Scheduling irrigation for cotton

Title : Cotton Irrigation

Objective: To understand the components of the water balance equation :

Et= I+R-D+delt SM

Et= evapotranspiration
R= Rainfall
D= drainage
delta SM = change in soil moisture


1. Down load the climate data for 1997. Use the climate data from Leyendecker science center.  Retrieve the climate data and copy and past it into the irrigation scheduling excel spreadsheet. If you use Explore as the browser, you need to past the climate data into Notepad  and then copy from Notepad to Excel. Also, make sure that you have select Cotton as the crop. The Penman spreadsheet uses the calculated GDD from the weather site and the correct Gdd's are calculated based on cutoff temperatures for each crop.  Use the data then text to column option in excel to convert the climate data from text to column. Plant Cotton on May 1 1997 at the Leyendecker farm  on a sandy loam soil.  Change the water holding capacity of the soil in 8 H of the spreadsheet. Get harvest data from Depart of Agriculture to determine what sort of yield you should be getting if you are farming and not using irrigation scheduling. .

2.  Schedule irrigation's using spreadsheet  and Penman's equation  (penman instructions) for five levels of  Management Allowed Depletion (MAD). Click on the html page of the spreadsheet for instructions. Select a MAD of 50, 60, 70,80,90 % by changing the value in cell H10. The spreadsheet that uses Penman's equation is different than the spreadsheet that uses Samani's equation. Do not mix them up. You will see that the climate data has 15 columns that match with the title of the 15 columns in the Penman's spreadsheet.  Do not select the crop coefficient option when retrieving the climate data because the spreadsheet does this calculation and you will past more than 15 columns into the spreadsheet which will cause it to fail.

3. Schedule irrigation's using a spreadsheet  for Samani"s equation and Samani's calculate PET using the pet -et calculator. for  the same five levels of  Management Allowed Depletion (MAD). Past the climate data into the pet-et calculator, select the crop option for cotton and past the results into the spreadsheet for Samani's equation. The latitude for the Leyendecker science center is acquired from the description of the site and is   Latitude: 32 12' 4.44"N.  You can get the Latitude also from the web .The kt should be set for 0.185 . The pet-calculator returns

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

If you do not select a crop then the crop coefficient , et and cum et will not be calculated but if you look in the spreadsheet , you will see that the spreadsheet needs these values because their are no formulas in these column. You have to past all 8 columns into the spreadsheet for it to work.

A Samani calculator that puts the data in a penman spreedsheet format can be used.


4. Print out the results for all the run using Penman's and Samani's spreadsheet   and turn in along with a report showing the relationship between yield and Evapotranspiration, yield and selected MAD, and Yield and irrigation efficiency. Compare Samani's and Penman's results  Plot results on  an excel spreadsheet for the report  and past plots into  the word document.


see Penman and Samani for results.