PLANNING AND SYSTEM SELECTION

Transcription

1 CHAPTER 3 PLANNING AND SYSTEM SELECTION Kenneth H. Solomon (California Polytechnic State University, San Luis Obispo, California) A. M. El-Gindy (Ain-Shams University, Cairo, Egypt) Sagit R. Ibatullin (Research Institute of Water Economy,Taraz City, Kazakhstan) Abstract. This chapter reviews the process of planning for and selecting an irrigation system. Decision making involves the identification and evaluation of alternatives, and making and implementing the decision. The initial problem situation is improved through the cause-and-effect feedback loop, and a learning feedback loop influences future decision making. Irrigation system selection involves both objective and subjective components of problems, goals, and values, and available augmentations to the problem situation. An irrigation system in the broadest sense includes not only hardware, such as equipment, land improvements, water conveyance, and control structures, but technology and support infrastructure as well. These may include guides for equipment installation, operation, and maintenance, and plans for water management and irrigation-related crop husbandry, training, and extension. Planning for an irrigation system must consider a wide range of physical, economic, human, and social factors. Irrigation system selection must consider the capabilities, costs, and limitations of potential irrigation methods. This chapter presents a brief review of such factors for the most common surface, sprinkler, and microirrigation systems. Keywords. Decision making, Irrigation systems, Microirrigation, Planning, Selection, Sprinkler irrigation, Surface irrigation. 3.1 INTRODUCTION The process of planning for, and selection of, an irrigation system should be based upon an understanding of the goals and objectives to be met, a study of the resources available, a creative determination of the possible alternatives, and an evaluation of these alternatives according to economic and other criteria. This chapter presents factors to be addressed during these activities. Conceptual models for decision making and for irrigation system selection provide a framework for the review of individual factors. Irrigation system components beyond just hardware must be considered. Water supply and system capacity must be sufficient to meet requirements. Economic

2 58 Chapter 3 Planning and System Selection evaluation of irrigation options depends on the perspective of the evaluator and the goals and values of all those involved with the irrigation development. Capabilities, limitations, and evaluation factors for the most common types of irrigation systems are briefly discussed A Conceptual Model for Decision Making The decision-making process is illustrated in Figure 3.1. The process starts with a problem situation to be improved or solved. Therefore, a decision must be made. The situation at hand contains not only the actuality of the problem, but the potentiality of a number of alternative courses of action to improve the situation or solve the problem. These action alternatives exist whether or not we are clever enough to recognize them. Experience, judgment, and creativity increase our ability to identify the alternatives that exist. Part of this process is to recognize that certain constraints are acting on the situation with which we must deal. Notice that in the example of Figure 3.1, only alternatives B, C and D were identified. Alternatives A and E are in fact possibilities, but they were not thought of. Next, value system(s) are applied to evaluate those alternatives that have been identified. Though it may be helpful to quantify these evaluations as much as possible, value systems contain non-quantifiable elements. So, the final decision must take into account not only the quantitative score for each alternative, but some qualitative judgments as well. In the diagrammed example, for instance, alternative B was selected over C, even though C has a higher quantitative score: apparently the intangibles involved outweigh the six-point difference in score. Like many human endeavors, decision making is a cyclical process. In this case, two main feedback loops are identified. The primary feedback loop is the cause and effect loop. This is the way in which the chosen action will alter the current situation. Other aspects of the situation may be altered as well. The results of our decision will influence the list of alternatives for further action, either with regard to this problem or some future one. Problem Situation Action Alternatives A B C D E Identify Alternatives B C D Evaluate Alternatives Alt. Score B C D Make Decision Select Reject Reject B C D Experience Judgment Creativity Constraints Value System(s) Monitor Results of Decision Learning Process Cause and Effect Figure 3.1. A conceptual model of the decision process.

