Using Cutoff Ratio to Fine-Tune Furrow Irrigations (Save $14/acre)

March 17, 2017

C. Dean Yonts - Former Extension Irrigation Engineer

Article reviewed and updated by Chuck Burr, Nebraska Extension Educator

Added profit: $14/acre

A demonstration project in Clay County indicated a water savings ranging from 0.1 to 3.2 acre-inches with an average of 1.4 acre-inches when using the recommended cutoff ratio. At a water pumping cost of $10 per acre-inch, this would result in a savings of $14 per acre.

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Gravity irrigation, by nature, is somewhat inefficient. Runoff and deep percolation are realities that furrow irrigators must manage to keep irrigation application costs at a minimum. Application efficiency for furrow irrigation depends on soil type, slope, and texture, but is often in the range of 50%.

The first irrigation of the season is typically the most inefficient. Often the crop roots have only penetrated 18 to 24 inches when the field is irrigated for 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. This is due to the rough soil surface, clods, and residue in the furrow that slow water advance and increase infiltration.

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. One management tool to improve efficiency is 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.

Cutoff Ratio = Average Advance Time/Set Time

Research has been conducted to determine the "best" cutoff ratio for various types of furrow irrigation systems and soil types. Choosing the appropriate cutoff ratio depends on soil factors and irrigation system configuration. Table 1 lists the target cutoff ratios for a number of irrigation system/soil texture combinations.

By examining target cutoff ratios for coarse texture soils you can see that smaller ratios are recommended. 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 textured soils. 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 under 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. In this situation, efficiency is generally improved.

Table 1. Target cutoff ratio based on soil and system considerations.
	Sandy Soils	Loamy Soils	Clayey Soils
Without Reuse	0.50	0.70	0.90
With Reuse	0.20	0.40	0.50
Blocked Ends	0.70	0.85	0.95

The one telltale sign that indicates many furrow irrigators do not use the Cutoff Ratio is when flags mark furrow numbers at the end of the field. If irrigators used the Cutoff Ratio, even on clayey soils without reuse, water should advance to the end of the field in under 11 hours, on a 12-hour set time (12 hours x 0.90 cutoff ratio = 10.8 hours). If water advanced in less than 11 hours, there would not be a need to count rows (i.e., no need for flags) and leave gates open on furrows where water had not advanced to the end of the field.

Calculating Your Actual Cutoff Ratio

To use the cutoff ratio effectively, you will 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 ratio from Table 1, open fewer gates on the next set. This will cause more water to enter each furrow and will likely increase advance time.

With the proper cutoff ratio and gross application, you can achieve more uniform water application and minimize both deep percolation and runoff. Experiment with different combinations of furrow stream size and set time to find the optimum settings for a particular irrigation in a particular field. The best combination is one that moves water to the end of the furrow within the requirements of the cutoff ratio, is less than the maximum non-erosive stream size, and results in gross applications that are not excessive.