Extreme Heat and Irrigation: Don’t Let Your Crop Get Behind
Written by Wiley Ross Greene
Cotton plants showing drought stress and wilted leaves in a dry, sandy field with limited soil moisture.
The extreme heat we are experiencing across Southeast Georgia is creating some difficult irrigation management decisions, particularly for cotton and peanuts that are at or near peak water demand. With limited soil moisture reserves in many fields, growers need to be careful not to let crops get behind, because irrigation systems may not have the capacity to catch back up.
I recently worked with Dr. Wesley Porter, UGA Extension Precision Agriculture and Irrigation Specialist, to put together some recommendations for managing irrigation during these conditions. We discuss crop water demand, depleted soil moisture profiles, irrigation frequency, system capacity, electric load-management interruptions, and the importance of using current soil moisture and weather information when making irrigation decisions.
One of the biggest points is simple: during extreme heat and peak water use, it is much easier to stay ahead of crop water demand than it is to catch back up after the crop and soil profile have fallen behind.
Extreme Heat and Irrigation – Dr. Wes Porter and Ross Greene
I have received a lot of questions about the best way to manage irrigation during extremely hot conditions. I told someone recently that science tells me one thing intuition tells me another. I think we have to find a balance in between the two. It’s difficult to have a one size fits all answer because of planting date, soil type, soil conditions, recent rainfall or lack thereof, and irrigation system capacity. I am trying to bring all of that into perspective and provide some recommendations that will at least give you something to consider.
Forecasts across much of Georgia are calling for an extended period of high temperatures, with daytime highs approaching or exceeding 100°F in some areas. In combination with these extreme conditions, there are no guaranteed chances of rain across the state especially as we move into the week of August 16th. Unfortunately, this extreme heat is arriving at a critical time for many of Georgia’s Cotton and Peanut acres. Many fields are currently at or near peak crop water demand, when adequate soil moisture is particularly important for protecting yield potential. At the same time, soil moisture reserves deeper in the profile remain limited or have been recently depleted in many areas following dry conditions earlier this season. The combination of peak crop water demand, depleted soil moisture, and extreme temperatures could make the next several days especially challenging for irrigation management.
Water Demand Will Most Likely Exceed Normal Recommendations
During periods of extreme heat, crop water usage will exceed the typical rates used in Checkbook recommendations. Under the conditions currently forecast, daily evapotranspiration (ET) and crop water use could exceed 0.30 inch per day. For comparison, a crop using 0.30 inch per day requires about 2.1 inches of water per week just to replace what is being evaporated off. If you are looking at Weather Station ET alone, this does not account for the crop coefficient. Which during this time of season is around 1.2 times ET. If an irrigation system is operating at approximately 85% application efficiency, the amount that must actually be pumped can approach 2.5 inches per week in the absence of effective rainfall. For many irrigation systems, keeping up with that level of demand will be difficult. In addition, the temperatures will decrease application efficiency, I would suggest if possible to make applications starting in the evenings through mid-morning to increase efficiency and decrease evaporative losses. Again, understanding this may not be possible in most situations.
Peak Water Use Makes This Especially Important for Cotton and Peanuts
The timing of this heat is important. Many cotton and peanut fields across Georgia are currently in reproductive growth stages with high water requirements. Peanuts experience sustained high water demand during pod development and pod fill, while cotton reaches its greatest water demand during bloom and boll development. Moisture stress during these critical periods can reduce yield potential, and water applied later cannot necessarily recover yield that has already been lost. UGA irrigation guidance emphasizes staying ahead of crop demand rather than waiting until visible drought stress develops. Once the soil profile becomes significantly depleted during peak water use, irrigation alone may not be capable of catching the crop back up.
The Soil Profile Matters
One of our biggest concerns going into this period of extreme heat is that deeper soil moisture has already been depleted in many fields. Normally, irrigation supplements rainfall and water stored within the soil profile. When that reserve is depleted, however, the irrigation system must supply current crop demand while also trying to rebuild soil moisture. That can be especially challenging in the sandy Coastal Plain soils common across South Georgia. These soils have relatively limited plant-available water storage, meaning several days of high crop water use can quickly exhaust available moisture. In addition, during the current crop growth stage most of the moisture extraction is occurring at the deeper depths, making the refill near impossible without rainfall. Growers should closely monitor soil moisture and begin irrigation before visible crop stress occurs.
