By Steve Melvin and Derrel Martin

Deciding when to irrigate and how much water to apply are ongoing challenges for irrigators. Soil water data can assist in applying enough water to produce top yields, while minimizing water use and pumping costs.

This is accomplished by providing information on the amount of water stored in the crop root zone and how the water is distributed in the soil profile. These data help determine how long crops can go before irrigating and the depth of water the root zone can hold at the time of irrigation.

Starting in early August, soil water data also can help decide how much water from rain and irrigation will be needed until the crop matures, helping producers leave room in the soil to store the precipitation received over the winter — similar to the information provided by a fuel gauge.

Soil water monitoring equipment can generate ample management data. However, without effective strategies to analyze the information, it has limited value. Scheduling decisions require data analysis and interpretation.

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

Each of the four techniques has its own advantages and disadvantages. They can be used as stand-alone methods or in combination with the other methods.

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, included in EC3036, make it much easier to analyze soil water data and quickly make accurate scheduling decisions.

Using soil water trends

Soil water trends can help assess irrigation needs by evaluating the change in soil water status over time to determine if 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.

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 driving between the lines. 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. However, this method doesn't give detailed information about the amount of water in the soil, how much water the root zone will hold, or how soon irrigation should be applied.

The method relies on determining a range of soil water contents that provides adequate water for the crop and minimizes deep percolation losses. The goal is to maintain the soil water within an acceptable range over time. If the water content stays within that range, irrigation is adequate.

If the soil dries out of the zone over time, irrigation is insufficient to keep up with crop needs. If soil water increases above the target range, excess irrigation is being applied. The key is determining the range of water contents that is acceptable. Soil water sensor conversion charts are designed to provide guidance in setting the ranges.

Maintaining a depleted layer

The second trend-based method involves depleting water in the 12- to 24-inch layer until the soil reaches about 70% plant available water during the vegetative crop stage. Keep in mind water infiltrates through the soil surface when irrigation is applied with a center pivot or rain occurs. Therefore, the top foot of the soil profile will become wet after rain or irrigation and may stay wet during extended periods of the summer, providing enough water for the crop.

The second foot of the soil profile only gains water if the top foot is fairly 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, drainage from the top foot can be stored in the 12- to 24-inch layer.

So, irrigators can be confident that deep percolation is minimized. The method simplifies data interpretation because irrigation is managed to maintain the 12- to 24-inch layer in the desired water zone.

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 Aug. 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 with sandy soils — may need to keep the entire soil profile very close to field capacity during the vegetative stages to help meet crop water demands during the reproductive stages.

Using soil water volume

More involved scheduling methods use 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 determination of the amount of water stored in the active crop root zone.

The water content then can be transformed into the percent of available water or the amount of soil water. The volume of soil water helps determine how long it will be 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 or irrigation will be needed to satisfy water requirements until crop maturity.

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 copy can be obtained at your local Nebraska Extension office or at go.unl.edu/ec3036.

Melvin is a Nebraska Extension educator, and Martin is a biological systems engineering professor at the University of Nebraska-Lincoln.