Irrigation Chapter 11 - Furrow Irrigation Management
Irrigation consumes 95 percent of all the groundwater pumped in Nebraska and 47 percent of all the surface water diverted. Surface irrigation is the oldest method of irrigation and is used on more land in the world than either sprinkler or micro-irrigation methods. In the United States, surface irrigation accounts for approximately 43 percent of the irrigated acres. Furrow irrigation is one method of surface irrigation.
Furrows are sloping channels cut into the soil surface, into which relatively large, but nonerosive, streams of water are directed. As water moves down the furrow, it infiltrates into the soil laterally as well as vertically. Furrow irrigation can be implemented very efficiently on soils and fields well suited to this method. Furrow irrigation can, however, be very inefficient if the soils and other factors are not properly considered in developing and managing the system.
The soil’s infiltration rate is especially critical in the operation of furrow irrigation systems. If the infiltration rate is too high, the depth of water that infiltrates near the furrow inlet will be much larger than at the end of the field. The infiltration rate of the soil (Chapter 3) ultimately determines the feasibility of furrow irrigation. To efficiently operate a furrow irrigation system, management practices must be adapted to the field conditions at the time of the irrigation. The infiltration rate, roughness of the soil surface, and moisture content of the soil can and do change during the growing season and from year to year. This chapter discusses both conventional and surge-flow furrow irrigation management.
Authors: Brian Benham, Former University of Nebraska Irrigation Engineer,now Extension Specialist and Associate Professor, Director, Center for Watershed Studies, Virginia Tech
Dean Yonts, University of Nebraska Lincoln Biological Systems Extension Engineer, Panhandle Research Extension Center, Scottsbluff, NE.
Dean Eisenhauer, University of Nebraska Lincoln Biological Systems Engineer, Lincoln, NE.
Conventional Furrow Irrigation
Uniform application of water is not possible with a conventional furrow irrigation system. Uniformity can be improved, however, through a basic understanding of the system and the willingness to make the necessary management changes. The number of gates opened or tubes set (the set size) has a significant impact on how fast the water advances across the field and the amount of water being applied. Soil texture, slope, and surface conditions (whether the furrow is smooth or rough, wet or dry) all influence how quickly water advances down the furrow.
Set size should change during the season and year-to-year to match changing soil conditions. Using a small set (relatively few gates open) and a long set-time can result in a uniform irrigation, but will produce excessive runoff. Running too many furrows, however, can result in slow water advance and virtually no runoff, which will cause poor water distribution and deep percolation losses (Figure 11.1a). Efficient irrigation is obtained by almost filling the crop root zone, applying water uniformly (Figure 11.1b), and either minimizing or utilizing runoff.
Evaluating and Changing Current Practices
The correct amount of water to apply with each irrigation depends on the amount of soil water consumed by the plants between irrigations, the water-holding capacity of the soil, and the root depth. The rate at which water goes into the soil (infiltration rate) varies from one irrigation to the next and from season to season. The infiltration rate is affected by the condition of the soil surface. If the soil has recently been tilled and the surface is loose, the infiltration rate may be very high. If the soil has been packed by heavy rain or by water flowing over the surface, a seal may form, reducing the rate of infiltration. One common problem in furrow irrigation is that too much water is applied, especially during the first irrigation.
Based on information in Chapter 9, apply water when the crop has used about one-half of the available water in the root zone. For high efficiency, do not overfill the root zone. Overfilling leaches agricultural chemicals and fertilizers, wastes water, and increases costs. Leave room in the soil for 0.5 to 1.0 inch of rainfall.
Crops often are irrigated for the first time when the roots have penetrated about 18 to 24 inches. For the first irrigation a light application is usually all that is needed to refill the active root zone. During a normal year in Nebraska, off-season precipitation will have replenished the soil profile below this depth. Usually, on medium-textured soils, 1.5 to 2.0 inches of water is all that is necessary to replenish the soil water 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 first calculating the rate at which water is flowing into each furrow. To determine the individual stream size in gallons per minute (gpm) per furrow, divide the pump discharge (in gpm) by the number of furrows with flowing water (Equation 11.1). 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 average furrow stream-size is known, the average gross depth of water applied over the field area (inches of water) can be calculated (Equation 11.2).