3 Design and Operation of Farm Irrigation Systems 59 An additional feedback loop is the learning process. By monitoring the results of a decision, we learn things. This learning increases our experience, judgment and perhaps creativity. It may alter the constraints (or the way we perceive the constraints) of the situation. Learning may allow us to identify action alternatives that we did not see before either for the problem just considered, or for future problems. The learning process will also have an effect on our value systems. Learning, living, and growing are inseparable and go hand in hand with the evolution of our value systems. Thus the learning process will affect the way in which we evaluate alternatives and make future decisions A Conceptual Model for Irrigation System Selection Decision making within an irrigation context, based on our understanding of the decision process in general, is illustrated in Figure 3.2. The selection process is drawn onto a conceptual blackboard, divided into four quadrants: right to left into Objective vs. Subjective; and top to bottom into the Existing Situation vs. things Brought to the Situation. (Note that the items listed under each general heading are for illustration only. The lists are not intended to be exhaustive, nor will all items listed necessarily apply to any given situation.) The objective part of the existing situation contains resources and problems. The resource category is subdivided into physical and people-related resources. Problems may be associated with resources (amount, timing, place, quality) or with constraints of various types which prevent some otherwise desirable action. The subjective part of the existing situation contains the goals and priorities we are trying to meet with this irrigation decision, and the values by which the alternatives and results will be judged. To the existing situation we bring items that we hope will solve the problems observed; the existing situation is augmented. This augmentation always involves several components: physical things such as irrigation equipment; people-related things such Objective Subjective Resources Physical People Soil/Water Irrigators/Others Climate Skills Cultivars Experience Energy Education Infrastructure Motivation Financial Markets Irrigation Augmentation Other Equipment Management Plans Training Programs New Problems Problems Resources: Not Enough Too Much Bad Timing Poor Quality Constraints Equipment Management Plans Training Programs New Problems Results Augmented Situation Allows New Matchup of Problems & Capacities for Solution Goals & Priorities Food Nutrition Jobs Exports Income New Net Benefits + Advantages Disadvantages Values Profit Health Employment Independence Technology Tradition Figure 3.2. A conceptual model for irrigation system selection. The items listed under each general heading are for illustration only. The lists are not intended to be exhaustive, nor will all items listed necessarily apply to any given situation.

4 60 Chapter 3 Planning and System Selection as management plans and training; and inevitably, we bring along some new problems to be dealt with. For example, irrigation adds a drainage requirement, or advanced equipment adds training and maintenance requirements. The results of our actions are formed out of the mix of the elements in the existing situation and the items brought to the situation in the augmentation. The augmented situation includes new capacities for solving the original problem, as well as new problems. However, the results box in Figure 3.2 is not the end. The results should be viewed in light of the goals and priorities, and the values of the decision makers to determine the net benefits. The net benefits box spans both the objective and subjective halves of the conceptual blackboard, reminding us (as in the model for decisions) that what we seek has both quantitative and qualitative aspects. This is the framework in which irrigation system selection should take place. These models do not constitute a selection process itself, but are rather a reminder of all the things beyond the engineering details that must be considered An Irrigation System More Than Just Hardware An irrigation system is commonly thought of in terms of its hardware aspects: infield equipment, water conveyance and control structures, and land improvements such as grading. It is important to realize, though, that a complete irrigation system includes additional aspects of technology and support infrastructure. A given technology includes not only relevant hardware, but the collection of knowledge necessary to accomplish the desired function. Technology includes facts, relationships, and rules for making decisions. In order to accomplish its function, an irrigation system must include equipment and a variety of related software. Specifically, the irrigation technology package must include guides for design, installation, operation, and maintenance of the equipment; plans for irrigation scheduling and water management; and plans for the crop husbandry practices impacted by irrigation. It must also include plans for irrigation-related training, research, development, and extension. Just as equipment requires operational knowledge and plans, it requires a support infrastructure as well. This infrastructure is partly physical and partly institutional. Physical elements include dams, canals, offices, and facilities for research and demonstration. Institutional elements include water rights establishment and enforcement bodies, irrigation districts, farmer cooperatives, repair and maintenance services, and extension organizations. The infrastructure is required to make available the water, other agronomic inputs, services, and technological information necessary for successful irrigation. In developed irrigation markets, the widespread existence and availability of technical irrigation information and infrastructure may lead to the misconception that irrigation system selection consists solely of equipment selection. Consider, for example, the introduction of microirrigation to a developing irrigation market. In this case, it is obvious that a choice of microirrigation equipment without a matching choice to fund and implement a suitable package of microirrigation information, plans, and the necessary support infrastructure is doomed to failure.

