By: Vasudha Sharma, Extension irrigation specialist
Deciding when to irrigate to optimize production and reduce environmental pollution is a daily judgment call that requires you to consider several factors. Many of these factors change as the crop develops. Below are some general guidelines to consider when developing a water management plan and setting allowable soil water deficit limits.
Early season strategies
In the spring, always make sure the soil in the germination and early-growth root zone is moist when planting. If necessary, irrigate to wet this zone. Typically in MN, irrigation at planting is not required. As the plant grows, moist soil is necessary for proper root development, as roots will not grow through a dry layer of soil. A dry layer will result in a shallower rooting depth than desirable. For corn, experience shows that the soil water deficit can be as high as 60 to 65 percent in the early vegetative growth stage (germinating to 10th leaf) without affecting plant development. Root zone at this time may only be half to two-thirds of the crop's potential. Holding back on irrigating during the early vegetative growth stages promotes deeper root growth, increases the opportunity to store rainfall when it occurs and decreases the risk for leaching valuable nutrients.Mid-season strategies
As the crop nears its critical growth period or its usual peak water-use period, reduce the selected allowable soil water deficit to minimize the risks of not meeting the crop's water needs and causing yield losses. For most crops, this may mean changing soil water deficit limit to a 30 to 40 percent before entering the critical growth stage - such as the 10th to 12th leaf stage for corn, or early flower for soybeans. During these critical periods of high water use, regularly project the next two to three days of water needs to plan ahead and avoid stressing any part of the field before it’s irrigated. For example, when using a center pivot, which takes three days to cover the field, project what the water deficit will be after three days and use that to determine when to start irrigating. To reduce the leaching potential of a rainfall event, always consider the weather forecast when scheduling the next irrigation.Other strategies to consider
Increase allowable soil water deficit
Another possible irrigation water management plan is to set the allowable soil water deficit equal to, or slightly greater than, the irrigation system’s normal net application amount. For example, if the typical application is 0.75 inches net, then choose a planning deficit limit of 0.75 to 1 inch. If this is greater than 50 percent of the available water capacity in the root zone, make the amount smaller, especially during the critical stages of crop growth, to reduce the risk of moisture stress. This strategy will require more irrigation applications than the variable deficit strategy described earlier.Consider crop water use
Crop water use is the amount of water given up to the atmosphere by a crop due to evaporation from the soil surface and transpiration through the plant leaves. Crop water use is also called evapotranspiration (ET). Daily ET changes throughout the growing season due to weather variation and crop development. The ET depends on many factors including crop type, growth stage and climatic conditions. Parameters that have a major effect on a crop's daily water use include maximum and minimum temperatures, solar radiation, humidity and wind, and soil moisture.Various tools to estimate ET are:
- NDAWN Crop water use (Minnesota and North Dakota)
- Daily ET estimates (for SE Minnesota and Wisconsin)
- Crop water use (ET) tables in the MN-ND Checkbook irrigation scheduling spreadsheet
- Irrigation management assistant (IMA) is automated irrigation software that works on the principles of water balance irrigation scheduling method. At this time, the tool is available for Benton, Otter Tail, Becker, Hubbard, Wadena, Todd, Morrison, Sherburne, Dakota and Pope Counties of Minnesota.
Consider pumping capacity
A system’s pumping capacity refers to the ability of the irrigation system to refill the soil profile with water. Knowing this capacity enables you to better judge when to start an irrigation so that it may be completed before any part of the field exceeds the allowable soil water deficit.Pumping capacity can be expressed in terms of either:
- Pumping rate in gallons per minute (gpm) divided by the number of acres irrigated (gpm per acre). For example, the pumping capacity of a traveling gun covering 100 acres and pumping 500 gpm is 500 divided by 100, or 5 gpm per acre.
- Average daily application amount (inch per day).
You can determine the average application amount based on a 24-hour pumping day from the table below, which shows various pumping capacities and application efficiencies.
|
Pumping capacity |
65% efficiency |
75% efficiency |
85% efficiency |
|
net in./day |
|||
|
4 gallons per minute (gpm) per acre |
0.14 |
0.16 |
0.18 |
|
5 gpm per acre |
0.17 |
0.20 |
0.23 |
|
6 gpm per acre |
0.21 |
0.24 |
0.27 |
|
7 gpm per acre |
0.24 |
0.28 |
0.32 |
|
8 gpm per acre |
0.28 |
0.32 |
0.36 |
|
9 gpm per acre |
0.31 |
0.36 |
0.41 |
Limited or under designed pumping capacities
For irrigation systems with limited or under designed pumping capacities for a specific crop and soil type, there are limited water management strategy alternatives for reducing the risk of moisture stress. For example, research on irrigated corn in west central Minnesota has shown that producers should set the allowable deficit to no more than .75 inches to reduce the risk of stress with an under designed system (Bergsrud et al., 1982). This deficit should start in mid-vegetative stage (about 10th leaf) and continue until late dent.For detailed information on irrigation management strategies and irrigation scheduling methods, visit UMN Irrigation Extension Page.
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