Handling weather data and calculating Reference Evapotranspiration

By Ayse Kilic

This assignment will provide experience in obtaining weather data from the High Plains Regional Climate Center (at Univ. Nebraska-Lincoln) and processing those data using the REF-ET software to calculating reference ET using the ASCE Standardized Penman-Monteith equation.

  1. Each student should select one HPRCC weather site within the State of Nebraska where the first letter of the station is the same as the first letter of the student’s last name. If no station exists, select the closest letter.
  2. For your station, goto the HPRCC web site and select the “AWDN” weather station that was recording weather data during 2012 AND 2014. You will download two year’s data for daily weather measurements for 2012 and 2014. Instructions for downloading data from the HPRCC site and download REF-ET are at the end of this assignment. Note that if you wish, you can download 2012-2014 at the same time and process it with REF-ET and then focus on 2012 and 2014.
  3. For your weather station, download the daily data (in SI units) and process it using REF-ET. The products should be the out and in2 data files. The out file contains the input weather data (solar radiation, Tmax, Tmin, Tdew and wind speed) plus the reference ET. When you run the REF-ET program, ask it to output the 2005 Standardized ASCE Penman-Monteith reference ET for BOTH ETr (alflalfa reference) and ETo (grass reference). The in2 file contains nearly 100 ‘intermediate’ calculations of parameters that go into the reference ET calculations, such as solar angle, atmospheric transmissivity, vapor pressure, delta, gamma, etc. This file can be helpful to check your hand calculations.
  4. After running REF-ET, run the QAQC program and create graphics of Rs vs. time, Tmax and Tmin and Tdew vs. time, RHmx and RHmin vs. time and wind speed vs. time.
  5. Comment on how well the measured Rs agrees with the theoretical clear sky Rso curve.
    Do you think that the data need to be adjusted? By how much, and in which direction?
  6. Turn in:
  7. Printouts of the out file from REF-ET and the original text file from HPRCC
  8. Printouts of the four graphics created by QAQC in Excel.
  9. The name of your station, and the latitude, longitude, elevation.
  10. The date and value for the day having the maximum ETr for each year
  11. The date and value for the day having the minimum ETr for each year
  12. Do the values for (d) and (e) seem to be reasonable?
  13. Create a graph of ETr divided by ETo vs. day of the year.
  14. Plot monthly ETr for both years on the same graph vs month. Comment on whether ETr is higher or lower in 2012 than it is 2014 and why.

Based on the graph in g, on average, during May – August, how much larger is ETr than ETo? Give two important reasons why this difference occurs.

  1. What does “AWDN” mean?
  2. Include a photograph of your station or a typical HPRCC AWDN station.

Downloading daily weather data for AWDN stations from the HPRCC site & Use of Ref-ET

It is recommended that you use FireFOX (Mozilla) or Explorer to download the data. Chrome may not store the data as a text file.

  1. Go to the web site.
  2. Select ‘services’ and ‘on-line data services’ from the top bar.
  3. Click on “classic online services”
  4. Enter the user name and password
  5. At the bottom of the screen, click the disclaimer and agreement
  6. From the sheet, select: “Select a station”
  7. Select your state from “Identify a state”
  8. Select “click here to search for a station”
  9. Click on the name of a station and select “Click here to add the highlighted station….”
  10. From the “current station list”, select your station
  11. Click on “Raw data report” on the left
  12. Select your station from the list
  13. Keep ‘output format’ as ‘tabular’ and uncheck the enable “Quality Control Flags”
  14. Select “System International” units
  15. Select “Sample interval” as daily
  16. Select “Click here to select these options”
  17. On the next screen, change the start and end dates to 1/1/2012 and 12/31/2014
  18. Select “Click here to submit data request”
  19. You should see a web page full of daily weather data that has two or three lines of text at the top and then one line per day of weather data
  20. In FireFox, select the top right “Firefox” button and select “Save page as”. In the file save as dialog box, point to where you wish to save the data. Change the ‘name’ of the file from “dr3.cgi” to something more descriptive, such as “Scottsbluff_NEAWDN_daily_weather_2012_2014.txt” Save the data as the ‘txt’ type.
  21. This should create a text file that can be read by REF-ET, however:
  22. The downloaded file may not always contain both the ‘line-fee’ and ‘carriage control’ characters. Therefore, REF-ET attempts to read the data file as one line. To correct for this,before running REF-ET, open the weather data file in WordPad and save it. By saving it, that will insert both characters and then REF-ET will know that the data are on separate lines.

  1. IF the downloading or quality of the station’s data does not seem to be good, then make a note of this and select a better station.

REF-ET Download and operate instructions. April 2014

(REF-ET calculates reference ET from weather data).

  1. Go to the Univ. Idaho site:
  2. Click on “as a compressed archive” to download to your computer
  3. Enter your name, location, etc. to register (there is no cost)
  4. Select the download button
  5. Fill out the form.
  6. Select the download url and save the file to your hard drive.
  7. Using windows explorer, go inside the zip archive and run the Ref-ET setup program.
  8. Agree to the license agreement (free), use the default destination folder, and install all six files, including the documentation. The REF-ET and QAQC programs will be installed.
  9. To run REF-ET, click the Windows start button (lower lefthand of display), click ‘all programs’ and select REF-ET.
  1. For instructions on running REF-ET, go to the folder that contains REF-ET on your computer, (for example, “C:\Program Files (x86)\Ref ET\” and open the Ref ET V4.1 Users Manual.pdf file. (or you can download the pdf from the REF-ET web site).
    -- Instructions begin on page 13 of the manual. There will also be an in-class demonstration.
    Note that REF-ET will read the ‘txt’ data file from the HPRCC web site that you will download (you may need to open the data file in WordPad and resave it to get the carriage control/line feeds to work correctly). REF-ET will also help you create the ‘def’ file that ‘defines’ the data in the text file (what is in there, which columns it is in, what the units are). Please be careful in selecting units and parameters. Solar radiation is not the same as net radiation, etc.

Note that the RHmean in the HPRCC daily data is the average RHmean for the day. You will want to provide REF-ET with that data parameter, as well as both maximum and minimum daily air temperature, solar radiation and wind speed.

You can use the station tool on HPRCC to get location information (lat., long. Elevation) information. You can dig around on the HPRCC site to find the heights of the anemometer and air temperature/humidity sensor.

REF-ET will create an “out” file that contains the basic weather data and reference ET calculations. It will also create an in2 file. The in2 file contains nearly 100 ‘intermediate’ calculations of parameters that go into the reference ET calculations, such as solar angle, atmospheric transmissivity, vapor pressure, delta, gamma, etc. This file can be helpful to check your hand calculations.

Descriptions of the files are given in the REF-ET users manual.

  1. After successfully running the REF-ET program, you can run the QAQC.exe program. That program ‘reads’ the ‘out’ and ‘in2’ text files created by REF-ET and it plots the solar radiation, air temperature, dewpoint and wind speed data in a spreadsheet for your evaluation. A Word file for documentation of QAQC is in the same folder as REF-ET’s manual.

1