NWSETO: User's Guide and Program Documentation
ET003
Quick Answer back
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