Average stream size (gpm) = Pump discharge (gpm)
Number of furrows flowing
Gross depth (in.) = 1155 x ave. stream size (gpm) x Time water applied (hr)
Furrow length (ft) x Watered furrow spacing (inches)
For example, consider the following situation:
|Pump discharge = 750 gpm||Set size = 100 gates flowing|
|Furrow length = 1,320 ft.||Watered furrow spacing = 30 inches|
|Set-time = 12 hours|
|Note: If irrigating every other furrow, wetted furrow spacing is doubled.|
Calculate the stream size using Equation 11.1.
100 furrows = 7.5 gpm per furrow
Calculate the gross depth applied using Equation 11.2.
1155 x 7.5 gpm x 12 hours
1320 feet x 30 inches = 2.6 inches
This type of information will help you 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 amount should not exceed 1.5 to 2 inches. On medium to fine textured soils, it should not exceed 2.5 to 3 inches.
Applying the right average application over the field area does not guarantee efficient irrigation. Water also must be applied uniformly from the upstream to the downstream end of the field. (Figure 11.1b). Crop yields can be reduced on both ends of the field if one end receives too much water and the other end receives too little water. 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.
Set-Time and Stream Size
The appropriate set-time and stream size depend on the slope, intake rate, and length of run. Runoff, and the uniformity of water infiltrated along the furrow, can be 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 11.3).
Cutoff ratio (CR) = Average advance time to end of furrow
The choice of an appropriate cutoff ratio depends on soil factors and irrigation system configuration. Table 11.1 lists the target cutoff ratio for several irrigation system/soil texture combinations. For example: on a loamy soil and no reuse system the target cutoff ratio is 0.70. So, given a 12-hour set-time, the desired advance time for this system (8.4 hours) can be found using Table 11.1 andEquation 11.3.
Average advance time = CR x set-time
8.4 hr. = 0.7 x 12 HR
where CR = cutoff ratio
The easiest way to change the advance time is to alter the furrow stream size (change the number of furrows in each set). This will affect the cutoff ratio and hence the uniformity of water application. Remember that altering set size without altering set-time will change the gross water application.
|Sandy Soils||Loamy Soils||Clayey Soils|
When selecting the furrow stream size, consider furrow erosion. In general, the maximum nonerosive stream size decreases as furrow slope increases. Estimate the maximum non-erosive stream size for a field using Equation 11.4. Generally, the furrow steam size should be less than maximum non-erosive stream size but still large enough to obtain relatively uniform water application. Another limit on furrow stream size is that most furrows cannot transport more than about 50 gpm without overtopping. Very small stream sizes may limit infiltration too severely and should be avoided.
Max. non-erosive stream size (gpm) < _ 12.5
Average field slope (%)
With the proper cutoff ratio and gross irrigation application, you can achieve uniform water application and minimize deep percolation and runoff. Experiment with different combinations of furrow stream size and set-time to find the best combination for a particular irrigation in a particular field. The best combination is the 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 demonstrate the cutoff ratio concept, consider an example where the first irrigation of the year has the following conditions:
|Soil = sandy||System = no reuse|
|Pump discharge = 760 gpm||Set size = 80 gates flowing|
|Furrow length = 2,600 ft.||Watered furrow spacing = 30 inches|
|Set-time = 24 hours||Observed advance time = 15 hours|
First, using Equations 11.1, 11.2, and 11.3, calculate the furrow stream size, gross application, and observed cutoff ratio:
Stream size = 760 gpm = 9.5 gpm per furrow
Gross application = 1155 x 9.5 gpm x 24 hr = 3.4 inches
2600 ft x 30 in
Cutoff ratio = 15 hours = 0.63
These calculations indicate two items that need to be evaluated. First, the cutoff ratio is too high and should be reduced, according to Table 11.1 (from 0.63 to 0.50). Second, the gross water application (3.4 inches) is excessive for the first irrigation on a sandy soil. One way to decrease the gross application is to reduce the set-time. One way to produce an acceptable cutoff ratio is to increase the furrow inflow rate or stream size (open fewer gates per set).
