1.1 The perspective and objectives of irrigation
1.2 Irrigation methods and their selection
1.3 Advantages and disadvantages of surface irrigation
A reliable and suitable irrigation water supply can result in vast improvements in agricultural production and assure the economic vitality of the region. Many civilizations have been dependent on irrigated agriculture to provide the basis of their society and enhance the security of their people. Some have estimated that as little as 15-20 percent of the worldwide total cultivated area is irrigated. Judging from irrigated and non-irrigated yields in some areas, this relatively small fraction of agriculture may be contributing as much as 3040 percent of gross agricultural output.
Effective agronomic practices are essential components of irrigated systems. Management of the soil fertility, cropping selection and rotation, and pest control may make as much incremental difference in yield as the irrigation water itself. Irrigation implies drainage, soil reclamation, and erosion control. When any of these factors are ignored through either a lack of understanding or planning, agricultural productivity will decline. History is absolutely certain on this point.
Irrigated agriculture faces a number of difficult problems in the future. One of the major concerns is the generally poor efficiency with which water resources have been used for irrigation. A relatively safe estimate is that 40 percent or more of the water diverted for irrigation is wasted at the farm level through either deep percolation or surface runoff. These losses may not be lost when one views water use in the regional context, since return flows become part of the usable resource elsewhere. However, these losses often represent foregone opportunities for water because they delay the arrival of water at downstream diversions and because they almost universally produce poorer quality water. One of the more evident problems in the future is the growth of alternative demands for water such as urban and industrial needs. These uses place a higher value on water resources and therefore tend to focus attention on wasteful practices. Irrigation science in the future will undoubtedly face the problem of maximizing efficiency.
Irrigation in arid areas of the world provides two essential agricultural requirements: (1) a moisture supply for plant growth which also transports essential nutrients; and (2) a flow of water to leach or dilute salts in the soil. Irrigation also benefits croplands through cooling the soil and the atmosphere to create a more favourable environment for plant growth.
The method, frequency and duration of irrigations have significant effects on crop yield and farm productivity. For example, annual crops may not germinate when the surface is inundated causing a crust to form over the seed bed. After emergence, inadequate soil moisture can often reduce yields, particularly if the stress occurs during critical periods. Even though the most important objective of irrigation is to maintain the soil moisture reservoir, how this is accomplished is an important consideration. The technology of irrigation is more complex than many appreciate. It is important that the scope of irrigation science not be limited to diversion and conveyance systems, nor solely to the irrigated field, nor only to the drainage pathways. Irrigation is a system extending across many technical and non-technical disciplines. It only works efficiently and continually when all the components are integrated smoothly.
1.2.3 Topographical characteristics
1.2.5 Water supply
1.2.7 Social influences
1.2.8 External influences
There are three broad classes of irrigation systems: (1) pressurized distribution; (2) gravity flow distribution; and (3) drainage flow distribution. The pressurized systems include sprinkler, trickle, and the array of similar systems in which water is conveyed to and distributed over the farmland through pressurized pipe networks. There are many individual system configurations identified by unique features (centre-pivot sprinkler systems). Gravity flow systems convey and distribute water at the field level by a free surface, overland flow regime. These surface irrigation methods are also subdivided according to configuration and operational characteristics. Irrigation by control of the drainage system, subirrigation, is not common but is interesting conceptually. Relatively large volumes of applied irrigation water percolate through the root zone and become a drainage or groundwater flow. By controlling the flow at critical points, it is possible to raise the level of the groundwater to within reach of the crop roots. These individual irrigation systems have a variety of advantages and particular applications which are beyond the scope of this paper. Suffice it to say that one should be familiar with each in order to satisfy best the needs of irrigation projects likely to be of interest during their formulation.
Irrigation systems are often designed to maximize efficiencies and minimize labour and capital requirements. The most effective management practices are dependent on the type of irrigation system and its design. For example, management can be influenced by the use of automation, the control of or the capture and reuse of runoff, field soil and topographical variations and the existence and location of flow measurement and water control structures. Questions that are common to all irrigation systems are when to irrigate, how much to apply, and can the efficiency be improved. A large number of considerations must be taken into account in the selection of an irrigation system. These will vary from location to location, crop to crop, year to year, and farmer to farmer. In general these considerations will include the compatibility of the system with other farm operations, economic feasibility, topographic and soil properties, crop characteristics, and social constraints (Walker and Skogerboe, 1987).
The irrigation system for a field or a farm must function alongside other farm operations such as land preparation, cultivation, and harvesting. The use of the large mechanized equipment requires longer and wider fields. The irrigation systems must not interfere with these operations and may need to be portable or function primarily outside the crop boundaries (i.e. surface irrigation systems). Smaller equipment or animal-powered cultivating equipment is more suitable for small fields and more permanent irrigation facilities.
