Introduction to surface irrigation2

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2.1.3 Evolution of the practice


Although surface irrigation is thousands of years old, the most significant advances have been made within the last decade. In the developed and industrialized countries, land holdings have become as much as 10-20 times as large, and the number of farm families has dropped sharply. Very large mechanized farming equipment has replaced animal-powered planting, cultivating and harvesting operations. The precision of preparing the field for planting has improved by an order of magnitude with the advent of the laser-controlled land grading equipment. Similarly, the irrigation works themselves are better constructed because of the application of high technology equipment.
The changes in the lesser-developed and developing countries are less dramatic. In the lesser-developed countries, trends toward land consolidation, mechanization, and more elaborate system design and operation are much less apparent. Most of these farmers own and operate farms of 1-10 hectares, irrigate with 20-40 litres per second and rely on either small mechanized equipment or animal-powered farming implements.
Probably the most interesting evolution in surface irrigation so far as this guide is concerned is the development and application of microcomputers and programmable calculators to the design and operation of surface irrigation systems. In the late 1970s, a high-speed microcomputer technology began to emerge that could solve the basic equations describing the overland flow of water quickly and inexpensively. At about the same time, researchers like Strelkoff and Katapodes (1977) made major contributions with efficient and accurate numerical solutions to these equations. Today in the graduate and undergraduate study of surface irrigation engineering, microcomputer and programmable calculator utilization is, or should be, common practice.
Microcomputers and programmable calculators provide several features for today's irrigation engineers and technicians. They allow a much more comprehensive treatment of the vital hydraulic processes occurring both on the surface and beneath it. One can find optimal designs and management practices for a multitude of conditions because designs historically requiring days of effort are now made in seconds. The effectiveness of existing practices or proposed ones can be predicted, even to the extent that control systems operating, sensing and adjusting on a real-time basis are possible.

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2.2 Surface irrigation methods





2.2.1 Basin irrigation (http://www.fao.org/docrep/t0231e/t0231e04.htm#2.2.1 basin irrigation)
2.2.2 Border irrigation (http://www.fao.org/docrep/t0231e/t0231e04.htm#2.2.2 border irrigation)
2.2.3 Furrow irrigation (http://www.fao.org/docrep/t0231e/t0231e04.htm#2.2.3 furrow irrigation)
2.2.4 Uncontrolled flooding (http://www.fao.org/docrep/t0231e/t0231e04.htm#2.2.4 uncontrolled flooding)




The classification of surface methods is perhaps somewhat arbitrary in technical literature. This has been compounded by the fact that a single method is often referred to with different names. In this guide, surface methods are classified by the slope, the size and shape of the field, the end conditions, and how water flows into and over the field.
Each surface system has unique advantages and disadvantages depending on such factors as were listed earlier like: (1) initial cost; (2) size and shape of fields; (3) soil characteristics; (4) nature and availability of the water supply; (5) climate; (6) cropping patterns; (7) social preferences and structures; (8) historical experiences; and (9) influences external to the surface irrigation system.
2.2.1 Basin irrigation


Basin irrigation is the most common form of surface irrigation, particularly in regions with layouts of small fields. If a field is level in all directions, is encompassed by a dyke to prevent runoff, and provides an undirected flow of water onto the field, it is herein called a basin. A basin is typically square in shape but exists in all sorts of irregular and rectangular configurations. It may be furrowed or corrugated, have raised beds for the benefit of certain crops, but as long as the inflow is undirected and uncontrolled into these field modifications, it remains a basin. Two typical examples are shown in Figure 3, which illustrate the most common basin irrigation concept: water is added to the basin through a gap in the perimeter dyke or adjacent ditch.

There are few crops and soils not amenable to basin irrigation, but it is generally favoured by moderate to slow intake soils, deep-rooted and closely spaced crops. Crops which are sensitive to flooding and soils which form a hard crust following an irrigation can be basin irrigated by adding furrowing or using raised bed planting. Reclamation of salt-affected soils is easily accomplished with basin irrigation and provision for drainage of surface runoff is unnecessary. Of course it is always possible to encounter a heavy rainfall or mistake the cut-off time thereby having too much water in the basin. Consequently, some means of emergency surface drainage is good design practice. Basins can be served with less command area and field watercourses than can border and furrow systems because their level nature allows water applications from anywhere along the basin perimeter. Automation is easily applied.
Basin irrigation has a number of limitations, two of which, already mentioned, are associated with soil crusting and crops that cannot accommodate inundation. Precision land levelling is very important to achieving high uniformities and efficiencies. Many basins are so small that precision equipment cannot work effectively. The perimeter dykes need to be well maintained to eliminate breaching and waste, and must be higher for basins than other surface irrigation methods. To reach maximum levels of efficiency, the flow per unit width must be as high as possible without causing erosion of the soil. When an irrigation project has been designed for either small basins or furrows and borders, the capacity of control and outlet structures may not be large enough to improve basins.

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Border irrigation can be viewed as an extension of basin irrigation to sloping, long rectangular or contoured field shapes, with free draining conditions at the lower end. Figure 4 illustrates a typical border configuration in which a field is divided into sloping borders. Water is applied to individual borders from small hand-dug checks from the field head ditch. When the water is shut off, it recedes from the upper end to the lower end. Sloping borders are suitable for nearly any crop except those that require prolonged ponding. Soils can be efficiently irrigated which have moderately low to moderately high intake rates but, as with basins, should not form dense crusts unless provisions are made to furrow or construct raised borders for the crops. The stream size per unit width must be large, particularly following a major tillage operation, although not so large for basins owing to the effects of slope. The precision of the field topography is also critical, but the extended lengths permit better levelling through the use of farm machinery.

