LINCOLN, Neb. — A review of soil water data logs from farmers in the Upper Big Blue Natural Resources District indicates irrigators tend to overwater more on wetter years and sometimes even underwater on dryer ones. The tendency should not be a surprise because humans are creatures of habit. We get out of bed every day on the same side, we sit at the same place at the table for breakfast, etc. Farmers are no different — they tend to plant about the same number of seeds per acre each year, they apply about the same amounts of fertilizer each year, they plant the rows the same direction, etc. Well, you get the point, and without any compelling data, why make changes. However, irrigation scheduling should be an exception to this approach because rain amounts and timing are different each year.
The amount of irrigation applied last year, or the year before, may have very little to do with the amount needed this year. The long-term average of irrigation application depth and timing is relatively meaningless for the decisions producers need to make on any given day in the current growing season. However, without any additional data, all an irrigator can do is put on about the same amount of water as in the past and slightly adjust it for dry years or wet years.
The Upper Big Blue NRD requires growers in six areas that have an increased median nitrate-nitrogen level in the groundwater to use soil water monitoring equipment in one irrigated field and report the data collected annually. Most of the producers impacted by this rule use Watermark sensors; thus, this study has largely focused on Watermark data for easy comparison. The soils in the NRD are a silt loam or similar soil texture, making the field-to-field data comparable as well. The assumption was made that the farmers using Watermark sensors would represent all irrigators in the area. The NRD removes names from the information prior to group analysis to protect privacy and so the location for each field is just somewhere in the Upper Big Blue NRD.
The Watermark sensors generate data reported in centibars and have a range from 0 to 240 centibars (cb). The unit refers to the force required by the plant to pull the water out of the soil and into the plant. The soil water level where deep percolation slows to a low rate in a silt loam soil is about 70 cb; thus, it is recommended to keep at least one of the sensors in the second or third foot in this range or dryer. Field capacity in a silt loam is about 30 cb, so the goal is not to refill the soil above this level with irrigation in the second or third foot.
The top foot will go above field capacity after every irrigation or heavy rain. Yield losses would not be expected until all the sensors were approaching 140 cb, making the 70 cb level very achievable without any chance of causing yield loss. The report indicated that the field was this dry on a given day and not how many days the soil was drier than 70 cb. The 70 cb level is equal to about 70% of plant available water (PAW) and stress should not start until below 50% of PAW during the summer and 40% in late August and September. Soil types such as those with high sand or clay content would require different levels other than 30 and 70 cb.
The analysis focused on dividing the data into six categories based on the soil water levels in the heart of the season (July and August) and on Sept. 15. The description of the six categories is as follows:
- Good — One sensor drier than 70 cb early and one sensor drier than 70 cb on 9/15.
- Fair — One sensor drier than 70 cb early and one sensor drier than 30 cb on 9/15.
- Wet Late — One sensor drier than 70 cb early and no sensor drier than 30 cb on 9/15.
- Wet Early — No sensor drier than 70 cb early, but one sensor drier than 30 cb on 9/15.
- Wet All Season — Sensors 2 and 3 not drier than 70 cb all year, but one sensor drier than 30, but < 70 cb on 9/15.
- Very Wet All Season — Sensors 2 and 3 not drier than 70 cb all year and no sensor drier than 30 cb on 9/15.
The analysis found the fields in 2017, ‘18, and ‘19 was irrigated to a level preventing water stress all season, however 15-20 % of the fields may have been under watered late in the season in the year in the very dry years of 2020 and ‘21. In general, about one third of the irrigators in the study are doing a good job of applying the correct amount of water to minimize deep percolation while producing top yields. The other two thirds could have saved money and water without lowering yield. In fact, about one third could have saved a lot!
|Weighted Avg All Years||Weighted Avg 2017, 2018, 2019|
|Wet All Season||22%||10%||0%||5%||4%||12%||16%|
|Very Wet All Season||14%||4%||27%||8%||4%||9%||12%|
University of Nebraska Extension irrigation scheduling recommendations encourage irrigators to allow the crop to continue using more and more of the stored soil water starting in August and continuing into September when the crop matures. The recommendation is to lower the soil water level from the usual summer water condition of a minimum of 50% plant available water in the top three feet of soil to 40% in the top four feet after the dough stage (R4) is reached. Thus, the stored soil water content should be significantly lower when the crop matures in September than earlier in August.
The data shows many irrigators are applying more water late in the season than is needed. Some years, a significant rain can cause the soil to be wetter in September, but it is usually due to applying more irrigation water than needed.
