Description ofSystems

The practice of surface irrigation is ancient and is used on more than 90% of the world's irrigated area. The sustainability of irrigated agriculture depends on improvements and innovations in surface irrigation methods, their appropriateness for the different systems, and their adoption in field practice.

Surface irrigation methods include several processes of water application to irrigated fields [16]: furrow, basin, border, contour ditches (wild flooding), and water spreading. The two last processes have several variants. They basically consist of directing water diverted from ditches or watercourses onto sloping fields. They are primarily used to irrigate pasturelands and are generally very inefficient. The main surface methods are basin, furrow, and border irrigation.

Basin Irrigation

Basin irrigation is the most commonly used system worldwide. It consists of applying water to leveled fields bounded by dikes. Two different types are considered: one for paddy rice irrigation, where ponded water is maintained during the crop season; and the other for other field crops, where ponding time is short—until the applied volume infiltrates.

In traditional rice irrigation, small basins are flooded before planting and are drained only before harvesting. The depth of water in the basins can be very large in case of floating rice but usually it should be kept between 5 and 10 cm [17]. Very frequent water applications are used. In sloping lands, the basin dikes usually are built on the contour and fields often irrigate and drain from field to field in cascade. In flat areas, the basins are commonly rectangular and often have independent supply and drainage. When water-saving irrigation is practiced in tropical areas where water ponding is not required for temperature regulation, water is applied only to keep the soil near saturation [18]. In modern rice basins, laser leveling is used, basin size often exceeds 1 ha, and each basin has independent supply and drainage facilities. Rice transplanting may be replaced by mechanical seeding, sometimes in dry soil.

For nonrice crops, basin irrigation can be divided into two categories: traditional basins, of small size and with traditional leveling; and modern precision-leveled basins, which are laser leveled and large, and have regular shapes (Fig. 5.11). Especially with

Figure 5.11. Basin irrigation system.

traditional basins, shape depends on the land slope and may be rectangular in flat areas and follow natural land contours in steep areas. For row crops, and especially horticultural crops, the basins often are furrowed, with the crops being planted on raised beds or ridges. For cereals and pastures, the land is commonly flat inside the basin. Tree crops sometimes have raised beds around the tree trunks for disease control.

Basin irrigation is most practical where soil infiltration rates are moderate to low so that water spreads quickly across the basin and water-holding capacity of the soil is high so that large irrigations can be given. Basin irrigation depths usually exceed 50 mm. Inflow rates for basin irrigation have to be relatively high (>2 L s-1 per meter width) to achieve quick flooding of the basin and therefore provide for uniform time of opportunity for infiltration along the basin length. Basins must be leveled precisely for uniform water distribution because basin topography determines the recession of the ponded water. Figure 5.12 shows the irrigation phases for basins and furrows. In basins, because of large inflow rates, the advance time, as well as the time of cutoff, is short. The depletion phase is large, to allow for the infiltration of the ponded water.

Surface drainage often is not provided with basin irrigation. This simplifies the layout of the fields and the water delivery channels, but can result in waterlogging and soil aeration problems if soil infiltration is low and rainfall is high or irrigations are large. Where rainfall may be high during cropping, a network of surface drainage channels should be provided. Because there is normally no runoff from basins, quantifying irrigation

Figure 5.12. Irrigation phases, times of advance tadv, cutoff tco, and recession trec, infiltration opportunity time t , and infiltrated depths Z for (a) level basin irrigation and (b) sloping furrow irrigation.

applications requires only measurement of inflows and irrigation time. A desired net application can be preset if flow rate is known.

Water usually is delivered to basins through open channels. In traditional systems, water is diverted from the earthen channels into the fields through cuts in the banks. Improved systems with concrete or steel outlet and check gates greatly improve water control and reduce the labor requirement. Concrete lining of field channels also improves water control and reduces seepage losses and maintenance. The labor requirements of basin irrigation can be low if the basins are large and precisely leveled and water control is good.

