7.2 Surge flow
7.4 Adaptive control systems
7.5 Water supply management
Surface irrigation is the historical choice of irrigators worldwide and will undoubtedly remain so. Surface practices have for the most part changed very little in centuries. Two questions arise in this connection. First, is there a need to do things differently, and second, is there a means to do so?
The stimuli to discover and implement improved surface irrigation practices are numerous and varied. Perhaps the most important are population growth, urbanization and industrialization because existing water resources have largely been committed for these uses. Water shortages are often addressed as arbitrary restrictions in supply but it would be wiser to introduce practices to conserve water. Since agriculture usually imposes the largest water demand, improved irrigation efficiency should become the centrepiece of conservation strategies. Energy resources and labour availability are declining in most countries. Reduced energy supplies may tend to restrict the use of sprinkler and trickle systems. A lack of labour will prompt the adaptation and use of automation as part of the operation of irrigation systems. The immediate and long-term futures of most irrigated regions also appear to depend heavily on improved irrigation practices.
The probability that major technical or operational innovations for existing surface irrigation systems will be made is low. Over the thousands of years during which surface irrigation has been practiced, the alternatives for diverting the water onto the field have been clearly identified. There are, however, four areas where significant innovations have or will be made; these are: (1) precision land levelling in order for water movements to be more uniform and manageable; (2) automation for headland facilities; (3) supervisory and adaptive or feedback control systems; and (4) water supply control and management.
In precision land levelling, the most important innovation has been the laser guidance and control system applied to mechanized land levelling equipment. Precision has increased by at least a factor of 10 and results are impressive in terms of efficiency and production. This topic was discussed in Section 6 and need not be mentioned here except to conclude that the technology's expansion into the developing countries will make a significant improvement in surface irrigation.
Automation of surface irrigation headland facilities is difficult. Each irrigation behaves differently which limits the standardization necessary for effective automation. Thus, a great deal of research and development notwithstanding, surface irrigation automation has not been widely successful. However, a series of new concepts has emerged in the last decade that offers a better opportunity. Some of the important US references on automation include Haise et al. (1980), Dedrick and Erie (1978), and Humpherys (1969, 1971, and 1983). Perhaps one of the more interesting is the 'surge flow' concept developed at Utah State University. For many years automation has attempted to manage discharge directly. These efforts have not been very successful, but the surge flow concept manages discharge indirectly by regulating on an off time and by so doing has made the management problem tractable. A similar innovation involving indirect flow regulation is 'cablegation' described by Kemper et al. (1985). Both surge flow and cablegation will be described below to illustrate the idea of new approaches to automation. They are by no means the only automating measures now available, but are illustrated here to show the reader two alternatives for improving water control by controlling time rather than discharge.
Control systems and water supply management are opportunities to deal with the uncertainty associated with variable infiltration and will be considered separately below.
7.2.1 Effects of surging on infiltration
7.2.2 Effects of surging on surface flow hydraulics
7.2.3 Surge flow systems
In 1979, Stringham and Keller (1979) reported a new approach for automating surface irrigation systems in which problems with slow advance and excessive surface runoff occur. The approach was called 'surge flow' to describe the hydraulic regime of the flow over the field. In 1986, a US patent was granted to Professors Keller and Stringham of Utah State University for the concept. A trademark registration was issued for the term 'Surge Flow' although by the time of writing this bulletin, the term has become widespread as a surface irrigation water management concept. Consequently, the use of 'surge flow' in this guide will not attempt to distinguish the proper use.
Under the surge flow regime, an irrigation is accomplished through a series of individual pulses of water onto the field such that, instead of the typically found advance-wetting-depletion-recession trajectory shown in Figure 1 in normal surface irrigation conditions, it looks like that in Figure 71. Thus instead of providing a continuous flow onto the field for say six hours, a surge flow regime would apply six 1 hour 'surges'. Each surge is characterized by a cycle time and a cycle ratio. The cycle time is comprised of an on-time and an off-time related by the cycle ratio which is the ratio of on-time to the cycle time. The cycle time can range from as little as one minute to as much as several hours. Cycle ratios typically range from 0.25 to 0.75. By regulating these two parameters, a wide range of surge flow regimes can be produced which can significantly improve irrigation efficiency and uniformity.
Figure 71. Typical surge flow advance-recession trajectory
It is perhaps worth noting that surge flow, while appearing quite simple, is nevertheless an advanced irrigation technology.- The design and evaluation require a level of hydraulics beyond this guide and the equipment needed to implement surge flow fully is often feasible only in large farming operations. This is not to imply that surge flow cannot or should not be considered in developing countries, only that special adaptations will be necessary.