The type of irrigation system selected is an important economic decision. Some types of pressurized systems have high capital and operating costs but may utilize minimal labour and conserve water. Their use tends toward high value cropping patterns. Other systems are relatively less expensive to construct and operate but have high labour requirements. Some systems are limited by the type of soil or the topography found on a field. The costs of maintenance and expected life of the rehabilitation along with an array of annual costs like energy, water, depreciation, land preparation, maintenance, labour and taxes should be included in the selection of an irrigation system.
Topography is a major factor affecting irrigation, particularly surface irrigation. Of general concern are the location and elevation of the water supply relative to the field boundaries, the area and configuration of the fields, and access by roads, utility lines (gas, electricity, water, etc.), and migrating herds whether wild or domestic. Field slope and its uniformity are two of the most important topographical factors. Surface systems, for instance, require uniform grades in the 0-5 percent range.
The soil's moisture-holding capacity, intake rate and depth are the principal criteria affecting the type of system selected. Sandy soils typically have high intake rates and low soil moisture storage capacities and may require an entirely different irrigation strategy than the deep clay soil with low infiltration rates but high moisture-storage capacities. Sandy soil requires more frequent, smaller applications of water whereas clay soils can be irrigated less frequently and to a larger depth. Other important soil properties influence the type of irrigation system to use. The physical, biological and chemical interactions of soil and water influence the hydraulic characteristics and filth. The mix of silt in a soil influences crusting and erodibility and should be considered in each design. The soil influences crusting and erodibility and should be considered in each design. The distribution of soils may vary widely over a field and may be an important limitation on some methods of applying irrigation water.
The quality and quantity of the source of water can have a significant impact on the irrigation practices. Crop water demands are continuous during the growing season. The soil moisture reservoir transforms this continuous demand into a periodic one which the irrigation system can service. A water supply with a relatively small discharge is best utilized in an irrigation system which incorporates frequent applications. The depths applied per irrigation would tend to be smaller under these systems than under systems having a large discharge which is available less frequently. The quality of water affects decisions similarly. Salinity is generally the most significant problem but other elements like boron or selenium can be important. A poor quality water supply must be utilized more frequently and in larger amounts than one of good quality.
The yields of many crops may be as much affected by how water is applied as the quantity delivered. Irrigation systems create different environmental conditions such as humidity, temperature, and soil aeration. They affect the plant differently by wetting different parts of the plant thereby introducing various undesirable consequences like leaf burn, fruit spotting and deformation, crown rot, etc. Rice, on the other hand, thrives under ponded conditions. Some crops have high economic value and allow the application of more capital-intensive practices. Deep-rooted crops are more amenable to low-frequency, high-application rate systems than shallow-rooted crops.
Beyond the confines of the individual field, irrigation is a community enterprise. Individuals, groups of individuals, and often the state must join together to construct, operate and maintain the irrigation system as a whole. Within a typical irrigation system there are three levels of community organization. There is the individual or small informal group of individuals participating in the system at the field and tertiary level of conveyance and distribution. There are the farmer collectives which form in structures as simple as informal organizations or as complex as irrigation districts. These assume, in addition to operation and maintenance, responsibility for allocation and conflict resolution. And then there is the state organization responsible for the water distribution and use at the project level.
Irrigation system designers should be aware that perhaps the most important goal of the irrigation community at all levels is the assurance of equity among its members. Thus the operation, if not always the structure, of the irrigation system will tend to mirror the community view of sharing and allocation.
Irrigation often means a technological intervention in the agricultural system even if irrigation has been practiced locally for generations. New technologies mean new operation and maintenance practices. If the community is not sufficiently adaptable to change, some irrigation systems will not succeed.
Conditions outside the sphere of agriculture affect and even dictate the type of system selected. For example, national policies regarding foreign exchange, strengthening specific sectors of the local economy, or sufficiency in particular industries may lead to specific irrigation systems being utilized. Key components in the manufacture or importation of system elements may not be available or cannot be efficiently serviced. Since many irrigation projects are financed by outside donors and lenders, specific system configurations may be precluded because of international policies and attitudes.
The preceding discussion of factors affecting the choice of irrigation systems at the farm level is not meant to be exhaustive. The designer, evaluator, or manager of irrigation systems should be aware of the broader setting in which irrigated agriculture functions. Ignorance has led to many more failures or inadequacies than has poor judgement or poor training.
