New Mexico State University
Climate Center's

Potential and Actual Crop Evapotranspiration Wizard
Using Samani's Equation

This system calculates the Potential Evapotranspiration for a given set of data for a given latitude. This page is a compilation of several databases, some global some local to NM, and is fairly involved... not complicated. So if you are new to this system please read the instructions fully. We have made every effort to assure that this page is user-friendly, however if you do have a problem or a simple question you can contact me at webmaster@weather.nmsu.edu.

Step 1: Getting daily weather data: Data can be acquired from the following places or you can enter the data by hand into the Wizard. The data needed is Date, maximum & minimum temperature, and latitude.The equation solved by the Wizard is:
ET0 = 0.0135(KT)(Ra)(TD)1/2(TC+17.8)    where TD = Tmax-Tmin (0C), and TC is the average daily temperature (oC),  Ra = extraterrestrial radiation (mm/day )   Samani and M.Pessarakli, 1986

Getting the latitude

1 The latitude for the site location must be acquired from the internet site having the maximum and minimum temperature.

2. For the NOAA stations latitude can be acquired from this document,
Note: Value is given as a 4 digit number, i.e. 3241, type this in the box below with a colon ':' between the first two and last two digits,
i.e. 32:41.

3. Now you have all the information that this equation requires! .

Step 2: Filling out the PET Wizard


Column 1 starts on this side. For dates with the format "DD/DD/DD" it is considered one column, all others separated
by white space, commas, or other are individual.
1.What is the Format
of the Date?
MM/DD/YYYY
YYYY/MM/DD
DD/MM/YYYY
Other Date *
Specify columns
Month Day Year

*Year is not needed.
Note: use for dates
that are separated by
delineator.

 2.Column Number?
(Starting with one)
Date *
Max Temp
Min Temp


* If date is not one column
please choose "other" as
the format of the date and
fill in the columns that each
month and day appear
in.

 3. What is the Latitude this data
was collected at?
Please use DEG:MIN format i.e.34:12

4. The Data Above are in?
English
Metric

 5. How is Data Separated?
White space (including tabs)
Comma
Other:

6. Calibration Coefficient
(Default for NM)
kt =
Notes on this element

Table of Cities
7. Crop Specific Evapotransperation Option
Click box to generate Crop Coefficient and ET
Preset Defaults:

(Temp Unit is automatically selected by temperature type
selected above.)
Ratio
Intercept
C1
C2
C3
Heat Units Maximum:
Heat Units Minimum:
Heat Units Base:
8. Base Offset for GDD:

Use this to add previously calculated
GDD's into current time-span.		 Notes:
Samani's EQ uses monthly intervals
from 1 to 12. However we have adjusted
this to better reflect a smooth curve.

Works best with Internet Explorer

Older Versions of Netscape wont work,
please use the most current version.



Program Info:

Status		 
|  DATE  |  MAX TMP | MIN TMP |  GDD  |PET in/ |Crp cf |ET in/ |Cum ET|
                                           mm              mm   in/mm


Notes on kt coefficient:

Reference: Z. A. Samani and M.Pessarakli, 1986: Estimating Potential Crop Evapotranspiration with Minimum Data in Arizona,
Transactions of the ASAE Vol. 29, No. 2, pp. 522-524

Figure 1 give a comparison between Samani method of calculating Pet and Penman's method for Las Cruces NM

If you have any questions please contact webmaster@weather.nmsu.edu
Copyright © 1996 New Mexico State University
Last Updated: Nov 18 1999		 Department of Agronomy and Horticulture
Box 30001 / Dept.3Q / Las Cruces, N M 88003-8003
Telephone: (505)646-3405
FAX: (505)646-6041