water
you irrigate with.New Mexico State University
Climate Center's
Potential and Actual Crop Evapotranspiration Wizard
Using Samani's Equation Java Version
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 precipitation (optional).
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! Click here to finish.
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
Now select the crop you wish to run the simulation with.
| Samani's (PET) calculator formatted spreadsheets |
| Alfalfa |
| Salt Cedar |
| Soybeans |
| Barley |
| Chile |
| Corn |
| Cotton |
| Douglas Fir |
| Lettuce |
| Mesquite |
| Onions |
| Potatoes |
| Pumpkins |
3. In the open spreadsheet, adjust the soil values to
match your soil profile and the default amount of water
you irrigate with.
| max root zone depth (inches) | 48 | Enter the maximum root depth that the crop will grow in inches |
| Root growth coefficient inches/degree | 0.01 | This should be adjusted so that the maximum root depth is reached when the the crop coefficient reaches the maximum value of .01 or greater |
| irrigation amount (inches) | 3.6 | Amount of irrigation water that you specify that will be applied at each irrigation automatically by the program . In the spreadsheet under column U this amount will be placed on the day the crop should be irrigated. If the amount or date is different , then enter the correct amount in column AB and the program will not irrigate until a new irrigation is needed. |
| soil water holding capacity inches/ft | 1.5 | This is dependent on the soil type . See the soil table for value for each soil type |
| Management allowed depletion% | 50 | This is the amount of water to be taken out of the soil profile before irrigation. It is crop dependent and should be set at 50% if no information is available. See the table by Sanders for suggested Management Allowed Depletion (MAD) for your crop. MAD=100%-ASM (Available Soil Moisture) |
| Beginning root depth | 6 | This is the soil depth that contains water for evapotranspiration assumed to be at field capacity at planting. The soil profile down to the maximum rooting depth is assumed to be at field capacity. If this value is set to zero then if root grows fast enough then irrigation will not occur because the plants are getting enough water from the soil moisture in the lower depth. However, if the root growth function does not allow the root for shallow rooted plants to grow fast enough then irrigation will have to occur every couple of days. To stop this from occurring will cause some soil moisture stress but save a lot of water. This is accomplished by setting the beginning root depth at some value 6- 12 inches. |
Highlight the first cell that contains the date from the sample data, then press Ctrl+V or do Edit -> Paste to put the desired weather data into the spreadsheet.
Next, Click on Data on the toolbar on top then select Text to Columns. In the following menu, make sure Fixed Width is selected, click Next, click Next again, and finally click Finish. It will ask if you would like to replace the contents of the destination cells, click Yes. Now, with all of the cells that are highlighted on the left represent the data that you just pasted in. Anything below these highlighted cells is sample data and should be deleted so that it doesn't accidentally interfere with the simulation.
To use the spreadsheet to schedule irrigation , set
the MAD so no stress occurs and then irrigate on of before the date that the
Model irrigates. Put the amount you irrigated into the spreadsheet (Column AB)
and look for the next irrigation date computed by the model (Column U).
The spreadsheet has been broken into three parts due to it's length.
Max temp= maximum temperature in F:
Min temp= minimum temperature in F:
Max Rh= maximum relative humidity:
Min Rh= minimum relative humidity:
Soil Temp= soil temperature at 4 inches
Solar Rad= solar radiation in ly/day
Max Wind and Ave Wind are wind speed in mph
Etr: This is the modified Penman reference evapotranspiration (ET) based on climate data.
Rain inches= rainfall in inches must be changed for current location
Gdd Corn Gdd Cotton Gdd Generic are growing degree days for corn cotton and the generic is the crop selected , in this case chile. The gdd for corn and cotton are to give a range of reasonable Gdd values. If the generic value is 3 times these values, something is wrong with the input data.
Root
depth inches is the depth the roots have reached.
Kc
ET = estimated crop water use for the day under the current soil water
stress conditions.
Water in root zone – note: it is assumed that soil water is at field
capacity at the beginning of season.
Predicted irrigation in inches is the irrigation applied by the model when the selected MAD is reached. If you have irrigated in column AB correctly then the model will not irrigate. If you do not want the model to irrigate set the MAD at 95 and the crop will die before the model irrigates
Drainage = water in
excess of that required to bring root zone moisture to field capacity. Only
calculated for the irrigations applied for the model
Column (W,) is the drainage from applied irrigation by user
Column (y) is the soil moisture stress variable. Et/ Etmax where Etmax is non stressed Et. The variables goes from 1 to 0. At a value of .75 the crop is transpiring only at 75% of potential and crop yields are being lost.
Column Z and AA are the daily non stressed potential Et and Accumulated Stressed Et
Column AC is for notes.
The Graphs at the bottom of the workbook show the water budget and yield calculations.
Figure 1 gives 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: May 29 2003 |
Department of Agronomy and Horticulture Box 30001 / Dept.3Q / Las Cruces, N M 88003-8003 Telephone: (505)646-3405 FAX: (505)646-6041 |