During a drought, irrigation can obviously help the crop avoid being stressed, but there are limits to how much irrigation can help. 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 preirrigation. Irrigation systems are sized with this in mind. As a general rule, on soils with high water-holding capacity, such as silt loams, an irrigation capacity of about 0.25 inches/day is needed to have a reliable irrigation system. On soils with low water-holding capacity, 0.32 inches/day may be needed to achieve the same reliability.
Typical peak crop water use rates average about 0.35 inches/day, so 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. Single daily crop water use rates can approach and occasionally exceed 0.50 inches/day. During drought periods, crop water use rates would be higher than normal.
Many irrigated producers are experiencing the double jeopardy of decreased rainfall and higher-than-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. What irrigation options are available in those situations to minimize crop yield loss? Increasing the flow rate (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.
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 in/day or greater.
Irrigation capacity (IC) is determined by dividing the system flow rate in gallons per minute (GPM) by 450 (450 GPM = 1 ac-in/hr), then multiplying that rate by the number of hours of daily pumping, and then dividing by the number of irrigated acres. This is the gross irrigation capacity. Multiply the gross irrigation capacity by the system efficiency to obtain the net IC.
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 GPM/450 x 24 hrs/day
Gross IC x Efficiency
Example: To determine how many acres can be irrigated for a target IC of 0.35 in/day, with a system flow rate of 600 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 currently experiencing water stress might be able to be salvaged in this scenario.
Three software tools may also be of help to decide the best course of action. These include:
- KanSched: an ET-based irrigation scheduling tool
(see download link at http://www.ksre.ksu.edu/mil)
- Crop Yield Prediction (http://www.ksre.ksu.edu/mil/YieldEstimator.htm)
- Crop Water Allocator (http://www.ksre.ksu.edu/mil/cwa)
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 some insight about different scenarios by examining output for multiple runs with different assumptions. However, selecting the number of acres to water in salvage should be based on estimated crop water use and irrigation capacity as described above.
Before making any decisions about changing irrigation practices, please read the following article (No. 5) on crop insurance-related concerns.