1.7,   6.2, 129.7,  13.0,  63.2,  53.5,  0.08


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

   1-Jan-04 ,   1,   7.3,  11.5,   4.2,   1.8,   6.0,   1.5,  -0.9,   1.8,   1.8,   0.9

   2-Jan-04 ,   2,   4.2,  11.1,   4.9,   1.7,   6.3,   1.1,  -0.4,   1.7,   1.2,   0.5

   3-Jan-04 ,   3,   8.9,  14.2,   4.6,   2.2,   6.1,   2.9,  -0.6,   2.2,   3.0,   1.3

   4-Jan-04 ,   4,   5.7,  14.6,   6.3,   1.4,   8.0,   0.7,  -1.0,   1.4,   1.4,   0.7

   5-Jan-04 ,   5,   9.7,  18.1,   6.9,   5.1,   6.8,   2.7,  -0.8,   5.1,   5.7,   2.3

   6-Jan-04 ,   6,   9.5,  14.0,   3.8,   2.3,   6.1,   2.2,  -1.1,   2.3,   3.1,   1.3

   7-Jan-04 ,   7,   8.4,  15.3,   2.6,   2.5,   6.0,   1.7,  -1.1,   2.5,   2.3,   1.0

   8-Jan-04 ,   8,   3.2,   8.6,   5.6,   1.8,   6.0,  -0.7,  -1.0,   1.8,   0.5,   0.3

   9-Jan-04 ,   9,   1.9,   8.2,   5.2,   1.5,   6.5,  -0.6,  -0.6,   1.5,   0.2,   0.1

  10-Jan-04 ,  10,   2.5,  13.1,   8.0,   3.1,  10.1,  -0.1,  -0.6,   3.1,   0.6,   0.3


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.

CONVERSIONS

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




REFERENCES

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.

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.

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