However, the data shows that on the dry years — 2021 and 2022 — 15-20% of the irrigators let their fields get very dry late in the season, maybe even dry enough to have resulted in lower yields. So, on dry years, data driven decisions could lead some irrigators to apply more water.
|Percent of fields that became drier moving from August to Sept. 15||2017||2018||2019||2020||2021||Weighted Avg All Years||Weighted Avg 2017, 2018, 2019|
|Soil in the 15-25 in zone||28%||42%||46%||55%||56%||43%||35%|
|Soil in the 25-36 in zone||25%||42%||46%||79%||52%||46%||33%|
|Soil in both zones||34%||53%||62%||79%||63%||52%||43%|
The data does not give any insight into why about two-thirds of the fields get wetter on typical years, but it could be because the irrigation routine is set in July when the plants are transpiring at their peak, the days are long, and the temperatures are high. Then, as the daylight hours shorten and the temperatures get cooler in late summer, many keep irrigating at July levels even though crop water use for corn has gone from an average of two inches/week at silking to 1.25 inches/week at full dent. Again, as creatures of habit, without good soil water monitoring data to direct our irrigation scheduling, we just keep on doing what we have been doing. Other crops, including soybeans, have a similar dramatic drop in crop water use.
Leaving the soil as dry as possible without lowering yields saves money on pumping costs and leaves room to store offseason precipitation. Most years adequate precipitation will be received from October through May to refill the soil profile on fully irrigated fields in the entire state. Even in the Panhandle, a lot of years the 2-4 inches of recharge that fully irrigated fields can hold will be received. In addition to financial savings, leaving the soil drier will help reduce harvest delays due to mud if rain is received in the fall.
Most farmers I work with try very hard to keep the cost of production as low as possible without compromising yield. So why do some spend extra money pumping more water than needed? Every farmer has hundreds of decisions to make each day and when to start the next irrigation can become just another decision that needs to be made quickly before moving on. We have had farmers tell us that they pump a little extra water just as insurance because they are not quite sure if the field is wet enough to produce optimal yields. But think about this: if your fuel delivery guy pumped 500 gallons of diesel in your tank to fill it, then pumped an extra 100 gallons that overflowed the tank, just to make sure it was full, how happy would you be with him? At a very real level, that is what many irrigators are doing when pumping water into a soil profile that is already full, because the extra water leaches below the crop root zone, taking nitrogen with it. Making data-driven decisions will help irrigators feel confident they are getting optimal yields without putting on extra water just for insurance purposes.
The conclusion is that many irrigators could save money and water by using data from irrigation scheduling systems to make data-driven decisions. Otherwise, irrigators follow their natural tendencies to put on about the same amount of water as in the past and just slightly adjust it for dry years or wet years.
Consider spending a bit more time and money to get good data and analyze it to make better irrigation scheduling decisions. Pumping water is the largest energy bill on most irrigated farms in Nebraska. For example, a 125-acre diesel ($4.75/gal) powered pivot pumping from 200 ft could cost around $1,600 to put on one inch of water. In addition, over-irrigating can carry valuable inputs below the root zone, reducing yield and contaminating groundwater. For more information on yield losses take time to read the following NebGuide: Plant Growth and Yield as Affected by Wet Soil Conditions Due to Flooding or Over-Irrigation.
Every irrigator can make excellent irrigation scheduling decisions by getting the right information and polishing their skills in analyzing the data. Today it is easier than ever to install equipment that will automatically record and analyze the data before sending it to your computer or smartphone. Your diligence will be rewarded with higher profitability and satisfaction that you are helping protect the environment.
Many irrigators already use soil water monitoring equipment or ET data to make good data-driven decisions. The only thing they need to do is continue what is working and hone their analysis skills.
Now is the time to get the probes in the ground. While other tasks may seem more pressing, early installation of sensors is important to ensure proper operation during the later critical growth phases. Early installation helps to minimize root and leaf damage and makes it easy to get around the field with the pickup or ATV to install the equipment. Keep in mind that the plants next to the probes are an integral part of the sensor and must be protected so they can represent all the other plants in the field. Do not install the sensors when the soil is wet and make as few footprints as possible to prevent soil compaction. Irrigators wanting to purchase equipment should keep in mind that many NRDs across the state offer incentives to purchase irrigation scheduling equipment.
Support for People Wanting to Improve Their Irrigation Management
For those who have not collected data in the past or would like to hone their scheduling skills, take some time now to figure out what will work best for your operation. Many resources are available to help. A great five-part video series on this topic can be found on the CropWatch YouTube channel, How to Schedule Irrigations with Soil Water Data.
Other articles include:
- Irrigation Sensor Installation Tips
- Value of Using Sensors to Manage Irrigation and Tips for Proper Installation
- Irrigation Scheduling: Checkbook Method
- Soil Water Sensors for Irrigation Management
- How Much Irrigation is Needed on Corn in the Vegetative Growth Stage?
— Steve Melvin, Extension Educator Irrigated Cropping Systems, University of Nebraska-Lincoln