|Parameter||Calculation Method||Our Example|
|Target cutoff ration||Table 11.1||0.50|
|New advance time||Target cutoff ratio x new set time||0.50 x 12 hrs = 6.0 hrs|
|Advance time ration||New advance time original advance time||6.0 hrs / 15 hrs = 0.40|
|Furrow ratio||Figure 11.2||0.60|
|New number of gates||Original number of gates x furrow ration||80 x 0.60 = 48 gates|
|New stream size||Equation 11.1||760 / 48 = 15.6 gpm|
|New gross application||Equation 11.2||(1155 x 15.6 x 12) / (2600 x 30) = 2.8 in|
Referring to calculations shown in Table 11.2 (our example), we have decreased the set-time from 24 hours to 12 hours. Using the desired cutoff ratio of 0.50 (Table 11.1), 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 use Figure 11.2 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. 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 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 11.2 indicate that the furrow stream size increased from 9.5 gpm per furrow to 15.6 gpm per furrow, and that the gross water application 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, using Equation 11.4, if the furrow slope is less than 0.8 percent, the 15.6 gpm stream size is within the non-erosive limits. Changes could be made in subsequent sets to continue to decrease the gross application. In this example, we 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.
Blocked End Furrow Irrigation
In Nebraska, diking or blocking the lower end of a field is a common practice on fields having less than 1% slope. This is especially true when a reuse system is not being used. Blocking furrow ends can result in non-uniform water distribution, agrichemical leaching and excessive deep percolation at both the upstream and downstream ends of the field. Management practices, soil characteristics and the field slope all impact these problems. Figure 11.3 (a) illustrates a typical blocked-end system infiltration profile and (b) a well-managed blocked-end system infiltration profile.
Table 11.3 lists the target cutoff ratios for a number of soil/field slope combinations when using blocked ends. Similar to managing conventional irrigation systems, choosing the appropriate cutoff ratio when using blocked end furrow irrigation is helpful in determining the desired average advance time. For example: on a loamy soil with average slope of about 1% and a set time of 12 hours , the desired average advance time should be about 9 hours (9hrs = 0.75 X 12 hrs).
General Slope Description
Length of Run
Irrigation runs which are too long result in water being lost to deep percolation at the upstream end of the furrow by the time the downstream end is adequately irrigated (see Figure 11.1a). As described in Chapter 3, the rate at which water penetrates into the soil varies with the slope steepness, soil texture, spacing of furrows, and soil compaction. Generally the length of irrigation runs should not exceed 600 feet on sandy soils and about 1300 feet on medium textured soils. On some lower intake rate soils, the length of run may be as long as 2600 feet and still distribute water uniformly.
The time required for water to advance down the furrow increases dramatically with furrow length. This is illustrated in Figure 11.4. Here, the time to advance water 2500 feet is almost three times longer than the time for 1300 feet. If you have a problem getting rows through in a reasonable 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 the 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 reuse systems. Nonrectangular fields present a different problem because 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 nonrectangular fields.
Every-Other Furrow Irrigation
Irrigating every other furrow is beneficial on soils with moderate infiltration rates and high water holding capacities. Irrigating only every-other furrow supplies water to one side of each furrow ridge and applies water to more area in a given time than irrigating every furrow. Another benefit of irrigating every- other furrow is the ability to store rainfall in a soil that was recently irrigated. If water has been applied to every furrow, the entire soil surface is wet and the root zone may have been refilled prior to rain.
Figure 11.5 shows the lateral and downward infiltration of water for two soils where every-other furrow is irrigated. The distance between watered furrows should never exceed 6 feet. A soil probe can 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 below the soil surface. For 30-inch row spacing or better, the furrow bottom in the dry furrow should remain dry.
Every-other furrow irrigation should not be used on steep slopes or on soils with extremely high or low intake rates. On steep slopes, the water flowing down the furrow contacts only a limited amount of soil surface, causing low intake rates.
