Study Reviews Farmer Irrigation Scheduling Tendencies in Dry Years

Crop sensor
Figure 1. Early installation of sensors helps minimize root and leaf damage and makes it easy to get around the field with the pickup or ATV to install the equipment.

Study Reviews Farmer Irrigation Scheduling Tendencies in Dry Years

A review of soil water data logs from farmers in the Upper Big Blue Natural Resources District indicates irrigators tend to overwater more in wetter years than dry ones. On very dry years like 2022, many farmers apply about the right amount of water. However, some still over irrigate, and some tend to under-irrigate.

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 reasons, why make changes, right? However, irrigation scheduling should be an exception to this approach because the rainfall amounts and their timings 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 make slight adjustments if the weather is dry or wet.

To illustrate this, let’s look at some data from the Upper Big Blue NRD. The 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 that measure soil matric potential; thus, this study has largely focused on Watermark data for easy comparison. The soils in the NRD are mostly 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.

To protect privacy, the NRD removes names from the information prior to group analysis; therefore, the location for each field is an unspecified area in the Upper Big Blue NRD.

The Analysis

The analysis focused on dividing the fields into one of six categories based on the soil water levels in the heart of the season (July and August) and on Sept. 15. The six categories were developed based on the readings from the Watermark sensors. The sensors generate data reported in centibars and have a range from 0 (wet) to 240 (dryer) centibars (cb). Centibar is a unit of measure that refers to the force required by the plant to pull the water out of the soil and into the plant. Typically, installations use a set of three sensors, with one sensor installed at six to 12 inches, one at 18-24 inches, and one at 30-36 inches. The categories are as follows:

  1. Good — At least one sensor out of the three depths drier than 70 cb early and one sensor drier than 70 cb on 9/15.
  2. Fair — At least one sensor drier than 70 cb early and one sensor drier than 30 cb on 9/15.
  3. Wet Late — At least one sensor drier than 70 cb early but no sensor drier than 30 cb on 9/15.
  4. Wet Early — No sensor drier than 70 cb early, but one sensor drier than 30 cb on 9/15.
  5. Wet All Season — Both sensors measuring the second and third foot not drier than 70 cb all year, but one sensor between 30 to 70 cb on Sept. 15.
  6. Very Wet All Season — Both sensors measuring second and third foot not drier than 70 cb all year and no sensor drier than 30 cb on Sept. 15.

These six categories were determined because the soil water level where deep percolation losses of water and nutrients 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. 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. 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 (30cb) after every irrigation or heavy rain. However, if the second foot gets above 30 cb, it would indicate that over-irrigation is occurring. The data in the chart 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 found that all the fields in 2017, ‘18 and ‘19 were irrigated to a level preventing water stress all season, however 15% to 20% of the fields may have been underwatered late into the season just a little bit (it may or may not have affected the yield) late in the season in the dry years of 2020, ’21 and ‘22.

In general, about one-third of the irrigators in the study on the normal to wet years are doing a good job of applying the correct amount of water to minimize deep percolation of water and nutrients and save pumping costs 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! On the other hand, about 60% to 80% are doing a good job during the dry years as shown in Table 1.

Table 1. Average percent of fields by year fitting into the six categories. The dry years 2020, ‘21 and ‘22 are different than the other years.
86 Reports
50 Reports
15 Reports
39 Reports
53 Reports
60 Reports
Weighted Avg Normal to Wet Years
'17, '18, '19
Weighted Avg Dry Years
'20, '21, '22
Good 28% 40% 20% 64% 57% 70% 31% 64%
Fair 10% 18% 7% 21% 6% 8% 13% 11%
Wet Late 13% 16% 40% 0% 6% 3% 17% 3%
Wet Early 13% 12% 7% 3% 25% 15% 12% 15%
Wet All Season 22% 10% 0% 5% 4% 2% 16% 3%
Very Wet All Season 14% 4% 27% 8% 4% 2% 12% 4%

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.

Using the 40% recommendation, 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. The data shows that in 2017, 72% of fields were over-irrigated late in the season, and even in the drought year of 2022, 36% of fields were over-irrigated late in the year.

However, the data shows that in the dry years, 2020, ’21 and ‘22, 15-20% of the irrigators let their fields get dry late in the season, maybe even dry enough to have resulted in small yield losses. So, in dry years, data driven decisions could lead some irrigators to apply more water.

Table 2. Percent of fields that became wetter moving from August to Sept. 15. The dry years 2020, ’21 and ‘22 fields are much drier than the other years in the fall.
Percent of fields that became wetter moving from August to Sept. 15201720182019202020212022Weighted Avg Normal to Wet Years
'17, '18, '19
Weighted Avg Dry Years
'20, '21, '22
Soil in the 15-25 in zone 72% 58% 54% 45% 44% 36% 65% 42%
Soil in the 25-36 in zone 75% 58% 54% 21% 48% 20% 66% 30%
Soil in both zones 66% 47% 38% 21% 37% 18% 37% 19%

The data does not give any insight into why so many farmers over-irrigate late in the year, 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 moving through August and into September.

The Extremely Dry Winter/Spring 2022-23

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 fields that were fully irrigated the previous year in the entire state. A lot of years, even in the Panhandle, the two to four inches of recharge that fully irrigated fields can hold will be received from rainfall or snowfall. In addition to financial savings, leaving the soil drier will help reduce harvest delays due to mud if rain is received in the fall.

However, the winter and spring of 2022-23 has been an exception for most of Nebraska, with off-season precipitation well below normal. This has highlighted the only downside to leaving the soil dry at the end of the irrigation season: In years when we have a very dry off-season, we will need to pump some water in June to refill the profile before the high water demand days of July arrive. The data showed that some fields were at field capacity at the end of the irrigation season in 2022, but many of the silt loam soil fields would require four to five inches of water to refill the profile. The problem is, without soil water monitoring equipment, one will not know if their field is already refilled to field capacity or not. Other lighter soil texture fields will take less water to refill them.

Carefully Consider Decisions to Apply Water in May and June

Decisions to irrigate in May and June need to be considered very carefully. Long-term averages show May and June to be two of the highest rainfall months in Nebraska. The rainfall is usually more than needed to refill the soil profile for fields that were irrigated the previous year.

Additionally, we need to factor in the cost to pump the irrigation water and the danger of leaching nitrogen and sulfur below the root zone, so irrigation can get expensive very quickly. One inch of water that moves below the root zone, whether from rain or irrigation, will take at least five to 10 lbs. of nitrogen with it. This illustrates that putting nitrogen fertilizer on closer to when the plant will use it with a sidedress application or through chemigation should be considered a best management practice.

The conclusion is that many irrigators could save money, water and nitrogen by using data from irrigation scheduling systems to make data-driven decisions to help them feel confident they are getting optimal yields without putting on extra water just for insurance purposes. 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 or wet years. 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.

First Steps

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 sensors when the soil is too 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 at: How to Schedule Irrigations with Soil Water Data.

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Nebraska Extension has educators and specialist across the state that would be happy to help you develop a plan as well. Feel free to send an email to Steve Melvin if you would like to set up a time to talk.

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