9.5: Surface Water Supplies

To the irrigator, there is great value and need for a dependable water supply that is flexible with respect to the frequency of available water, the rate of water delivery, and how long the water is available. These expectations for a surface water supply are more easily accomplished by pressurized delivery systems than by open channels. Nevertheless, the predominant means of delivering irrigation supplies from irrigation projects is by open channel. Pressurized delivery systems include pipelines which may vary from being underground and permanent to portable, temporary pipe on the soil surface.

Open Canals

Conveyance canals or ditches are frequently used to deliver water from surface storage or wells. A system of open canals often distributes water great distances from its source to the field. Figure 9.9 shows a large open canal and smaller lateral canals.

Losses of water by seepage from canals can be a major concern. Water seeping out the bottom of the canal is especially high in earthen canals at the beginning of an irrigation season when soil intake rates are high. Figure 5.8 gives approximations of canal seepage losses depending upon soil texture for unlined canals. Proper soil compaction at optimum moisture content can almost eliminate seepage in some soils.

Irrigation canals are sometimes lined to minimize seepage losses. In addition to reducing seepage, canals are lined to ensure against interrupted operation resulting from channel failure; to provide a more efficient cross section by increasing sideslopes, by reducing the roughness coefficient, by eliminating vegetative growth, and by reducing maintenance. Canals can be lined with a variety of materials. The most common lining material by far is concrete, but other materials include brick, rock masonry, asphalt, soil cement, rubber, colloid clay, and plastic. Concrete meets all the requirements for a lining better than any other material. Its principal disadvantages are high initial cost and possible damage from soil swelling and shrinking, soil chemicals, and freezing and thawing. Concrete can be applied in a variety of ways but continuous pouring with slip-form equipment is the most common.

The purpose of irrigation delivery systems is to provide water to the field in a timely and reliable manner (Figure 9.10). To improve reliability and increase flexibility frequently requires some type of automation of the delivery system. Water is delivered by one of three possible scheduling techniques: demand, arranged, and rotation. A “demand” schedule allows for complete flexibility on the frequency, rate, and duration of water delivery. A common example is a municipal water system; the user can open the faucet at any time (flexibility in frequency), receive a low or high flow rate (flexibility in flow rate), and take the water as long as desired (flexibility in duration). An “arranged” schedule requires the user to request the rate and duration of a water delivery in advance. The advance notice required to receive and to turn the water off is typically one to two days. Arranged schedules often require that the water be turned on or off at a specific time of the day. In a “rotation” schedule, all flow entering a small canal is delivered to only one field. The length of time water is delivered to a field depends upon its size. After delivering water for the prescribed period to one field the flow is shifted to the next.

In addition to the reliability that water is delivered when and as promised, there are two other aspects. One aspect is that the flow remain at the prescribed rate; the second is that flows and water levels in the canal are controlled so that canal structures and soil banks are not damaged. Many water delivery systems are now automated and there are many types of automated systems. Methods of automatic control differ based upon the control of flow rate or water level in the canal, the control based on measures at the upper end or the lower reaches of the delivery system, and the control being local or remote. More information on developing surface water supply systems for irrigation, including small earth dams, is presented in Huffman et al. (Huffman et al., 2013).

Figure 9.9. Canal delivery system in western Nebraska, which delivers water from Seminoe Dam (inset).

Figure 9.10. On-farm ditch providing water to small fields near Delhi, India (top), lateral canal with a weir to provide sufficient head (water surface elevation) for siphon tubes in Nebraska. (Bottom photo courtesy of Steve Melvin, Nebraska Extension.)

Pressurized Delivery Systems

Pipelines are used extensively to deliver water, especially when the capacity required is low enough for standard pipe sizes or the advantage of a closed delivery system outweigh those of a canal system (Figure 9.11). There are pipeline delivery systems where the pipe is 10 feet or more in diameter. Some of the advantages of buried pipelines include: few problems from damage caused by animals; no vegetative problems; land over the pipeline can be utilized; buried pipelines do not obstruct cross traffic; pipelines do not have to follow elevation contour lines; lower maintenance costs; less hardware required for controlling flows; and less threat of drownings.

Disadvantages of pipelines compared to canals include: initial cost may be higher than canals; and pipelines may plug from sediment or debris more easily.

Pipelines for water delivery systems are increasing in popularity. The conversion is especially rapid in expanding urban areas. Some irrigation districts use monolithic (cast-in-place) concrete pipe for low-pressure conditions. Reinforced concrete pipe are being used uphill and downhill from a supply canal. The uphill pipelines are supplied by pumps while the downhill laterals are normally gravity fed.

In some locations, the downhill laterals have sufficient slope and length to develop the pressure required to operate sprinkler systems without booster pumps. Many pipelines operate with a pressure head of 2 feet or less and lead directly to surface irrigation systems or booster pumps to provide the head for sprinkler or microirrigation systems.

Figure 9.11. Installation of a buried pipeline for irrigation water delivery.