As the remainder of this guide deals with specific surface irrigation issues, one needs to be reminded that much of the engineering practice is art rather than science. Experience is often a more valuable resource than computational skill, but both are needed. It is a poor engineering practice that leaves perfectly feasible alternatives just beyond one's perspective.
The term 'surface irrigation' refers to a broad class of irrigation methods in which water is distributed over the field by overland flow. A flow is introduced at one edge of the field and covers the field gradually. The rate of coverage (advance) is dependent almost entirely on the differences between the discharge onto the field and the accumulating infiltration into the soil. Secondary factors include field slope, surface roughness, and the geometry or shape of the flow cross-section.
The practice of surface irrigation is thousands of years old. It collectively represents perhaps as much as 95 percent of common irrigation activity today. The first water supplies were developed from stream or river flows onto the adjacent flood plain through simple check-dams and a canal to distribute water to various locations where farmers could then allocate a portion of the flow to their fields. The low-lying soils served by these diversions were typically high in clay and silt content and tended to be most fertile. The land slope was normally small because of the structure of the flood plain itself.
With the advent of modern equipment for moving earth and pumping water, surface irrigation systems were extended to upland areas and lands quite separate from the flood plain of local rivers and streams. These lands tend to have more variable soils and topographies, are usually better drained, and may be naturally less fertile. Thus, these lands usually require greater attention to design and operation.
Surface irrigation offers a number of important advantages at both the farm and project level. Because it is so widely utilized, local irrigators generally have at least minimal understanding of how to operate and maintain the system. In addition, surface systems are often more acceptable to agriculturalists who appreciate the effects of water shortage on crop yields since it appears easier to apply the depths required to refill the root zone.
The second advantage of surface irrigation is that these systems can be developed at the farm level with minimal capital investment. The control and regulation structures are simple, durable and easily constructed with inexpensive and readily-available materials like wood, concrete, brick and mortar, etc. Further, the essential structural elements are located at the edges of the fields which facilitates operation and maintenance activities. The major capital expense of the surface system is generally associated with land grading, but if the topography is not too undulating, these costs are not great. Recent developments in surface irrigation technology have largely overcome the irrigation efficiency advantage of sprinkler and trickle systems. An array of automating devices roughly equates labour requirements. The major trade-off between surface and pressurized methods lies in the relative costs of land levelling for effective gravity distribution and energy for pressurization. Energy requirements for surface irrigation systems come from gravity. This is a significant advantage in today's economy.
Another advantage of surface systems is that they are less affected by climatic and water quality characteristics. Even moderate winds can seriously reduce the effectiveness of sprinkler systems. Sediments and other debris reduce the effectiveness of trickle systems but may actually aid the performance of the surface systems. Salinity is less of a problem under surface irrigation than either of these pressurized systems.
There are other advantages specific to individual regions that might be mentioned. Surface systems are better able to utilize water supplies that are available less frequently, more uncertain, and more variable in rate and duration. The gravity flow system is a highly flexible, relatively easily-managed method of irrigation.
There is one disadvantage of surface irrigation that confronts every designer and irrigator. The soil which must be used to convey the water over the field has properties that are highly varied both spatially and temporally. They become almost undefinable except immediately preceding the watering or during it. This creates an engineering problem in which at least two of the primary design variables, discharge and time of application, must be estimated not only at the field layout stage but also judged by the irrigator prior to the initiation of every surface irrigation event. Thus while it is possible for the new generation of surface irrigation methods to be attractive alternatives to sprinkler and trickle systems, their associated design and management practices are much more difficult to define and implement.
Although they need not be, surface irrigation systems are typically less efficient in applying water than either sprinkler or trickle systems. Many are situated on lower lands with heavier soils and, therefore, tend to be more affected by waterlogging and soil salinity if adequate drainage is not provided. The need to use the field surface as a conveyance and distribution facility requires that fields be well graded if possible. Land levelling costs can be high so the surface irrigation practice tends to be limited to land already having small, even slopes.
Surface systems tend to be labour-intensive. This labour need not be overly skilled. But to achieve high efficiencies the irrigation practices imposed by the irrigator must be carefully implemented. The progress of the water over the field must be monitored in larger fields and good judgement is required to terminate the inflow at the appropriate time. A consequence of poor judgement or design is poor efficiency.
One sometimes important disadvantage of surface irrigation methods is the difficulty in applying light, frequent irrigations early and late in the growing season of several crops. For example, in heavy calcareous soils where crust formation after the first irrigation and prior to the germination of crops, a light irrigation to soften the crust would improve yields substantially. Under surface irrigation systems this may be unfeasible or impractical as either the supply to the field is not readily available or the minimum depths applied would be too great.