Furrow Irrigation

In furrow irrigation, small regular channels direct the water across the field (Fig. 5.13). These channels, called furrows or corrugations, serve both to convey water across the field and as the surface through which infiltration occurs. Because conveyance and infiltration are two opposing purposes, designing and operating furrow systems always requires balancing the trade-off between quickly conveying the water across the field and maintaining the flow long enough to infiltrate adequate water. Efficient furrow irrigation nearly always requires irrigation times longer than advance times, and thus drainage of tailwater runoff at the downstream end. Runoff typically varies from 10% to 40% of the applied water and should be collected, stored, and reused as indicated in ASAE EP408.2 [19]. Without tailwater reuse, furrow irrigation efficiency is unlikely.

Irrigation furrows usually are directed along the predominant slope of the field. Cross-slope or contour furrows reduce the furrow slope but, when the cross slope exceeds the

Figure 5.13. Furrow irrigation with siphon tubes from a concrete-lined ditch.

Figure 5.13. Furrow irrigation with siphon tubes from a concrete-lined ditch.

furrow slope, precise leveling and well-constructed furrows are necessary to prevent water crossover and the resulting uncontrolled channeling and gullying across the field. Furrows are used on slopes varying from 0.001 to 0.05 mm-1. Low slopes require soils with low infiltration rates. Slopes greater than 0.01 usually result in soil erosion.

Because furrow slope and soil infiltration rate are usually beyond the control of the designer, furrow design primarily involves determining appropriate furrow length and flow rate. Lengths are commonly between 200 and 400 m. Short lengths are required when infiltration rates are high, and may be required where slopes are steep and soils are erodible. Long lengths allow larger, more efficient, fields. Subdividing long fields with gated pipes or temporary ditches allows efficient irrigation on large fields.

Furrow irrigation is used most commonly for row crops planted on beds or ridges. Furrows may be formed between each plant row or bed, or between alternate rows. Alternate furrows may be used for any given irrigation. This increases the effective MAD because of the wider spacing and thus the greater area irrigated from each furrow. Furrow spacing is limited by the amount of lateral movement of infiltrated water. Fine-textured soils and deep root zones allow widely spaced furrows. Furrow spacing in broadcast and row crops varies from 0.6 to 1.6 m. Furrows in orchards may be up to 3 m apart. Small furrows or corrugations also are used in close-growing crops such as small grains, pastures, and forage. These small furrows often do not completely contain the flows but help to direct them more quickly and uniformly across the field.

Furrow inflow rates are selected to balance the runoff loss that results from high inflow rate and rapid advance against the nonuniform distribution that results from low inflow rate and slow advance. The soil infiltration rate is the most important factor in the relationship between inflow rate and irrigation performance. Because infiltration varies seasonally and is difficult to predict, furrow irrigation systems must be designed to operate over a fairly wide range of inflow rates, and efficient furrow irrigation requires monitoring of the irrigation process and adjusting the inflow rates as required. Consequently, furrow irrigation is relatively labor intensive. Inflow cutback can improve performance greatly by allowing both high initial flows for rapid advance and low final flows to minimize runoff loss. Cutback irrigation seldom is practiced because most irrigation water is supplied at a constant rate and managing the remaining water after the flow cutback is difficult. Cutback also increases labor. Tailwater reuse reduces the need for cutback.

Water is supplied to furrows from ditches or gated pipes along the upper end of the field. In some traditional systems, water is supplied to furrows from earthen ditches through periodic cutouts through the bank that supplies a small ditch that delivers water to 5 to 10 furrows. This feed-ditch method is labor intensive, difficult to control, and results in nonuniform water application. The preferred and more common way to deliver water to furrows from ditches is with siphon tubes (Fig. 5.13). Siphon tubes are typically about 1.7 m long and made from rigid aluminum or polyethylene. Siphon tube use requires that the water level in the supply ditch be at least 10 cm above the field elevation. Water in the ditch is dammed or "checked up" to the required height with flexible (canvas or reinforced polyethylene) dams across the earthen ditches or rigid dams or weirs in concrete-lined ditches. Siphon-tube flow rate is adjusted by changing the elevation of the downstream end of the tube, by switching to a larger or a smaller tube, or by using two or more tubes together. The advantage of siphon tubes over spiles through the bank is that they do not require plugs or gates and do not leak.

