Surface Irrigation

Objectives:

1. Increase understanding of irrigation efficiency, losses, and distribution uniformity associated with surface irrigation.
2. Increase understanding and application of best management practices to improve efficiency and uniformity of surface irrigation.

Key Points:

1. Surface irrigation uses gravity flow to spread water over a field. With flood irrigation, the entire land area to be irrigated is covered with water. Furrow irrigation utilizes small channels or ditches between planted rows to convey water across a field.
2. Using pipe systems to convey and distribute water increases on-farm irrigation efficiency, provides better irrigation control, and reduces labor costs.
3. The correct amount of water to apply at each irrigation depends on the amount of soil water used by the plants between irrigations, the water-holding capacity of the soil, and the depth of the crop roots. Applying the right amount of water to an irrigation set does not guarantee efficient irrigation. Water also must be uniformly applied from one end of the irrigation run (field) to the other.
4. Best management practices to consider include precision land leveling, gated pipe, surge flow irrigation, irrigation scheduling, recirculating irrigation runoff (tailwater re-use), and alternate furrow application.

Assess your knowledge:

1. Describe flood, furrow, and level basin irrigation.
2. Which factors affect the uniformity of water application?
3. Name three advantages of using pipe systems to convey and distribute water.
4. Describe two other best management practices that can reduce water losses.

Surface irrigation uses gravity flow to spread water over a field. Surface systems are the least expensive to install, but have high labor requirements for operation compared to other irrigation methods. Skilled irrigators also are needed in order to achieve good efficiencies. Even if properly designed, surface systems tend to have low water application efficiencies than more advanced irrigation technologies.

Surface Methods

With flood irrigation, the entire land area to be irrigated is covered with water. There may be no method of controlling water flow other than the topography of the land.

Furrow irrigation utilizes small channels or ditches between planted rows to convey water across a field. As water infiltrates through the furrow, it is then moved within the soil both laterally and vertically to saturate the soil profile.

With level basin irrigation, water is applied over a short period of time to a completely level area enclosed by dikes or borders. The floor of the basin may be flat, ridged or shaped into beds. Basin irrigation is most effective on uniform soils precisely leveled when large stream sizes relative to basin area are available.

Selection and Applications

Application Rates

The correct amount of water to apply at each irrigation depends on the amount of soil water used by the plants between irrigations, the water-holding capacity of the soil, and the depth of the crop roots. The rate at which water goes into the soil varies from one irrigation to the next and from season to season.

In general, to avoid completely refilling the root zone in sandy textured soils, gross application amounts should not exceed 1.5 to 2 inches. On medium to fine textured soils they should not exceed 2.5 to 3 inches.

Applying the right amount of water to an irrigation set does not guarantee efficient irrigation. Water also must be uniformly applied from one end of the irrigation run (field) to the other. 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-Stream Size

Select a stream size appropriate for the slope, intake rate, and length of run. Runoff and the uniformity of water infiltrated along the furrow are related to the cutoff ratio. This is the ratio of the time required for water to advance to the end of the furrow to the total set time used for the irrigation. A cutoff ratio of 0.5 is desired. For example, for a 12-hour set time, the advance time should be about six hours. The easiest way to change the advance time is by altering the furrow stream size, i.e. by changing the size of the irrigation set. This will affect the cutoff ratio and hence the uniformity of water application.

The best combination of furrow stream size and set time moves water to the end of the furrow within the requirements of the cutoff ratio, is less than the maximum erosive stream size, and results in gross applications that are not excessive.

Length of Run

Irrigation runs which are too long result in water being lost by deep percolation at the head of the furrow by the time the lower end is adequately irrigated. The length of irrigation runs should not exceed 600 feet on sandy soils and about 1300 feet on clay soils. However, on some low intake rate soils, the length of run may be as long as 2600 feet and water should still be distributed uniformly between the upper and lower end of the field. The time required for advance increases dramatically with furrow length. 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 allowable non-erosive stream size, shortening the row length is an alternative for reducing advance time.

Intake Rates

The rate at which water penetrates into the soil 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. At first, water will penetrate rapidly into the soil, but within one or two hours it will decrease to a rate which stays relatively consistent for the remainder of the irrigation. This fairly consistent rate is called basic intake rate. If the basic intake rate is 0.5 inches per hour or less, the length of run can be 1300 feet long. Higher intake rates require shorter water runs.

Distribution and Delivery Systems

Using pipe systems (rather than earthen ditches) to convey and distribute water to fields has several advantages:

• Increased on-farm irrigation efficiency. Avoid water loss due to deep percolation from earthen conveyance ditches.

• Better irrigation control. Fluctuations in irrigation-canal water levels are common. Using earthen ditches and siphon tubes requires intensive labor to avoid water spillage as a result of such fluctuations (for example, siphon tubes may lose their vacuums and stop working). In contrast, a pipe-irrigation system needs only to have an outlet opened to deliver water through the pipe to furrows; irrigation can be left unattended, even when fluctuations in water levels occur.

• Labor savings. In the Rio Grande Valley, water is distributed through canals coming from the river and is delivered at different outlets (called turnouts). Systems are designed to deliver one “head” of water at each turnout (one head equals approximately 3 cfs or 1,346 gpm). One turnout is installed for each 40-acre field. Some field-blocks are larger than 40 acres, and several fields may be irrigated at the same time. With gated pipe or poly pipe irrigation systems, one irrigator can control six to eight irrigation fronts.

Surface Method Best Management Practices

Precision land leveling improves water application efficiency. Leveling land is cost effective on many sites, and will pay for itself by increasing yields and reducing water losses.

Gated pipe can result in a 35 to 60 percent reduction in water and labor costs. Gated pipe provides a more equal distribution of water into each furrow and eliminates seepage and evaporative losses which occur in unlined irrigation ditches. Gated pipe is available as the traditional aluminum pipe, the less expensive low-head PVC pipe, and the inexpensive “lay-flat” plastic tubing (also called “poly-pipe”).

Surge flow irrigation is a variation of continuous-flow furrow irrigation. Water is usually applied in cycles of one to three hours of alternating on-off periods. Surge works by taking advantage of the natural surface sealing properties of many soils. Surge often results in increased irrigation efficiencies and gives the grower the ability to apply smaller amounts of water at more frequent intervals. Automatic surge valves are also appealing because of reduction in labor.

Irrigation scheduling by use of evapotranspiration data is beneficial to irrigators by providing additional management information on their crop needs. Irrigation scheduling is a method of determining both the time of irrigation application and, within the limits of the flood system distribution, the size of application to make the most efficient use of water.

Recirculating irrigation runoff water (also called “tailwater reuse”) is a method of making more effective use of irrigation water and labor. Reuse of runoff water decreases the amount of water that needs to be pumped or delivered and can be used to improve water application efficiencies by approximately 20 percent. Growers who don’t have reuse systems often cut the stream size in the furrow to a very small flow in order to minimize runoff, possibly causing an uneven water distribution pattern.

Alternate furrow application supplies water to one side of each row. The result is applying water to more acres than irrigating every furrow from a given water source in a given time. Irrigating every other furrow is often beneficial on soils with high infiltration rates and low water-holding capacities. Finally, alternate furrow irrigation effectively reduces the wetted surface area from which evaporation can occur.

References

• Using Flexible Pipe with Surface Irrigation (L 5469)
• Managing Furrow Irrigation Systems (L-913)