By Steve Melvin, Extension Educator Irrigated Cropping Systems

Irrigation expenses are usually the biggest energy cost on the farm. On dry years, they become even higher. So how does one know when the system must be started and when it can be turned off? How should the low commodity prices this year affect these decisions?

Many producers fall into the mindset that these decisions just need to be made quickly and move on with the day. However, I often wonder if a person had to go to the bank, take out eight or ten $100 bills to feed into each pivot before it would start, would irrigators want more information about how much water is left in the soil and want to hone their analyzing skills?

Irrigation is unlike the decisions around many other crop inputs that are only made once. Crops need about the same amount of water each year; the big difference is that one never knows ahead of time how much rain will be received and when it will come. Adding to the challenge is the fact that each field will receive a different amount of rain.

Soil water monitoring equipment can generate ample management data, however, without effective strategies to analyze the information, it is of limited value. Scheduling decisions are really spending decisions that affect thousands of dollars each year, thus must be made with finely honed data analysis and interpretation skills to get a good return on the investment. The data is similar to the information the fuel gauge on the pickup provides. (Figure 1)

The irrigation scheduling strategies and procedures that are briefly described in this article are taken from a new Extension publication EC3036, Irrigation Scheduling Strategies When Using Soil Water Data.

A new video series also describes the strategies in more detail and can be found on the CropWatch YouTube channel: How to Schedule Irrigations with Soil Water Data. They are designed to help irrigators quickly and accurately plan their irrigation timing and application amounts. Each of the four techniques has its own advantages and disadvantages. They can be used as standalone methods or in combination with the other methods.

Many irrigators use all four techniques sometime during the year depending on how accurate they need to be during a particular crop growth stage and the amount of time available to make the decision. The publication uses Watermark sensor data as the example, but the techniques can be used with data from any commercially available soil water monitoring system.

In addition, Soil Water Sensor Conversion Charts have been developed to assist in interpreting data from devices that measure soil water tension, such as Watermark® sensors. The charts are included in EC3036. They make it much easier to analyze the soil water data and quickly make accurate scheduling decisions.

Soil water trends can help assess irrigation needs by evaluating the change in soil water status over time and soil depth to determine whether irrigation is keeping up with, or exceeding, crop water use. Data generated with a data logger taking readings two or more times per day are ideal for analyzing soil water trends. However, even manual readings taken weekly can help assess the soil water status.

Methods based on soil water trends require less calculation than techniques using the water content of the soil. Two scheduling techniques that use soil water trends include Management Zones and Maintaining a Depleted Layer.

Management Zones

Irrigation scheduling using management zones (soil is too wet, too dry, or about right) is the simplest method and is designed to enable quick decisions— much like just driving your truck between the lines on the highway. (Figure 2)

Almost all commercial companies will display their data in this format. It is especially useful when time is not available to analyze data more thoroughly. The downside of the method is it does not give detailed information about the amount of water in the soil, how much water the root zone will hold without losing water to deep percolation, or how soon irrigation should be applied.

The key to making the method work relies on determining a range of soil water contents that provides adequate water for the crop and yet minimizes deep percolation losses. The goal is to maintain the soil water within an acceptable range over the summer. As long as the water content stays within that range, then irrigation is adequate.

If the soil dries out of the zone, then irrigation needs to increase to keep up with crop needs. If the soil water increases above the target range, then irrigation should be stopped for a few days. The big challenge is determining what the “about right” water level should be. This is the biggest problem I see with the commercial soil water monitoring systems used on many farms today. Sometimes the “about right” zone get set at a very wet level.

The irrigator should not just except the default range, but continuously evaluate the level of the zone and change as needed. The previously mentioned Soil Water Sensor Conversion Charts are designed to provide guidance in setting the ranges. A video with a more complete description of the technique can be found at: Part 3 How to Schedule Irrigations with Soil Water Data: Irrigation Scheduling

Maintaining a Depleted Layer

The second trend-based method that works well on soils with uniform texture in the top 4 feet, involves depleting water in the 12- to 24-inch layer until the soil becomes dryer than the top 12 inches would be after a rain or irrigation. It is my preferred go-to method for irrigation scheduling because once a person gets a little experience using it, I find it to be a very quick and accurate scheduling technique.

During the vegetative crop stage is the easiest time to set the field up, but it can also be done during the reproductive stage as a slow gradual process. Corn and soybeans will have roots developed to 24 inches by the 12 leaf and early bloom stages respectively and 36 inches by silking and pod elongation.

An important concept to keep in mind is that water infiltrates into the soil at the surface when irrigation is applied with a center pivot or it rains. Therefore, the top layer of the soil will become very wet and may stay wet during extended periods of the summer assuming sufficient irrigation water is applied for the crop. The second foot of the soil will only gain water if the top foot is very wet.

Deep percolation essentially ceases when the soil water content in the second foot is lowered. If rains occur when the upper soil layer is wet, then drainage from the top foot can be stored in the 12- to 24-inch layer; thus, irrigators can be confident that deep percolation is minimized. The method simplifies data interpretation because irrigation is managed to keep the 12- to 24-inch layer in the desired dryer water level (Figure 2). The method can be done using any commercial system that show soil water trends for different soil depth over the summer.

Once the depleted zone is established during the vegetative growth stage and the crop reaches the reproductive stage, the field can be fully watered with very little deep percolation. If the 12- to 24-inch layer continues to get drier, keep the irrigation system running. If it starts to get wetter, stop irrigating for a few days. Ideally, after August 1 the depleted layer should slowly expand deeper with the crop using most of the subsoil water by the time it matures.

One word of caution: fields with lower capacity irrigation systems—especially in combination with sandy soils—may need to keep the entire soil profile very close to field capacity during the vegetative stages to help insure adequate water to meet crop water demands during the reproductive stages. A video with a more complete description of the technique is at: Part 4 How to Schedule Irrigations with Soil Water Data: Advanced Irrigation Scheduling.

Using Soil Water Volume

More involved scheduling methods utilize soil water data to estimate the amount of water (expressed as volume or depth) in the crop root zone. Knowing the amount of water in the root zone assists in applying enough water to produce top yields while minimizing pumping.

These scheduling methods require determining the amount of water stored in the active crop root zone. The water content can then be transformed into the percent of plant available water or the amount of soil water.

The volume of soil water helps determine how long until the crop needs to be irrigated and how much water the soil can hold when irrigating. Starting in early August, soil water data can help determine how much rain and/or irrigation will be needed to meet water requirements through the end of the summer and crop maturity, with the goal of utilizing most of the available soil water before the crop matures.

For a more complete description and examples of how to determine each of the scheduling methods, review EC3036, Irrigation Scheduling Strategies When Using Soil Water Data. A hard copy can be obtained by going to your local Nebraska Extension office. The videos can be viewed by going to go.unl.edu/IrrigationSchedulingVideos.