July 6, 2012

Irrigation management under hot, dry conditions like we're seeing now can become complicated when your water supply is limited due to well capacity or water allocations.  Careful management can help you more efficiently use the water you have to produce yield.  This year a few well-timed inches of water may make all the difference.

Estimating Well Capacity

The first thing to consider when discussing irrigation management is whether the irrigation system has sufficient capacity. While some extra capacity is good, too much capacity requires larger pipelines, pumps, and power supplies that may not make good economic sense.

To calculate the gross system capacity of your system use the following formula:

Gross system capacity, gpm/ac = System flow rate (gpm) divided by acres irrigated

For example, consider a pivot that covers 125 acres and has a well that delivers 500 gpm.

Gross system capacity = 500 gpm/125 ac = 4.0 gpm/ac

Some water never reaches the soil where it can be used by the crop so not all of the 4.0 gpm per acre is used to produce forage or grain.

To determine the minimum net system capacity required for your location and soil texture use Table 1. First, determine whether your field is in Region 1 or Region 2 of the map in Figure 1. Then find the corresponding column for your region in Table 1. Scroll down to the line that best represents the soil texture in the field. Read the minimum net system capacity required to meet crop water demands for your area. Compare the gross system capacity to the recommended minimum net system capacity for your area.

You will need to adjust these numbers for system efficiency and run time. For example if your pivot is 90% efficient, divide the minimum net capacity by 0.90. Minimum net system capacity is also affected by the down time of a pivot. If a pivot is under load control and is shut down 50% of the time, you must double the minimum net system capacity for your system. For more information check out the NebGuide G1851, Minimum Center Pivot Design Capacities in Nebraska.

Knowing your system capacity is helpful in determining how aggressive you can be in scheduling your irrigations. For example if you have a system with 6.9 gpm/ac (862 gpm and 125 acre pivot), you can be aggressive because you have more capacity than is required to meet peak ET anywhere in Nebraska. If you have a fine sand soil in western Nebraska and only have 4.6 gpm/ac (575 gpm and 125 acre pivot),you will need to keep the soil profile nearly full as you don’t have enough capacity to keep up with the 5.9 gpm/ac net capacity needed to meet requirements 9 out of 10 years. If the extreme conditions persist all summer long, the field will likely have yield loss.

Managing a Limited Capacity Well

So what do you do if you have a limited capacity well and are falling behind? Do you abandon part of the field or try to keep up the best you can? Assuming you have made it through the most critical growth stage of pollination (see Crop Watch article “How Does Hot, Dry, Windy Weather Affect Your Corn Plants Now?”), we need to look at the crop production function to help answer this question. As you look at the blue line on the graph (Figure 2), notice how steep the curve is when less water is applied and how flat the curve becomes as more and more water is applied. This is called the law of diminishing returns. The more water you apply, the less return you get per unit of water.

Figure 2. Effect of irrigation efficiency on crop response and consumptive use. Note the point of diminishing returns when more irrigation doesn't relate to increased yield at the same rate.

Based on the graph, if enough water is not available to fully irrigate the field, the next best option is to apply as much water as you can to the whole field as opposed to letting a portion of the field go to rainfed. However, in extreme situations (like in southern Kansas last year) where you do not have enough well capacity to keep the crop alive, it may be best to irrigate a portion of the field and harvest the remaining portion for silage or allow your cows to graze.

Managing with Water Allocations

Another source of limited capacity may be a water allocation. Most allocations are designed to meet crop water demands, however, some fields are serviced by canal systems that may not be able to deliver the full water allocation. Other fields may be located in an NRD that has allocations that do not quite meet crop water requirements. Do you use extra water this year, leaving the remainder for use in future years? Do you buy extra water from the water district if water is available.

The initial recommendation would be to maintain yields the best you can this year and to reduce some application late in the growing season when it has less effect on yield. Try not to borrow too much water from next year’s allocation. One way to evaluate the potential consequences of your situation is to use the Water Optimizer computer program. This will help you to look at different scenarios to make the best use of your available water supply. Water Optimizer is intended for pre-planting decision making and not for in-season management decisions.

Furrow Irrigation with Limited Water

Just a few comments on furrow irrigation with limited water. If you don’t have enough water to fully irrigate, it is best to minimize runoff and deep percolation. There is no value in water pumped that leaves the field and does not contribute to yield. You may have to reduce your set time to accomplish this. Use a soil probe to determine the soil depth where the wetting front has advanced to. If you have water moving below the active root zone, you would be better off with a shorter set time to reduce deep percolation. Other techniques like watering every other row and using surge irrigation also can increase the value of the water available.

Chuck Burr
Extension Educator, Phelps County
Bill Kranz
Extension Irrigation Specialist, Haskell Ag Lab, Northeast REC