Since its introduction in 1979, surge flow has been tested on nearly every type of surface irrigation system and over the full range of soil types. Results vary depending on the selection of cycle time, cycle ratio and discharge. But in nearly every case, the intermittent application significantly reduces infiltration rates and/or substantially reduces the time necessary for the infiltration rates to approach the final or 'basic' rate. To effect infiltration rates, the flow must completely drain from the field between surges. If the period between surges is too short, the individual surges overlap or coalesce and the infiltration effects are generally not created.
Research shows that the surging effect on infiltration is primarily due to the consolidation of the thin layer of fine material deposited in the bottom of the furrow or on the border or basin surface by the destruction of soil aggregate and erosion caused by the water flow. As the water drains from the field between surges, the negative pressure that develops in the soil consolidates the surface layer, collapsing the larger pores, attracting small particles into the lattice between larger particles, and orienting clay and silt into a layered structure. As a result the permeability of the field surface is reduced and thereafter infiltration rates are lowered. The reductions in surface permeability seem to be more pronounced in sandy loam soils than in clay loam soils. The rate of aggregate wetting and erosion affect the thickness and extent of the surface layer.
Evidence of the consolidation of the fine layer between surges can usually be observed in the field 5-15 minutes after the water has completely drained from the field. Tension cracks form between the layers of fine material and those less disturbed by the flow. When water is again introduced into the field, sediments are deposited in these cracks as they begin to swell shut, thereby further compacting the surface layer.
The effect of having reduced the infiltration rates over at least a portion of the field is that advance rates are increased. Generally, less water is required to complete the advance phase by surge flow than with continuous flow. Surging is often the only way to complete the advance phase in high intake conditions like those following planting or cultivation. As a result, intake opportunity times over the field are more uniform. However, since results will vary among soils, type of surface irrigation, and the surge flow configuration, tests should be conducted in areas where experience is lacking in order to establish the feasibility and format for using surge flow.
The hydraulic regime of a surge flow system is composed of two parts: (1) the distinct surge phase; and (2) the coalesced surge phase. The distinct surge phase is noted above. Each pulse of water advances and recedes over a portion or all of the field as shown in Figure 71. This phase is used during the advance phase for the entire field, i.e. during the time needed to wet the entire surface of the field. Surges during the distinct phase must be of sufficient duration and discharge to fill cracks and depression storage along the pathway so that there is enough volume and energy to continue advancing at an adequate rate over the succeeding field section, but short enough to limit cumulative intake and maximize or minimize the infiltration reduction.
In the coalesced phase, the individual surges run together, overlap and result in a nearly steady flow in the downstream sections of the field. In this situation, the flow rate below the point of convergence is about one-half of the instantaneous rate at the field inlet. If the cycle ratios are reduced, the flow in the continuous flow reaches will be correspondingly reduced. It is therefore possible to adjust the cycle ratios until practically no surface runoff occurs. The reader thus immediately sees the coalesced phase as being exactly equivalent to the cutback phase described in previous sections for furrow irrigation. Indeed, the original research of Stringham and Keller (1979) was directed toward the development of an alternative cutback method. The advantages of surge flow during the advance phase came as a welcome surprise.
Thus, by combining the distinct and coalesced phases of surge flow into one system, the solution of the long-standing surface irrigation dilemma is available, a high flow for the advance phase and a low flow for the wetting phase.
There are basically two field systems commercially available for surge flow, both limited at present to furrow irrigation. The first is shown in Figure 72 and will be described here as the 'dual line' system. Water is supplied to the field generally through a buried pipeline which connects to surface gated pipe through a riser and valve. The valve, shown schematically in Figure 73, is automated to switch the flow between two sets. Surging is accomplished by alternating the flow between the two sets. When these two are finished, the entire flow is directed to another riser and valve by the irrigator. The dual line system is in widespread use in the USA where irrigators already have a gated pipe furrow irrigation system in place. They only need to purchase the automated valve to implement fully a surge flow regime. The costs for these systems where the distribution and gated pipe already exist can be as low as US$50 per hectare.
Figure 72. Schematic diagram of a dual line surge flow furrow irrigation system (redrawn from Humpherys, 1987)
Figure 73. Configuration of one automated surge flow valve for the dual line system (redrawn from Humpherys, 1987)
The second field configuration is the single line system shown in Figure 74. A single gated pipe is connected to the water supply and individual outlets along with pipe are controlled by small hydraulic, pneumatic, or electric valves which are organized in banks and sets as shown and controlled by a single controller.
Figure 74. Schematic of the single line surge flow system (redrawn from Humpherys, 1987)
The single line system is economic for new systems where all of the field facilities need to be provided. It also tends to be more economic where only the gated pipe is available and the decision of the irrigator is whether or not to put in a buried supply line and then use the bi-directional valve or to put automated gates on the gated pipe and use the single line concept. In many cases, the single line system will be more flexible than a dual line system in terms of irrigating an entire field.
