In 2010, we completed a three year study evaluating N fertilization of center pivot irrigated rice at the Delta Research Center in Portageville, Missouri. Rice farmers have asked what amount and frequency of irrigation water is needed to optimize rice yields under center pivot. Now, our focus is shifting from nitrogen to studying water management under different soil and climate conditions. Field experiments are being conducted in a Missouri and South Africa. Treatments include 2 day and 3 day irrigation intervals with 80%, 100%, and 120% of calculated evapotranspiration (ETc) from weather station data. Figure 1 shows the test layout in a pivot circle of a 70 acre field in South Africa. The north one-half is being used for the irrigation test using a blend of Nerica 4, 11, and 14 rice seed imported from Benin, Africa.
Figure 1. Irrigation plan for center pivot rice field in South Africa.
To apply two frequencies of irrigation, plans were entered into the irrigation system. Plan A is programmed for 2 day interval irrigation. Plan B is for 3 day irrigation intervals and Plan C is for days when both (2 and 3) day irrigation are scheduled. The plans below are show with hypothetical irrigation amounts (Figures 2 -4). The actual irrigation amounts are entered based on the previous 2 or 3 day ET calculations. After the plans are stored in the system only the last column (Rate) is modified based on weather. Treatment amount changes (80, 100, and 120% ET) are done in specific zones using a variable rate controller added to the center pivot.
Figure 2. Plan A to apply 2 day irrigation frequency treatments.
Figure 3. Plan B to apply 3 day irrigation frequency treatments.
Figure 4. Plan C to apply both irrigation frequency treatments on the same day.
Below is an example calculation sheet for a 6 day cycle beginning with rainfall or irrigation on day 0. When a major rainfall occurs after the cycle begins, we start over again at day 0. To calculate crop ET at full canopy, we multiply weather station daily ET0 by 1.1. This value was then multiplied by 1.2 because the variable rate controller is designed apply 100% or less. In other words, the system is “tricked” by entering 120% ET as 100%, with the lower rates automatically applied in the correct zones.
Figure 5. Which irrigation plans are applied at specific times in two 6 day irrigation cycles.
Figure 6. Calculation sheet for each plan using hypothetical ET values from the weather station.
We have been in a two week raining period. This is not good for evaluating irrigation, but it has given me time to set up the weather station and start finalizing the irrigation programs show above. Unfortunately, I have not been successful uplinking radio signals from the Campbell Scientific weather station in South Africa to Columbia, Missouri where it will be loaded on a page available to the public on the world wide web. All the equipment is in place and functioning except for a router port and irrigation controller problem. Hopefully, repairs will be made by service providers this week in time for the drier weather predicted by a local climatologist.