Research indicates that every-other furrow irrigation results in yields that are comparable to those achieved when every furrow is irrigated. Table 11.4 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. Infiltration is not reduced by one-half compared to watering every furrow because of increased lateral infiltration.
|Soil||Every furrow||Every-other furrow|
|Albaton clay loam||157||154|
|Luton silty clay loam||152||159|
|Crete silty clay loam||153||156|
|Holdrege silt loam||179||177|
|Sarpy sandy loam||140||143|
|Ortello sandy loam||118||119|
|One sandy loam||114||107|
Recirculating irrigation runoff water makes more effective use of irrigation water and labor. Reuse of runoff water decreases the amount of water pumped or delivered and can be used to improve water application efficiencies by as much as 25 percent. Reuse systems are essential for efficient surface irrigation. The economic value of runoff water often will be the deciding factor in installing a reuse system; however, Nebraska law prohibits ground water irrigation runoff from leaving the field. Reuse of irrigation runoff water often is more feasible than adding labor to accomplish efficient irrigation and eliminate runoff.
Irrigators who do not have reuse systems often reduce the stream size to a small flow to minimize runoff. This causes uneven water distribution as explained earlier. One way to potentially manage irrigation runoff without a reuse system and actually improve water distribution is to use surge irrigation.
Surge irrigation was first studied as a method of reducing the amount of runoff that occurred during irrigation. It was discovered, however, that water moved to the end of the field more quickly when applied intermittently, or surged, than when applied continuously. Water can be applied intermittently by cycling irrigation water from side to side using a surge valve. A surge valve is simply a butterfly valve with a computer controlling the valve position and directing flow to either side of the valve. By alternating flow on each side of the valve, an intermittent wetting and soaking cycle is created in the furrow. This wetting and soaking action reduces the water intake rate of the soil. With the intake rate reduced, water will advance more quickly down the furrow. Faster advance and better runoff control, coupled with proper management, can result in a more uniform application, reducing the amount of water needed to effectively irrigate the field.
Prior to the development of surge irrigation, when water was not getting to the end of a field during a given set, the irrigator would move on and return in a few days to finish irrigating the partially watered sets. During the second watering, the irrigation water is usually moved all the way to the end of the field. Rather than making two irrigation sets several days apart, surge irrigation alternates flow between open gates on either side of the valve. Alternating water from side-to-side is done automatically for short durations. A typical surge irrigation system is shown in Figure 11.6.
A further discussion of surge irrigation requires a familiarity with the terminology used to describe surge irrigation.
Set —The total number of furrows with gates open at any time. Note that this is gates open, not flowing. Thus the total is for both sides of the valve.
Set-time —The total time that water runs between gate changes.
On-time —The total amount of time that water is actually applied (flowing into) a given furrow.
Advance time —The on-time required to advance water from the upstream to the downstream end of the furrow.
Cycle —One on-off sequence for one side of the valve. During this time, both sides of the valve receive water. One side is on-off, the other side is off-on.
Advance cycles —Cycles that result in water passing over dry soil. The number of advance cycles is usually from two to eight depending on field conditions. The time for each successive advance cycle is longer than the previous one.
Advance phase —The time during which the advance cycles occur.
Soak or cutback cycles —Cycles that occur after the last advance cycle. If the advance cycles were properly timed, water should reach the downstream end during the final advance cycle. Cutback cycles are usually of constant duration and continued until the desired application amount is applied. These cycles should be just long enough to produce the runoff required to properly irrigate the downstream reaches of the field. Cutback cycles that just advance water to the downstream end with no runoff (on open-end systems) will likely result in crop water stress over the downstream reaches of the field.
Change time —The on-time required to advance water to the portion of the field where wetting is desired during the first cycle time. This is one-half of the first advance cycle using the measured advance method.
How Surge Irrigation Works
When water first contacts the soil in an irrigation furrow, the infiltration rate is high. As the water continues to run, the infiltration rate at that point in the furrow is reduced to a near constant rate. If water is shut off and allowed to infiltrate, soil particles consolidate and form a partial seal in the furrow, further reducing the infiltration rate. When water is reintroduced into the furrow the result is more water movement down the furrow and less infiltration into the soil.
High infiltration rates can lead to poor irrigation system performance due to deep percolation and poor water distribution across the field. Surge flow, if properly managed, will improve irrigation performance by providing a more uniform application. Figure 11.7 shows the infiltration pattern of continuous (a) and surge (b) irrigation and the potential difference in uniformity of water application between the two systems.
