G97-1338-A
Proper furrow irrigation practices can minimize water application, irrigation costs and chemical leaching and result in higher crop yields.
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Often, irrigating an entire field as quickly as possible is the goal of a furrow irrigator. While irrigators may be satisfied just to get the water to the end of the furrows, consideration should be given to how much water is applied and how it is distributed.
| Figure 1. Infiltration profiles under conventional furrow irrigation. |
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Efficient irrigation is obtained by almost filling the crop root zone each irrigation, applying water uniformly (Figure 1b) and either minimizing or utilizing runoff. Runoff and the uniformity of the water infiltrated along the furrow are related to soil conditions, field topography and the irrigator's management practices.
The correct amount of water to apply at each irrigation depends on many factors, including the amount of soil water consumed by the plants between irrigations, the water-holding capacity of the soil and root depth. Infiltration rate, the rate at which water goes into the soil, varies from one irrigation to the next and from season to season. Infiltration rate is affected by soil surface conditions. If the soil was recently tilled and the surface is loose, the infiltration rate may be very high. On the other hand, if the soil has been packed by heavy rains or by water flowing over the surface, infiltration may be reduced. One common problem in furrow irrigation is too much water is applied, especially during the first irrigation.
In general, apply water when the crop has used about one-half of the available water in the root zone. For high efficiency, do not completely fill or overfill the root zone. Overfilling leaches agricultural chemicals and fertilizers, wastes water and increases costs. Leave storage room in the soil for .5 to 1.0 inch of rainfall.
The first irrigation often occurs when roots have penetrated about 18 to 24 inches. For the first irrigation, a light application is all that is needed to refill the active root zone. In Nebraska, precipitation usually replenishes the soil profile below this depth. On medium-textured soils, 1.5 to 2.0 inches of water is all that is usually necessary to replenish the soil moisture in the top 18 to 24 inches of soil.
To evaluate irrigation practices, estimate the gross depth and uniformity of application. The gross depth of water applied can be figured by calculating the rate that water is flowing into each furrow. Divide the pump discharge in gallons per minute (gpm) by the number of furrows flowing to determine the individual stream size (gpm per furrow):
| Equation 1 | |
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| Stream Size= | Pump Discharge (gpm) Number of Furrows Flowing |
Note: this calculation is only accurate if leaks in the delivery system are minimal. A key to good management is to maintain pipes and ditches to minimize leaks and seepage. Once the furrow stream-size is known, the average gross depth of water applied over the field area (inches of water) can be calculated as below:
| Equation 2 | |
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| Gross Depth (inches) = | 1,155 x Stream Size (gpm) x Time Water Applied (hours) Furrow Length (ft) x Wetted Furrow Spacing (inches) |
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The above information will help you to make better management decisions and improve the overall performance of your irrigation system. To avoid completely refilling a fully developed root zone on sandy-textured soils, the gross application amounts should not exceed 1.5 to 2 inches. On medium- to fine-textured soils, application amounts should not exceed 2.5 to 3 inches.
Applying the appropriate application over the field area does not guarantee efficient irrigation. Water also must be uniformly applied from the upstream to the downstream end of the field (Figure 1b). Crop yields can be reduced on both ends of the field if one end receives too much water and the other end too little. Set time and stream size are perhaps the most readily "manageable" irrigation parameters. Set times and stream sizes that accommodate field conditions will improve irrigation efficiency.
The appropriate set time and stream size depends on the slope, intake rate and length of run. Runoff and the uniformity of water infiltrated along the furrow are related to the cutoff ratio. The cutoff ratio is the ratio of the time required for water to advance to the end of the furrow to the total set time:
| Equation 3 | |
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| Cutoff Ratio = | Average Advance Time Set Time |
Choosing the appropriate cutoff ratio depends on soil factors and irrigation system configuration. Table I lists the target cutoff ratios for a number of irrigation system/soil texture combinations. For example: on a loamy soil and no reuse system with a 12-hour set time, the desired advance time should be about 8.4 hours (8.4 , 12 = 0.70 or 12 x 0.70 = 8.4 hours). The easiest way to change the advance time is to alter the furrow stream size (change the number of furrows flowing in each set). This will affect the cutoff ratio and hence the uniformity of water application. Altering set size without altering set time will change the gross application.
Table I. 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 |
When selecting the furrow stream size, consider furrow erosion. In general, the maximum non-erosive stream size decreases as furrow slope increases. Estimate the maximum non-erosive stream size for a field by dividing 12.5 by that field's average percent slope. As a rule of thumb, your furrow steam size should be less than the result of this formula, but still large enough to obtain relatively uniform water application. Another limit on furrow stream size: most furrows cannot transport more than about 50 gpm without overtopping. Very small stream sizes may severely limit infiltration and should be avoided.
With the proper cutoff ratio and gross application, you can achieve 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. To enhance furrow irrigation performance, irrigators can visit the nearest Cooperative Extension office for the latest printed and computer-based aids using the cutoff ratio concept.