Consider More Frequent Irrigation
Where the soil profile is depleted, growers should consider applications of approximately 0.5 to 0.75 inch per irrigation event, with applications occurring as frequently as the irrigation system, soil conditions and field setup will reasonably allow. Multiple irrigation events may be necessary to begin replenishing the soil profile while simultaneously meeting current crop demand. Maintaining higher soil moisture can also help moderate temperatures around the crop compared with extremely hot, dry soil. The goal is not simply to apply as much water as possible at one time. Instead, the priority is to maintain available soil moisture and prevent the crop from falling further behind. I know this doesn’t work everywhere, but like I stated earlier, night events every or every other night would be optimal in these conditions. You have to look at your crops, the maturity, the current soil moisture levels and try to make the best decision on how to allocate applications across your farm.
Account for Irrigation Downtime and Load Management
Growers should also consider how much time their irrigation systems are actually available to operate. Between competing for single water sources, large systems, potential limited water sources, and some electric cooperatives offering load-management programs that reduce electricity costs by interrupting power to irrigation systems during periods of peak electrical demand systems may not being able to run a full 24 hours per day and you will have to plan around this. During extreme summer heat, these interruptions may occur during the afternoon and can keep an irrigation system offline for several hours. Growers participating in these programs should account for those interruptions when planning irrigation cycles. A system that is already operating near its maximum capacity may have even less opportunity to meet crop water demand when several hours of pumping time are lost.
In addition, some systems may require manual restarting after a load-management interruption. Growers should know how their particular system responds when power is restored and make sure irrigation equipment resumes operation as soon as practical.
When possible, growers may need to take advantage of overnight, morning and other available pumping periods to maximize irrigation runtime and avoid falling farther behind.
Use Current Conditions, Not Just the Checkbook
Traditional UGA checkbook irrigation recommendations remain an excellent foundation for irrigation scheduling. However, checkbook values are based on historical water-use patterns and may underestimate actual demand during unusually hot weather. Weather-based ET information, soil moisture sensors and other irrigation scheduling tools can provide a better picture of current field conditions and help growers determine whether their irrigation program is keeping pace with crop demand. Any time that you can use a more advanced method for estimating irrigation management especially in these conditions it is recommended.
Key Points for the Coming Heat
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Cotton and peanuts in many fields are at or near peak water demand, making moisture stress particularly concerning for yield.
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Crop water use could exceed 0.30 inch per day during extreme heat.
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Fields with depleted soil moisture profiles are particularly vulnerable.
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Do not wait for visible crop stress before irrigating.
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Where appropriate, consider 0.5–0.75 inch applications as frequently as system capacity and field conditions allow.
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Account for irrigation system efficiency and actual pumping capacity.
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If participating in an electric load-management program, account for periods when the system may be shut down and verify that equipment restarts after power is restored.
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Monitor soil moisture and current weather/ET information whenever possible.
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Recognize that some irrigation systems simply may not have enough capacity to completely meet crop demand during prolonged extreme heat.
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There is a difference between heat stress and moisture stress, plants will experience heat stress even if there is adequate moisture in the soil, so monitor for these conditions too.
Bottom Line
The upcoming heat is arriving during a critical period for many Georgia cotton and peanut fields. With crops at or near peak water demand and soil moisture reserves already depleted in many areas, even a relatively short period of moisture stress could affect yield potential.
The priority over the next several days should be to stay ahead of crop water demand rather than trying to catch up after stress occurs. Keep irrigation systems operating as field conditions and system capacity allow, monitor soil moisture closely, and account for any downtime that reduces the number of hours available to irrigate. Once the crop and soil profile get behind during extreme heat and peak water use, it can be extremely difficult for irrigation alone to catch back up. A stable moisture environment is more optimal than pushing the crop through periods of stress or extreme stress.
If you have questions about management in conditions such and these, reach out to your local County Extension Agent.
University of Georgia is an Equal Opportunity Institution. If you need reasonable accommodation or language access services, contact the Evans or Candler County Extension office at (Evans 912-739-1292/[email protected] or Candler 912-685-2408/[email protected]), at least three weeks prior to the program date.