Cycle times used with surge irrigation vary with soil texture, slope and field length. Fine textured soils respond less to surge irrigation than do coarse textured soils. If field slope is so steep that it causes a rapid rate of advance, the effects of surge irrigation will be reduced. If the intake rate of a soil is low due to soil texture, tight soils or compacted layers, surge irrigation is likely to be ineffective in reducing the irrigation advance times below those for continuous flow; however, runoff control is still possible.
Surge flow irrigation reduces irrigation runoff by using short duration cycles after the water has reached the end of a field. This helps maintain high uniformity of water application and improves the overall irrigation performance. Another advantage to surge irrigation, unrelated to the improvements in irrigation system performance, is that the surge valve can be used to improve irrigation system management without a large increase in labor. The surge controller provides a two-set automated furrow irrigation system.
There are, however, two potential drawbacks associated with surge flow. First, surge flow may not always reduce the advance time of water down the furrow. If it does not, there are still benefits of labor savings and runoff reduction. Because infiltration rates are often lower with surge flow, a second concern is net water application. If infiltration rates are reduced with surge flow, less water may be stored in the root zone during an irrigation set. If this occurs, the irrigator must compensate by irrigating more frequently or increasing the set-time to avoid under watering. Proper irrigation scheduling may become even more important when surge irrigation is used.
Proper field leveling is also important with surge irrigation. Many irrigators with fields that have low spots or reverse grades have observed water ponding and infiltrating in these low areas to the extent that advance times are actually increased with surge irrigation. This has been observed more often on coarse textured soils than on fine textured soils. Some touch-up grading to eliminate back slopes may be necessary.
A surge valve with an automated controller will cost $1,000 to $3,000, depending on the size of the valve and the controller options. Using a surge valve often requires that somewhere in the system, pipe is used to convey water to the desired location of the valve. Normally, a surge valve is installed in a gated pipe system and regulates water between open gates on either side of the valve. If gated pipe is already used and the water source is conveniently located, a surge valve may be the only additional equipment required. Field layouts for surge irrigation systems are shown in Figure 11.8. An ideal situation is to have the irrigation well or water supply located near the middle of the gated pipeline (a). The valve could then be placed so there is equal land area or number of furrows on each side of the valve. Many times this is not possible and the water supply must be brought to the appropriate location using mainline pipe (b). Another method is to place the surge valve at the edge of the field and use two parallel lines conveying water down the field cross-slope (c). This is the desired layout if lay-flat plastic pipe is used to achieve constant downhill water flow. For cases (a), (b), and (c), water is alternated between open gates on either side of the surge valve. This requires that gates be opened on each side of the surge valve every time a new irrigation set is made.
Another alternative is to use buried pipe lines with risers spaced at intervals that will allow an irrigation set at each riser (d). This system does not require opening and closing the gates once they are set, but it does require moving the surge controller to each of the risers, unless the more expensive option of placing a controller on each valve is exercised. A drawback of this system is that the set size is fixed. Thus, the irrigator cannot change the number of gates flowing from irrigation-to-irrigation or year-to-year.
On irregular shaped fields (e), place the valve so an equal number of acres are on either side of it. With this option, the cycle times are the same for each side but the number of furrows on each side is inversely proportional to the furrow length. For example, if the furrows are 300 feet long on the left set, and 900 feet long on the right set, there would be one-third as many furrows irrigated per set on the right side. Another way of dealing with irregular shaped fields is to place the valve in the middle of the pipeline and have different cycle times for each side of the valve (f). The goal should be to apply the same amount of water on each side.
Finally, if there is adequate slope in the pipeline and the gated pipe does not flow full, the surge valve can be used as a gate valve to stop flow part way across the field (g). When released to the downstream side, the flow must be below the gates in the first section and thus, surge can be accomplished.