Adaptation for border and basin systems can be made by automating existing control structures and perhaps by a new control structure like that of Ismail and Westesen (1984). Single or dual line surge flow systems can also be utilized where open channel systems are present (Testezlaf et al. 1985).
The cablegation system illustrated graphically in Figure 75 was developed by the Soil and Water Management Research Unit of the US Department of Agriculture's laboratory at Kimberly, Idaho (Kemper et al. 1985). The system involves a pipe with fixed or adjustable outlets which is placed on a precise gradient. An adjustable plug is placed inside the pipe and connected by a cable to a winch-type unit at the pipe inlet. The winch unit includes a speed control feature.
Figure 75. Schematic diagram of a cablegation furrow irrigation system
Hydraulically, a cablegation system operates in the free surface flow regime upstream of the travelling plug except immediately adjacent to it. In the region near the plug, the flow is slowed and expands to fill the pipe. Thus, in the uniform open channel flow region of the pipe, the water surface is below the outlets which are therefore shut off from the field. Near the plug, the water level rises above the outlets to supply the field. The unique feature of the cablegation system is the high outlet flows nearer the plug. This feature gives the advance phase discharge needed to facilitate field coverage. As the plug moves downstream, the outlet flow is cutback to allow soaking time without causing excessive surface runoff.
Cablegation and surge flow are two examples of an alternative approach to managing surface irrigation. After years of trying to regulate discharges unsuccessfully, these two methods accomplish this end by managing time and equipment speed.
The most limiting problem associated with design and management of all types of surface irrigation systems is the fact that the infiltration characteristics are unpredictable. They change after each irrigation, from season to season, and following each cultivation. They change over a period of years as the content of organic matter changes, as salinity in both the water and soil changes, and as irrigation methods are altered. It should be clear that if infiltration rates were predictable, the time of advance and irrigation efficiency would be quite predictable and this would allow much better management and automation.
The premise of the adaptive control system is that infiltration and therefore advance time, cutoff time, and application efficiency can be forecast during the early stages of the irrigation and that actions can be taken shortly thereafter if the outcome of present settings is not going to be adequate. Using a volume balance hydraulic concept similar to that discussed in Section 4.3.4, Burt et al. (1982) outlined a procedure in which infiltration coefficients could be deduced from rate of advance during the first watering set and then used to refine the set size, flow and times during subsequent sets to improve efficiency substantially. Reddell and Latimer (1986) took the next step and coupled the volume balance inference of infiltration to real time conditions with a microcomputer located near the field in which sensor readings are processed to determine when the advance phase will be completed and how the system should be set for the cutback flow. Work soon to be reported by Busman (1987) and others now working on the computer software and field verification will indicate the application of advance hydraulic models to the same problem except the infiltration will be deduced from advance sensor readings near the field inlet. This will allow settings to be changed to improve the performance of irrigation on the current set as well as those subsequent.
It is clear from evaluating hydraulic principles that if the discharge onto a field varies from the design or values given by the manager, the performance will be significantly affected. If the discharge is reduced, it is likely that uniformity will suffer and deep percolation losses will increase. If the flow is unexpectedly increased, runoff losses will increase or ponding; on the field surface will be excessive. When the water supply is uncertain, irrigators are reluctant to invest heavily in costly agricultural inputs like high-yielding seed varieties, fertilizers and cultivation practices. When water deliveries to the farm are not timely, crop yields tend to decline due to crop stress or overwatering.
The irrigator usually has very little actual control of the problems noted above unless his water supply is from a well, he is near the headworks of an irrigation project, or he is very influential in the operation and maintenance of the irrigation facilities upstream of his farm. Thus, an overriding concern in developing efficient and effective surface irrigation systems is the operation of the irrigation project itself. The management of the collection, storage and conveyance systems in a project is a critical factor in the performance and production of the surface irrigation system at the farm level. To ignore this linkage is to invite low production, waterlogging and salinity, pollution of both surface and subsurface water resources, poverty of the agricultural sector, and numerous other well-known irrigation problems. Yet, this linkage has rarely if ever been established effectively, and as one would expect, the problems are easy to identify.
Irrigation project management for improved on-farm irrigation and efficiency is beyond the scope of this guide, but it brings into focus the future direction of water management. The technical principles of irrigation are fairly well developed, understood, and modelled. Most research and development efforts are aimed at refining and expanding engineering, soil and plant science, and economic knowledge of individual processes and interactions that are already well defined. The weakness therefore in irrigation science and application lies primarily in the management of the irrigation system as a whole and not the design and operation of the irrigation system's individual components (fields, farms, canals and watercourses, reservoirs, dams and headworks, etc.). The hydraulics of surface irrigation, for example, continue to receive research attention even though the fundamental relationships have been established long since. It is important that this research continue in-order that the application of the research be made more accurate and universal.