Irrigation has been practiced for at least 6,000 years as a way of increasing the growth of food and forage plants used by the cultivators and their livestock. Defined as the artificial application of water to the root zone of desirable plants, irrigation substitutes for natural rainfall that is deficient in quantity or irregular or untimely in occurrence. As such, it can allow particular crops to be grown where they would not otherwise grow, to survive droughts that would otherwise kill them or stifle their growth, and to take maximum advantage of other agronomic inputs, such as fertilizer, in producing the desired output – typically the leaf, the seed, or the root of the plant.
Background
Design of an irrigation system is a recursive process. The design process involves succeeding stages of information gathering, stakeholder consultation, and decision-making, which systematically close in on a final design.
Two types of irrigation assistance projects can be distinguished, differing in their comprehensiveness and complexity. The first of these comprises greenfield projects, in which an entire new irrigation system is developed on previously unirrigated land. These were common 20 or 30 years ago in many parts of the world. They are expensive and complex undertakings, often designed to open new areas to settlement and cultivation.
More common are rehabilitation and modernization projects, in which an existing irrigation system is brought back to its original condition (rehabilitation) or improved to restore functionality and meet new needs (modernization). Such projects have the important attribute of being able to build on sunk costs represented by past investments in infrastructure and the accumulated human and social capital represented by experienced farmers, private input supply networks, and local farmer-based organizations. This reduces the cost of new investment as well as the risks inherent in the project. Pure rehabilitation projects are rare, because conditions have usually changed since the project was originally designed and these changes need to be taken into account.
Farmer involvement in project design is important for two reasons. First, farmers are the ones who will use the water to generate income for themselves and economic output for the nation. For them to do this, the irrigation services provided by the project must match their needs. It behooves project planners to ascertain exactly what these needs are, not just in terms of volume of water, but also delivery schedules, water quality, and reliability. Since the irrigation project will have a monopoly on supply, farmers won’t have a choice of suppliers, and market mechanisms will not work to assure the quality of the irrigation inputs they use. Careful consultative planning prior to construction, and accountable management once the system is completed, are necessary for this.
Second, farmers will usually be the ones to operate the last stage of the water distribution system, linking larger canals with individual farms. In most developing countries, where farm sizes are a few hectares or less, the operating agency will not be able to manage the large number of small canals that comprise this last link in the distribution system, and groups of farmers will do this. Farmers may organize themselves informally for this task, or they may be more formally organized into a WUA or Irrigation District for this purpose. The implication for project design is that these farmer groups must be consulted and their capacity to operate and maintain the envisioned facilities assessed. Operating requirements of the system, as designed, must be kept within these capacity parameters.
Forming WUAs is often an important and integral part of sound irrigation project implementation. It is advisable to begin the organization process at the outset of the project, during detailed design, to establish an early connection with farmers and to lay the groundwork for their eventual assumption of management responsibilities, and to take advantage of their insights into existing needs and challenges. However, it may be difficult to establish a fully functioning WUA prior the commencement of system operation. WUA development should generally proceed in stages, beginning at project inception and continuing, in practice, beyond the commissioning of the system.
Elements of an Irrigation System
An irrigation system can be divided into six sub-systems. Five of these are based on the physical elements of the system, while the sixth comprises the institutions that govern and manage the system. These six elements are common to all irrigation systems.
The five physical elements form a chain, based on the movement of water from source to use. This chain comprises:
- the supply and delivery sub-system, which extracts the water from a natural source and transports it to an agricultural area
- the distribution sub-system which breaks up the supply and delivers it to individual farms
- the application sub-system which takes water from the distribution sub-system and introduces it into the soil volume in which agricultural plants have their roots
- the agricultural sub-system, in which plants extract the water placed in the soil by the application sub-system and employ it to produce a crop output
- the drainage system, which removes excess surface and sub-surface water from the fields
Agriculture
The agricultural sub-system is the most complex of the five “physical” elements, in many ways, because it involves living plants; vagaries of weather, pests, and diseases; use of other variable inputs, such as fertilizer; external economic forces which set prices for inputs and outputs; and a cultivator who makes, more or less independently, a wide range of farm management decisions. The agricultural sub-system is the engine that drives the entire irrigated agricultural process. It pays the bills and justifies the investment in the other system elements. Without an effective agricultural sub-system, the other elements of the system have no purpose.
A key element of the system design process is the estimation of water requirements for the agricultural sub-system. Water requirement include
- the water that the crops will transpire during the plant growth process,
- water which evaporates from wet soil and plant surfaces, and
- water required to leach soluble salts from the crop root zone.
In addition to these requirements, allowance must be made for losses which occur during extraction, delivery, and distribution.
Water Supply and Delivery
The water supply system includes the facilities which extract water from a natural waterway, groundwater aquifer, or other source and deliver it to the areas to be irrigated. It may also include facilities for storing water over periods of months to years, though artificial storage is not necessarily a part of every water supply system.
Surface Water Supply
Surface water can be withdrawn from a river or stream by gravity or by pumping. In the case of gravity withdrawal, a weir is usually constructed in the waterway to raise the level of water and lead it into an artificial channel extending away from the river. For engineered structures, a very important analysis will be a flood frequency analysis to assess the risks of failure of the structure in response to high flow events. Structural failure can cause serious property damage and loss of life downstream.
If water is pumped from the river, pumps have to be situated such that they will always have access to water, even when water levels in the river are low, and also be above expected high water levels to avoid damage from flooding. Pumps can be either diesel or electrically driven, and relative prices of fuel and electricity, as well as the hours of availability and reliability of the grid electricity supply, will be important considerations in making this choice.
