A simple spreadsheet for scheduling irrigation can be downloaded by clicking here.
(Spreadsheet updated Mar 16, 2016)
The methodology used in the spreadsheet is presented in Crop evapotranspiration – Guidelines for computing crop water requirements – FAO Irrigation and drainage paper 56, Chapter 8
To use the spreadsheet, you need to enter input wherever the cells are yellow. Here are some basic instructions:
Setting up the Spreadsheet
1. In the first Worksheet Tab (Soil Moisture Worksheet), enter the date of the first day of the crop season.
2. Enter soil field capacity and wilting point in percent. If you know the soil texture you can get the field capacity and wilting point online by clicking here.
3. Enter rooting depth for each day of the season. Maximum rooting depths for various crops are given in FAO 56 Table 22.
4. Management Allowed Deficit (MAD) is given in FAO 56 Table 22, however, the FAO refers to it as the Depletion Fraction (P). P and MAD are the same thing.
5. Enter soil volumetric moisture content as a percent on the first day of the season.
6. Enter the evapotranspiration (ETc) each day. Many meteorologic stations will provide estimates of daily reference evapotranspiration (ETo). You will need to multiply the ETo by the appropriate crop coefficient (Kc) for your crop to obtain the actual evapotranspiration (ETc).
7. Enter the rainfall or irrigation each day.
8. In the next Worksheet Tab (Application Rate), enter the date of the first day of the crop season.
9. Enter the area of the field in acres.
10. Enter the percent wetted area. If drip irrigation is used, only a portion of the field may become wet.
11. Enter the percent efficiency of the irrigation system.
12. Enter the pump flow rate in gallons per minute.
Using the Spreadsheet to Schedule Irrigation
The objective of the analysis is to minimize crop stress as indicated by the crop stress factor (Ks). If this parameter is less than 1, then the crop is in a state of water stress. To simplify the analysis the spreadsheet provides the column “Did Stress Occur?” When stress occurs, as indicated by “Yes”, then you should irrigate your crop. To see how much you should irrigate, see the column called “Irrigation Needed”. Enter the amount you want to irrigate in the column called “Applied Irrigation or Rainfall”. Finally, go to the Application Rate Worksheet Tab to see how many hours you should run your irrigation system to put on the desired amount of water.
Soil Moisture Graph Worksheet Tab. Any time that this graph falls below the green line (Threshold Moisture Content) the crop is in stress. Throughout the crop season the graph should never fall below this line if possible. It is also very important that the soil moisture not exceed the field capacity because this water will be wasted, either through deep percolation or surface runoff.
ETcadj Worksheet Tab. The evapotranspiration is shown in this worksheet. ETcadj is the crop ET multiplied by the crop stress factor, and represents the real ET.
Crop Stress Factor Worksheet Tab. This graphs shows the crop stress factor as a function of time throughout the crop season. This line should be as close as possible to 1 throughout the season. Any value lower than 1 indicates that stress occurred. NOTE ALSO!! In this worksheet a value of the relative seasonal crop yield is provided. The yield response factor (Ky) required to calculate the relative yield can be obtained from Table 24 of the FAO 56 document for various crops.
Cumulative ET vs Irrigation Worksheet Tab. The purpose of this graph is to show how close your applied irrigation was to the ET. Throughout the season and at the end of the season, the value of the cumulative irrigation should be as close as possible to the cumulative ET. (Note in this graph that the ET used is the ETc and not the ETcadj).
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