Hourly Reference
Evapotranspiration (ETo) Calculator
User's Guide for HRPM.EXE
ET007 Quick Answer
Copyright
(2001) Regents
of the
Created on
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
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
Station, date, Time, DOY,
Rs (W), T
(oC), U2 (m/s), Td (oC)
6,
6,
6,
6,
6,
6,
6,
6,
6,
6,
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
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,
6,
6,
6,
6,
6,
6,
6,
6,
6,
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.
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.,
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.