The discharge of most rivers varies significantly throughout the year and flows are often lowest during warm dry months, when the need for irrigation water is the greatest. Hence, this critical period of low flow and high demand requires careful study to design an irrigation system with the desired degree of reliability. This may lead to the decision to include storage in the system design by constructing a dam across the river or stream. Water is then released for irrigation use until the supply is exhausted. For any type of impoundment structure, it is important to carry out a flood frequency analysis and to construct a spillway that will pass the design flood.
An irrigation water source of growing importance is reclaimed urban wastewater. Where water is scarce, urban water systems will usually have priority over other uses. However, urban water systems “consume” only 10 to 20 percent of the water they withdraw, with the rest usually returned to natural watercourses, preferably after being processed in a wastewater treatment plant. These sources thus constitute an extremely reliable source of water for irrigation use, assuming that the water meets appropriate water quality standards. This synergism may also suggest projects which combine investments in wastewater treatment and irrigation system development.
Groundwater Supply
Groundwater can be withdrawn from open (dug) or drilled wells. Open wells are typically 1 to 2 meters in diameter, permitting the well diggers to work from inside the well as it descends. Open wells are typically dug in alluvium and are relatively shallow. Their large circumference provides a relatively large area into which groundwater can seep; however, they typically do not penetrate very far below the water table and their yields are thus limited. Most often, dug wells are utilized by an individual farmer or a small group of farmers.
Drilled wells are created by inserting a pipe down through the ground until it penetrates the water table. Drilling techniques vary from simple manually operated percussion or jetting systems to large truck-mounted rotary or cable tool rigs. Tubewells are usually cased with a steel pipe to prevent the sides from collapsing, though sometimes, for very shallow wells, inexpensive local materials such as bamboo are used for this purpose. The casing features slots or holes where it passes through water-bearing strata to allow water to infiltrate into the casing. Drilled wells can be of much higher capacity than dug wells, depending on local conditions, and can serve relatively large irrigation systems if aquifer yields are high.
Water Distribution
Once water reaches the edge of the area to be irrigated, it must be divided into smaller streams and distributed to individual farms and fields. Moreover, while water flow through the delivery facility from the source will generally be continuous, flows in the smaller distribution channels may be intermittent, responding either to requests from farmers or following a regular schedule.
The layout of open distribution channels can follow different patterns. Where the land to be irrigated is flat, a large slightly elevated main channel may have smaller channels branching to either side. On the other hand, if the land is sloping, the main channel may follow a hillside contour and deliver water only to the downhill side of the canal. Such contour canals are common when water is delivered to the flanks of hills on either side of a river valley, with the irrigated land commanded by the canal lying between the canal and the river below. In either case, and depending on the size of the area to be irrigated and the size of the farms being served, the canals branching from the main canal may subdivide into a hierarchy of ever smaller canals.
In addition to topography, a primary design parameter will be whether a particular level of canal is to provide continuous or intermittent flow. This matters a great deal, since a canal which flows intermittently will need to have a larger capacity than one which serves the same area on a continuous basis. This is so because if the canal is operated on a rotational schedule of, say, 10 days on and 10 days off, it must deliver enough water for 20 days of crop use in the 10 days during which it flows. It must thus have twice the capacity of one which will flow continuously. Often, flows in larger canals are continuous, and then at some point, delivery shifts to an intermittent one based on farmer demand or a rotational schedule to facilitate management by both the farmer and system operators.
Water Application
Once water reaches a farm, it is the farmer’s responsibility to apply it to his or her crops. As described earlier, the farmer needs water to meet crop evapotranspiration (ET) requirements, and possibly for land preparation and leaching of salts from the soil. To meet crop ET requirements and obtain optimal yields, the root zone of the crop must be kept within a fairly narrow range of moisture contents throughout the growing season. The depth of the root zone can vary from a few inches for some vegetable crops to many feet for tree crops, alfalfa, and others. Putting water into the root zone is the job of the water application system.
There are a number of different techniques for doing this. At one extreme, the entire surface of a field can be flooded to allow water to penetrate into the soil profile all across the field. At the other end of the spectrum, water can be inserted below the soil surface with a sub-surface drip system directly into the portion of the soil profile exploited by the roots of each plant, leaving the soil surface dry. Different techniques have different degrees of operational efficiency, and different costs. Costs tend to be inversely related to the efficiency of the technology, but because the application technology also affects crop yield and product quality, this relationship is not a simple one and requires careful analysis.
Drainage
Soils that are naturally well-drained may not require artificial drainage following the installation of an irrigation system. In other situations, however, the addition of irrigation water to an agricultural environment may require artificial drainage to remove excess water. The excess could be either surface water that runs off the ends of fields as a result of irrigation or heavy precipitation events, or groundwater that threatens to create waterlogged conditions in the crop root zone. Open drains leading back to a river may be dug to alleviate either situation and are usually the least expensive form of drainage.
However, open drains occupy extensive field areas, reducing the area available for growing crops. Also, in heavy soils, they may be ineffective at removing sub-surface water. In this case, tile drainage is required. “Tile” drainage today usually consists not of clay tile, but of perforated plastic pipe in long rolls which is installed in trenches, usually in a gravel envelope, and then reburied. These tile lines are connected to a collector and then allowed to flow into a natural waterway downslope.
References
Svendsen, Mark. 2012. Introduction to irrigation project design: a primer for non-engineers. USAID.
