During a drought, irrigation can obviously help the crop avoid being stressed, but there are limits to how much irrigation can help, said Danny Rogers, Kansas State Research and Extension irrigation engineer.

“A portion of the crop water budget, even in irrigated fields in western Kansas, is expected to be met by precipitation or stored soil water accumulated from off-season precipitation or pre-irrigation. Irrigation systems are sized with this in mind,” Rogers said.

Drawdown of the soil water content in the root zone is expected if irrigation is the only water source available for extended periods during the growing season—without being occasionally supplemented by rainfall, he said.

Many producers who irrigate are experiencing the double jeopardy of decreased rainfall and above-normal crop water use caused by high heat and dry winds. In some fields the crop may have already depleted the entire available root zone soil water reserve, he said.

What irrigation options are available in those situations to minimize crop yield loss?

“Increasing the flow rate, or irrigation capacity to a crop is generally not an option because the source of supply is a well with a fixed flow rate. The only practical but painful option is to abandon some acreage and concentrate the water on the remaining acreage,” Rogers said. 

The most likely scenario is that the soil water reserves are already depleted, so the area to try to salvage should allow an irrigation capacity of 0.32 inches per day or greater, he said.

The number of acres that can be irrigated for an irrigation capacity that will support crop growth can be calculated as follows:

Irrigated acres =          System flow rate (gallons per minute)/450 x 24 hrs/day

                                        Gross irrigation capacity x Efficiency

For example, to determine how many acres can be irrigated for a target gross irrigation capacity of 0.35 inch per day, with a system flow rate of 600 gallons per minute (GPM), and an irrigation system efficiency of 90 percent:

Irrigated acres =          600 GPM/450 x 24 hrs/day = 102 acres

                                        0.35 in/day x 0.9

Approximately 102 acres of a crop experiencing water stress might be salvaged in this scenario.

Three software tools may also be of help to decide the best course of action, he added. They include:

These tools were not designed specifically to look at the question of diverting water from one portion of the field to another, but could provide insight about different scenarios by examining output for multiple runs with different assumptions, Rogers said. However, selecting the number of acres to water in salvage could be based on estimated crop water use and irrigation capacity as described above.

Before making decisions about changing irrigation practices, Rogers suggested that growers consult with their crop insurance representative.