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2.2.3 Furrow irrigation

surface by channelling the flow along the primary direction of the field using
perimeter and spreads vertically and horizontally to refill the soil reservoir. Furrows are often employed in basins and borders to reduce the effects of topographical variation and crusting. The distinctive feature of furrow irrigation is that the flow into each furrow is independently set and controlled as opposed to furrowed borders and basins where the flow is set and controlled on a border by border or basin by basin basis.
Furrows provide better on-farm water management flexibility under many surface irrigation conditions. The discharge per unit width of the field is substantially reduced and topographical variations can be more severe. A smaller wetted area reduces evaporation losses. Furrows provide the irrigator more opportunity to manage irrigations toward higher efficiencies as field conditions change for each irrigation throughout a season. This is not to say, however, that furrow irrigation enjoys higher application efficiencies than borders and basins.
There are several disadvantages with furrow irrigation. These may include: (1) an accumulation of salinity between furrows; (2) an increased level of tailwater losses; (3) the difficulty of moving farm equipment across the furrows; (4) the added expense and time to make extra tillage practice (furrow construction); (5) an increase in the erosive potential of the flow; (6) a higher commitment of labour to operate efficiently; and (7) generally furrow systems are more difficult to automate, particularly with regard to regulating an equal discharge in each furrow. Figure 5 shows two typical furrow irrigated conditions.


Figure 5. Furrow irrigation configurations (after USDA-SCS, 1967)


(a) graded furrow irrigation system





(b) contour furrows

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2.2.4 Uncontrolled flooding


There are many cases where croplands are irrigated without regard to efficiency or uniformity. These are generally situations where the value of the crop is very small or the field is used for grazing or recreation purposes. Small land holdings are generally not subject to the array of surface irrigation practices of the large commercial farming systems. Also in this category are the surface irrigation systems like check-basins which irrigate individual trees in an orchard, for example. While these systems represent significant percentages in some areas, they will not be discussed in detail in this paper. The evaluation methods can be applied if desired, but the design techniques are not generally applicable nor need they be since the irrigation practices tend to be minimally managed.

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2.3 Requirements for optimal performance



2.3.1 Inlet discharge control (http://www.fao.org/docrep/t0231e/t0231e04.htm#2.3.1 inlet discharge control)
2.3.2 Wastewater recovery and reuse (http://www.fao.org/docrep/t0231e/t0231e04.htm#2.3.2 wastewater recovery and reuse)unlovers are always There is substantial field evidence that surface irrigation systems can apply water to croplands uniformly and efficiently, but it is the general observation that most such systems operate well below their potential. A very large number of causes of poor surface irrigation performance have been outlined in the technical literature. They range from inadequate design and management at the farm level to inadequate operation of the upstream water supply facilities. However, in looking for a root cause, one most often retreats to the fact that infiltration changes a great deal from irrigation to irrigation, from soil to soil, and is neither predictable nor effectively manageable. The infiltration rates are an unknown variable in irrigation practice.
In those cases where high levels of uniformity and efficiency are being achieved, irrigators utilize one or more of the following practices: (1) precise and careful field preparation; (2) irrigation scheduling; (3) regulation of inflow discharges; and (4) tailwater runoff restrictions, reduction, or reuse. Land preparation is largely a land grading problem which will be discussed in Section 5. Irrigation scheduling is a theme covered separately by several publications such as the FAO Irrigation and Drainage Paper 24 (Rev) by Doorenbos and Pruitt (FAO, 1977). The attention here then is focused on inflow regulation and tailwater control.
2.3.1 Inlet discharge control


Surface irrigation systems have two principal sources of inefficiency, deep percolation and surface runoff or tailwater The remedies are competitive. To minimize deep percolation the advance phase should be completed as quickly as possible so that the intake opportunity time over the field will be uniform and then cut the inflow off when enough water has been added to refill the root zone. This can be accomplished with a high, but non-erosive, discharge onto the field. However, this practice increases the tailwater problem because the flow at the downstream end must be maintained until a sufficient depth has infiltrated. The higher inflow reaches the end of the field sooner but it increases both the duration and the magnitude of the runoff.
There are three options available to solve this problem, at least partially: (1) dyke the downstream end to prevent runoff as in basin irrigation; (2) reduce the inflow discharge to a rate more closely approximating the cumulative infiltration along the field following the advance phase, a practice termed 'cutback'; or (3) select a discharge which minimizes the sum of deep percolation and tailwater losses, i.e., optimize the field inflow regime. Examples of these alternative practices are discussed and illustrated in Section 5. In this configuration, the head ditch is divided into a series of level bays which are differentiated by a small change in elevation. Water levels are regulated in two bays simultaneously so that the lower bay has sufficient head to produce an advance phase flow in the furrows while in the upper bay the head is only sufficient to produce the cutback flow. Thus, the system operates by moving the check-dam from bay to bay along the upper end of the field.
Two very recent additions to the efforts to control surface irrigation systems more effectively are the 'Surge Flow' system (Figure 6) developed at Utah State University, USA and the 'Cablegation' system developed at the US Department of Agriculture's Snake RiverWaterConservationResearchCente r in Kimberly, Idaho, USA. These systems will be dealt with in more detail in a later section.

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