Gated pipes (Fig. 5.14) are laid on the surface at the head end of the furrows and have adjustable outlets for each furrow. The most common type of outlet gate is a small rectangular slide that covers a rectangular slot in the pipe. Round outlets that adjust by rotating the circular cover and spigot outlets are also available. Rigid gated pipe usually is made from either 4-mm-thick aluminum or about 8-mm-thick PVC. PVC pipe must have ultraviolet inhibitors to prevent deterioration in the sun. Both have low-pressure, gasketed ends that are easy to connect and disconnect. Common pipe diameters are 15, 20, 25, and 30 cm and lengths up to 9 m are easy to maneuver. Advantages of gated pipe over open ditches are that they requires little or no field area and they are portable, and thus can be moved for tillage and harvesting operations.

Thin-walled (lay-flat) tubing made from PVC sheeting is used like rigid gated pipe. Both fixed and adjustable outlet gates for lay-flat tubing are available. Lay-flat tubing can hold only about 5 kPa of water pressure and generally lasts only one year. It is made in a range of diameters and wall thicknesses. It is a low-initial-cost alternative to rigid gated pipe but often has a higher annual cost because of its short useful life.

Figure 5.12b shows the irrigation phases for furrows. In basins, because of large inflow rates, the advance and ponding phases are relatively short and the depletion phase

Figure 5.14. Gated pipes in polyvinyl chloride (PVC).

is large to allow for the infiltration of the ponded water. In furrow irrigation, inflows are often selected (0.2 to 1.2 Ls-1) so that the advance phase is 20% to 50% of the irrigation time. Desirable advance-to-cutoff time ratios depend on the soil infiltration characteristics and whether runoff is reused. Depletion time is often short and sometimes may be ignored. Recession time is usually much shorter than advance time, and so, wetting time must be sufficient to adequately irrigate the tail end (low quarter) of the furrow.

Border Irrigation

In border irrigation, the field is divided into sloping strips of land separated by parallel border dikes or ridges. Water is applied at the upstream end and moves as a sheet down the border (Fig. 5.15). Border irrigation is used primarily for close-growing crops, such as small grains, pastures, and fodder, and for orchards and vineyards. The method is best adapted to areas with low slopes, moderate soil infiltration rates, and large water supply rates. These conditions allow large borders that are practical to farm.

Borders are most common and practical on slopes less than 0.005 mm-1. They can be used on steeper slopes if infiltration is moderately high and crops are close growing. Irrigation to establish new crops on steep borders is difficult because water flows quickly, is difficult to spread evenly, and may cause erosion. Design and management of very flat borders approximates conditions for level basins.

Border width is determined by cross slope and available flow rates. The elevation difference across a border should be less than 30% of the flow depth to ensure adequate

Figure 5.15. Border irrigation.

water coverage. Thus, border width is limited by field cross slope or by the amount of land movement required to eliminate cross slope. Deep flow levels, resulting from low slopes, high flow rates, or high crop roughness, allow more cross slope. Land leveling is critical to efficient border operation. Within these limitations, border widths should be multiples of the width of the machinery that will be used, to allow efficient machinery operation. In orchard and vineyards, width is determined by row spacing. Borders typically vary from 5 to 60 m wide.

Border length affects advance time and thus irrigation cutoff time. Longer borders require longer irrigation times and result in greater irrigation depths. Borders up to 400 m long are used where infiltration rates are moderately low and MAD is high.

In the ideal border irrigation, the recession curve parallels the advance curve, giving equal infiltration opportunity time all along the border. Thus, efficient border irrigation, like basin irrigation, requires large flow rates per unit width to advance the water quickly down the field. On sloping borders, this requires that inflow cutoff, and often even recession at the top end of the field, occur before advance completion at the tail end. If soil infiltration or crop roughness changes, inflow rates must change to maintain this balance. The downstream end of borders may be closed or open. Closed borders require accurate flow cutoff times to prevent excessive ponding at the tail, and may require drainage of excessive rainfall. Open borders require drainage systems and, preferably, tailwater collection and reuse. Water application to borders can be from cutouts or side gates from ditches or from large siphon tubes.

Border irrigation is gradually declining in use. Laser leveling allows some border irrigated fields to be converted to easier-to-manage basins. Some fields also have been converted to sprinkler irrigation, and orchards and vineyards may be converted to microirrigation.

Analyses in the following sections are oriented to these main surface irrigation methods, focusing on progress for modernizing surface irrigation systems with improved performance and labor and energy savings. This can contribute to the competitiveness of surface irrigation systems compared to pressurized systems.

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