Save $14 an acre in furrows
A demonstration project in Nebraska proves that you can save as much as 3 acre-inches of water in furrow-irrigated fields. On average, the water savings is 1.4 acre-inches using the cutoff ratio to determine when rows are fully watered. That 1.4 inches saved can cut watering costs by $14 per acre (at pumping cost of $10 per acre-inch).
The first irrigation is typically the most inefficient, explains Chuck Burr, University of Nebraska. “Often, roots have only penetrated 18 to 24 inches when the field is irrigated the first time. Crop water use is at a minimum, so there is little depletion of water in the soil at deeper depths.”
Although the soil surface is likely to be dry, the amount of water needed to refill the soil profile is usually 2 to 3 inches. However, the amount of water applied during the first irrigation is usually the greatest of all irrigation events, Burr adds. “This is due to the rough soil surface, clods, and residue in the furrow that slow water advance and increase infiltration,” he explains.
After the first or second irrigation, the furrow surface is smooth, and water advances at a much faster rate. Thus, the greatest savings from fine-tuning furrow irrigation sets occurs with the first or second irrigation.
One challenge facing most furrow irrigators is how to balance the amount of runoff leaving the field with the amount of water that percolates below the active root zone. A management tool to improve efficiency is employing the target cutoff ratio. The cutoff ratio is the ratio of the time required for water to advance to the end of the furrow divided by total set time.
The target cutoff ratios for a number of irrigation systems and soil texture combinations include the following:
• Without a reuse pit
Sandy soils: 0.5 inch
Loamy soils: 0.7 inch
Clay soils: 0.9 inch
• With a reuse pit
Sandy soils: 0.2 inch
Loamy soils: 0.4 inch
Clay soils: 0.5 inch
• Blocked row ends
Sandy soils: 0.7 inch
Loamy soils: 0.85 inch
Clay soils: 0.95 inch
By examining target cutoff ratios for coarse-texture soils, you can see that smaller ratios are recommended, Burr explains.
“Smaller ratios indicate a faster advance time so that the top end of the field would have a similar infiltration time as the lower end of the field. Shorter advance times would limit the deep percolation inherent with coarse-texture soils,” he says.
Conversely, with finer-texture soil, a greater advance time would encourage less runoff, since advance time and set time are similar. The reason that systems with reuse systems have such a low cutoff ratio (faster advance time) is that when the water runs out of the field, it is collected and used to irrigate the same field or another field.
Pumping water from a reuse pit to another field is less expensive than pumping water from the ground. By having fast advance times, deep percolation will be virtually eliminated and water that leaves the field in the form of runoff will be used again, at a reduced cost.
To use the cutoff ratio effectively, you need to calculate an actual cutoff ratio. Take observations in the field and keep track of the amount of time it takes for half of the rows to reach the end of the field. This is the advance time. Divide this number by the total set time (typically 12 or 24 hours). If the observed cutoff ratio is more than the recommended cutoff ratios at the left, open fewer gates on the next set. This will cause more water to enter each furrow and likely increase advance time.