ET003 Quick Answer
Copyright (2000) Regents of the University of California
Created July 2000 – Revised January 2, 2002
R. L. Snyder, Biometeorology Specialist
Department of Land, Air and Water Resources
University of California
Davis, CA 95616, USA
ABSTRACT
NWSETO is a program for the calculation of daily reference evapotranspiration (ETo) rates using forecast weather data. The program is written in compiled QuickBasic (Microsoft, Inc.) for use with an IBM compatible computer (MS DOS operating system). To obtain a copy, click on NWSETO.
INTRODUCTION
The program “NWSETO” is used to calculate daily reference evapotranspiration (ETo) rates using forecast weather data. Reference evapotranspiration (ETo) approximates the evapotranspiration of a 8 to 15 cm tall cool-season grass with adequate water supply to avoid water stress and adequate fetch to minimize localized advection effects on evaporation. The equations used in the NWSETO program were published by Hargreaves and Samani (1982) and Penman (1963).
The Hargreaves and Samani equation requires only the location latitude and daily maximum and minimum temperature to estimate ETo, whereas the Penman equation requires the location latitude, maximum and minimum temperature, maximum and minimum relative humidity, daily mean wind speed, and an estimate of net radiation from percentage cloud cover. Although the Hargreaves and Samani equation is less complicated, it does not respond to variations in humidity, wind speed, or radiation like the Penman equation. Therefore, it is more prone to errors.
The program is started by typing “NWSETO” and pressing the RETURN (Ret) key. The title page will appear on the screen. Press the Ret key to continue or the ESCAPE (Esc) key to exit. Pressing Ret will display the “SITE DESCRIPTION MENU”.
SITE DESCRIPTION MENU
When the site description menu appears, you can input a previously entered site name to recall stored information or input a new name and new information. Site information includes the following:
(1) Latitude [0 to 90 for north or 0 to -90 for south]
(2) Elevation (feet) [Note: 1 m = 3.28 feet]
(3) Wind sensor height (inches) [Note: 2 m = 79 inches]
A sample description menu display for the Davis, California lysimeter site is shown in Figure 1. A site name (not exceeding eight characters) must be input. This name serves as the filename for data storage. The latitude, elevation, and wind sensor height must be input before the program will continue to the next screen. Input a negative number for degrees south latitude. Wind speed is adjusted from the input height to a 2.0 m height for computations.
Other selections from the site description menu are shown in Figure 1. Press F3 to use the “file manager” to change the default directory, to list stored data files, and to copy or delete files. When listing datafiles, only those on the currently logged directory will be displayed. Site names with the extension “.NWS” are used for filenames to store input data. Background color can be changed for computers with color screens by pressing F5. Change the foreground color using F6. Press F8 to toggle (change back and forth) the units from English to Metric and Metric to English. Please note that a data set previously input cannot be changed to the other units without re-entry of the data. If data were previously stored under a filename using English units, the stored data cannot be recalled unless the units on the site description menu are set to English. Pressing F1 after inputting the site information will load the weather data input menu. If the site name, latitude, elevation, or wind sensor height are not input the program will not continue to the weather data entry spreadsheet.
WEATHER DATA ENTRY SPREADSHEET
A spreadsheet type of entry is used to input weather data into the program. Figure 2 shows sample input data and the resulting calculations. Input and output variables are listed across the top of the screen and 12 month and day combinations are listed down the left side. The Ret or arrow keys are used to move the cursor for data input and editing. Characters appear on the right-hand side of the cursor as data are input and they move to the left as more characters are added. Although the decimal point always appears in a data entry, the decimal key must be pressed before inputting fractions. Pressing the Backspace (BkSp) key deletes the right-hand character of an input variable. BkSp must also be pressed to delete a decimal point. Pressing DELETE (Del) deletes an entire data entry. Pressing Ret moves the cursor to the right one location or to the left side of the next row. Press Esc to return to the site description menu.
In Figure 2, the average temperature and humidity data for June were entered for June 15. The wind speed was varied from 2.0 to 10 mph using zero percent cloud cover to illustrate the effect of wind speed on the Penman equation and the lack of effect on the Hargreaves and Samani equation. Similarly, the percent cloud cover was varied with a constant wind speed 5.4 mph to illustrate the effect of wind speed on ETo rates for the two equations.
In some locations, daily dewpoint temperatures are archived in climatic data files rather than maximum and minimum relative humidity. Equations 1, 2, and 3 are used to calculate saturation vapor pressure at the dewpoint, maximum, and minimum temperatures, respectively (Tetens’, 1930). Estimates of maximum relative humidity (Hx) and minimum relative humidity (Hn) are calculated using equations 4 and 5.
1
2
3
4
5
REFERENCES
Hargreaves G. H., and Samani, Z. A. 1982. “Estimating potential evapotranspiraton .” Tech. Note, J. Irrig. and Drain. Engrg., ASCE, 108(3):225-230.
Penman, H. L. 1963. Vegetation and hydrology. Tech. Comm. No. 53, Commonwealth Bureau of Soils, Harpenden, England. 125 p.
Tetens, O. 1930. “Uber einige meteorologische Begriffe.” Z. Geophys., 6:297-309.
Directory: C:\IRRIG\
Date: 01-30-1992
SITE DESCRIPTION MENU
Site name (8 characters no extension) Davis
Latitude [ – for south ] 38.5
Elevation (feet) 61
Wind sensor height (in) 79
Units English
F1 – save & continue
F3 – file manager
F5 – change background color
F6 – change foreground color
F8 – toggle units
Esc – exit from program
Figure 1. A sample of the SITE DESCRIPTION MENU for Davis, California
WEATHER DATA INPUT SPREADSHEET FOR SITE: DAVIS
| Month | Day | Max Temp | Min Temp | Max Hum | Min Hum | Wind Speed | Cloud Cover | Pen ETo | HARG ETo |
| F | F | % | % | mph | % | in | in | ||
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 2.0 | 0.0 | 0.23 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 4.0 | 0.0 | 0.24 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 6.0 | 0.0 | 0.26 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 8.0 | 0.0 | 0.27 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 10.0 | 0.0 | 0.28 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 5.4 | 0.0 | 0.25 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 5.4 | 20.0 | 0.24 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 5.4 | 40.0 | 0.22 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 5.4 | 60.0 | 0.20 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 5.4 | 80.0 | 0.18 | 0.24 |
| 6.0 | 15.0 | 82.8 | 53.4 | 93 | 34 | 5.4 | 100.0 | 0.15 | 0.24 |
| – | – | – | – | – | – | – | – | – | – |
F9 – zero all data Esc – Exit
Figure 2. A sample of input data for June 15 with variable wind run and variable cloud cover. A typical wind speed for Davis is 5.4 mph.
Last updated on July 11, 2000