To demonstrate the use of Equations 1, 2 and the cutoff ratio concept, consider an example where the first irrigation of the year has the following conditions:
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First calculate the observed cutoff ratio, furrow stream size and gross application:
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| Figure 2. Graph for determining proper set size. |
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Referring to calculations shown in Table II, we have decreased the set time from 24 hours to 12 hours. Using the Desired Cutoff Ratio of 0.50 (Table I), we determine a New Advance Time of 6.0 hours. Taking a ratio of the new advance time to the original advance time yields a value of 0.40. Having found the Advance Time Ratio, and knowing our soil texture, we can enter Figure 2 and find the appropriate Furrow Ratio. The Furrow Ratio is the new number of gates to be opened divided by the original number of gates opened. In this example, the Furrow Ratio is 0.60. And the New Number of Gates that should be opened on the new set is 48. Reducing the gross water application was also a goal in this example. In order to obtain a real reduction in gross application, the ratio of original set time to the new set time (24 hours/12 hours = 2) must be greater than the ratio of the original number of gates opened to the new number of gates opened (80 gates/48 gates = 1.67).
The results in Table II indicate the furrow stream size was increased from 9.5 gpm per furrow to 15.6 gpm per furrow, and that the gross water application was decreased from 3.4 inches to 2.8 inches. For sandy soils, a 2.8-inch application is not unreasonably large and represents an 18 percent reduction from the prior set. Also, if the furrow slope is less than 0.8 percent, the 15.6 gpm stream size is within the non-erosive limits (12.5/0.80 = 15.6). Changes could be made in subsequent sets to continue to decrease the gross application. In this example, we have demonstrated: 1) how to improve the uniformity of irrigation by using the cutoff ratio; and 2) how to reduce the gross depth of application by reducing the irrigation set time proportionately more than the reduction in set size.
Table II. Example showing how the set time and cutoff ratio are used to improve the performance of a furrow irrigated set.
| Parameter | Calculation Method | Our Example | Your Example |
|---|---|---|---|
| Desired cutoff ratio | Table I | 0.50 |  |
| New Advance Time | Desired Cutoff Ratio x New Set Time |
0.50 x 12 hrs = 6.0 hrs | |
| Advance Time Ratio | New Advance Time Original Advance Time |
60 hrs / 15 hrs = 4.0 | |
| Furrow Ratio | Figure 2 | 0.60 | |
| New Number of Gates | Original Number of Gates x Furrow Ratio |
80 x 0.60 = 48 gates | |
| New Stream Size | Equation 1 | 760 / 48 = 15.8 gpm | |
| New Gross Application | Equation 2 | (1155 x 15.6 x 12) / (2600 x 30) = 2.8 in |
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| Figure 3. An example of how quickly water travels down a furrow. |
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The time required for water to advance down the furrow increases dramatically with furrow length (Figure 3). Here, the time to advance water 2,600 feet is three times longer than the time for 1,300 feet. If you have a problem getting rows through in a reasonable length of time (as determined by the cutoff ratio) and you are using the maximum non-erosive stream size, shortening the row length is an alternative for reducing advance time.
Short fields also pose a problem because application times required to infiltrate the desired amount of water may lead to excessive runoff. Again, the cutoff ratio technique will help overcome this problem. Short fields are also good candidates for runoff return systems. Non-rectangular fields present a different problem, as the length of run varies within the field. Applying the cutoff ratio technique on several sets across the field using the average furrow length for each set, may help overcome management problems on non-rectangular fields.
The water-penetration rate varies with the steepness of slope, soil texture, spacing of furrows and soil compaction. The rate at which soil will absorb water varies with time. Initially, water penetrates, or infiltrates, rapidly but within one or two hours infiltration decreases to a rate which stays relatively consistent for the remainder of the irrigation. This fairly consistent infiltration rate is called the basic intake rate. Longer field lengths can be used on soils with low intake rates; higher intake rates require shorter field lengths.
| Figure 4. Irrigated furrow wetting patterns. |
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Figure 4 shows the lateral and downward infiltration of water for two soils where every other furrow is irrigated. When the watered furrow spacing is too wide, there will be a dry area between the furrows and the crop may not get enough water (Figure 4, soil A). The distance between watered furrows should never exceed 6 feet. A soil probe may be used to check the penetration of water into the dry furrow after each set. Probe both the wet and dry ridge shoulders - both should be well wetted. The furrow bottom in the dry furrow should remain dry (Figure 4, soil B).
Every-other furrow irrigation should not be used on steep slopes or on soils with low intake rates. On steep slopes, the water flowing down the furrow is in contact with only a limited amount of soil surface, causing low intake rates.
Research indicates every-other furrow irrigation results in yields comparable to those achieved when every furrow is irrigated. Table III shows corn yields on various soil textures when irrigating every furrow and every other furrow using 12-hour irrigation sets. Irrigation water application may be reduced 20 to 30 percent by implementing every other furrow irrigation. Because of increased lateral infiltration, infiltration is not reduced by one-half compared to watering every furrow.
| Table III. Corn yields (bushels/acre) on various soils when irrigating every furrow and every other furrow with 12-hour sets. |
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Irrigators who do not have reuse systems often reduce the stream size in the furrow to a very small flow in order to minimize runoff. This causes uneven water distribution. One way to manage irrigation runoff while actually improving water distribution is to use surge-flow irrigation.
Electronic version issued January 1998
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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kenneth R. Bolen, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.
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