The irrigation on-times, during which water is applied to one side of the surge valve, are normally between 20 minutes and two hours. For each irrigation, an equal amount of off-time occurs during each cycle. This will not be the case when different cycle times are used to compensate for an irregular shaped field. Cycle time, the time it takes to complete both a full on-time and off-time sequence, is based on furrow length, soil texture, and field slope. The number of surge cycles used should be based on field length and field condition. Cycle on-times increase with each successive cycle. Long fields and fields with high intake soils will require more cycles; for example, five to six. Shorter fields with low intake soil will need fewer cycles, three to four.
A rule of thumb for surge irrigation is to advance water during each surge cycle a distance that is equal to that fraction of the number of surge cycles used. For example, if using four surge advance cycles, divide the field into four parts and advance the water one-fourth of the field distance during the first surge cycle. The time required to move the water the initial advance distance is the Cycle One on-time. For the second and subsequent on-times, multiply the Cycle One on-time by the factors given in Table 11.5. The table gives on-time factors for four-, five-, and six-surge advance cycles.
Following the final surge advance cycle, the surge valve begins the cutback phase. During cutback, the valve cycles the water at a shorter frequency between the two sides of the irrigation set until the desired depth of water has been applied. During a correctly timed cutback phase, some runoff will occur. This ensures that the lower portion of the field has been adequately watered. Table 11.5 gives the cutback cycle factors as well.
|Advance cycle no.||Fraction of field||On-time factor|
|On-time factors using four surge cycles.|
|Post advance or cutback||0.8 - 1.6|
|On-time factors using five surge cycles.|
|Post advance or cutback||1.2 - 2.3|
|On-time factors using six surge cycles.|
|Post advance or cutback||1.5 - 3.0|
The following example shows how to calculate surge cycle times for a 1,000-foot field with four surge cycles. Assume that water reaches a flag placed 250 feet down the furrow in 20 minutes. Using Table 11.1, calculate the four surge cycles and the cutback cycle:
Cycle number 1 1.0 x 20 20 minutes
Cycle number 2 1.9 x 20 38 minutes
Cycle number 3 2.4 x 20 48 minutes
Cycle number 4 2.9 x 20 58 minutes
Cycle number 5 1.2 x 20 24 minutes
Using one of the factory installed surge valve programs may be the easiest way to begin becoming familiar with surge irrigation and surge irrigation equipment. The preprogrammed on-time factors are similar to those given in Table 11.5.
Inexperienced surge irrigators may need to experiment to find the correct surge valve setting and the number of gates to open. Factors affecting surface irrigations like residue levels, furrow roughness, and soil moisture status vary from irrigation to irrigation and year to year. For this reason, surge valve settings and set size are rarely constant and must be monitored. Record keeping is essential. Remember that any change in the number of gates opened, which changes the flow rate into the furrow, will affect the surge valve settings and in time the set-time. Generally, surge irrigators should begin by opening about 30 percent more gates than would have been opened with a conventional set, with half of the open gates on the left side of the surge valve and half on the right. For example, if an irrigator normally opened 100 gates, they would now open 130 with 65 of these on the left side of the surge valve and 65 on the right. Irrigation socks may be required to prevent excessive erosion near the pipe since the furrow inflow rate is increased under this scheme. Remember that one goal of surge irrigation is to advance the water down the furrow as quickly as possible. Following the advance phase, soak or cutback cycles are used to complete the irrigation and control runoff.
Fine Tuning Surge Irrigation
After an irrigation set, use a soil probe to check the upstream and downstream end of the field for the depth of infiltration. Probe the soil on the side of the ridge at an angle inward toward the center of the ridge to determine moisture content beneath the plant. If very little infiltration has occurred at the down stream end, lengthen the cutback cycles to get more runoff or close some gates to get a larger stream size. In many cases surge irrigation reduces the quantity of water stored in the root zone. Given this, irrigations should be scheduled more frequently to reduce the risk of water stress on the plant.
Applying surge irrigation will require a change in management strategy. Surge irrigation often allows the irrigator to apply water more uniformly as well as more effectively; however, for the first few years, additional time may be required to fine-tune the surge system. Once this is accomplished, surge irrigation can help achieve more uniformly irrigated crops, water conservation, reduced pumping costs and improved water quality by reducing deep percolation. Surge irrigation is not for everyone, but past success suggests that most fields can be set up with surge valves and that furrow irrigators should consider it.