5 Design and Operation of Farm Irrigation Systems PLANNING FOR IRRIGATION Planning for irrigation consists of identifying and collecting information on relevant factors, followed by formulating and evaluating realistic options for irrigation systems. System here is taken in the broad sense, including elements of equipment, technical information, and infrastructure. As indicated in Figure 3.2, the initial planning phase certainly includes inventories of resources, problems, and the goals, priorities, and values of the decision makers. Table 3.1 lists a number of considerations that may have a bearing on irrigation decisions. A few of the key points listed are discussed further in the following sections Water Requirements and Water Supply A key point to consider in any irrigation plan is whether the water available is sufficient to adequately meet the water requirements for crop production. This comparison must be made in terms of both the total volume of water required and available during the year, and the peak rate at which water is used and supplied. The water supply must meet or exceed the minimum requirement less any effective precipitation. Effective precipitation is that portion of total precipitation which becomes available to support plant growth (ASAE, 1998a). The timing and rate of precipitation can influence its availability for plant growth. Rainfall during the growing season is ineffective if it runs off or percolates below the root zone. Precipitation falling outside the growing season is effective only to the extent that it is stored in the eventual crop root zone. Hence, precipitation resulting in runoff, deep percolation, or evaporation prior to the season is not effective. Precipitation and deep percolation occurring prior to the season may be effective in meeting part of the leaching requirement (see Section 3.2.2). The total volume of water required to produce a crop includes the water consumed during crop growth, water used to maintain a favorable salt balance in the root zone, and water used for certain husbandry practices such as germination, climate control, and beneficial vegetation such as windbreaks or cover crops. If the water supply does not meet the water volume requirement (reduced by any effective precipitation), irrigation options are limited. Either the supply must be augmented or the production plan altered, for example by using shorter-season varieties, eliminating water-consuming husbandry practices, or using a deficit irrigation schedule. The highest rate of crop water use depends both on climatic factors and on the timing of crop growth stages. Because weather varies from year to year, the peak-use rate will vary as well. It is expected that the peak-use rate determined from average values of weather variables will be exceeded one year in two. It may be prudent to study historical weather and base plans on a more conservative figure, for example the peak-use rate exceeded only one year in five, or one year in ten. The costs of both water supply and infield irrigation systems are sensitive to the peak flow rate, so it may not be economical to design for abnormally high peak-use rates that are expected to occur only rarely. The water supply and in-field irrigation systems should have the capacity to meet the design peak-use rate, adjusted to compensate for water application efficiency, and possibly adjusted again according to the strategies discussed in the following paragraphs. Note that application efficiency may vary throughout the year. For example, with surface irrigation systems, it may be difficult to uniformly apply the small amounts required by crops during early growth stages, thus reducing application efficiency. The application efficiency anticipated during the peak-use period should be used in this adjustment.

6 62 Chapter 3 Planning and System Selection Table 3.1. Factors for consideration in irrigation decision making. Physical Factors Crops, crop rotation Crop yields (especially if quality or quantity differs for different irrigation systems) Cultural practices Soils Texture, depth, uniformity Intake rate, water holding capacity Erosion potential Salinity Internal drainage Topography Water supply Water rights Source, delivery schedule Quantity available, reliability Current uses of water Water quality Salinity & other chemical constituents Suspended solids Climate (wind, heat, frost) Land value, availability Flood hazard Catastrophic weather events Water table Energy availability, reliability, and form Pests Infrastructure available Water supply & conveyance Agronomic inputs Equipment repair service Technological support Weather data Existing standards Equipment performance Design, installation Practices Common good practice Human Factors Labor Availability Skills, experience Education Potential for vandalism or theft Level of automatic control desired Economic Factors Crop value (price) Investment capital Foreign exchange, hard currency Credit Source & terms Interest rate For long term debt For operating loans Cash flow requirements Equipment life Costs & inflation Equipment Water, energy Other agricultural inputs Services Labor, various skill levels Supervision Management Operation & maintenance Repairs & replacement Warranties Incentives and subsidies Taxes Insurance Opportunity costs for inefficiencies Markets, export/import Impact of irrigation on related field and factory operations Potential for upgrades as conditions change Uncertainty Social Factors General values Specific goals and priorities Legal constraints Political issues Local cooperation and support Local and governmental expectations Environmental issues Quality standards Wildlife habitat Mitigation Health issues History of irrigation experience Biases and taboos

7 Design and Operation of Farm Irrigation Systems 63 Certain management strategies may allow additional adjustments in the design capacity of water supply and irrigation systems. Probably the most common of these is an increase in the design capacity to create an allowance for downtime, that is, time when the system is not operated so that it may be serviced, repaired, or maintained. For example, ASAE (1998b) recommended that the design capacity for microirrigation systems be sufficient to meet the peak-use rate in about 90% of the time available, or when operating no more than 22 hours per day. In some cases the schedule of electric power rates will impact downtime decisions. Another strategy is to plan for the use of stored soil water to meet the crop water needs during the peak-use period. Successful use of this strategy requires that the irrigation system be operated so that the available soil water reservoir is full (or nearly full) prior to entering the peak-use period. Peak-use water needs can be met by deliveries from the irrigation system and by planned depletion from the root zone. The amount that the design flow rate for the irrigation system can be decreased by this strategy depends on the amount of available water that can be stored in the crop root zone, and the duration of the peak-use period. This strategy carries the risk that an abnormally long peak-use period or unexpected system failures during or just prior to the peak-use period can result in unplanned stress to the crop due to insufficient water. In those areas where rainfall may be expected during the peak-use period, agronomic practices to increase the effectiveness of precipitation may reduce the required flow rate for irrigation and supply systems. Tillage methods that increase temporary surface storage of rainfall can increase effective precipitation by reducing runoff. Irrigation scheduling methods that don t completely refill the crop root zone can increase effective precipitation by reducing deep percolation. In cases where the water supply cannot match the peak-use rate, water storage in a pond or reservoir may solve the problem as long as the water supply volume is sufficient to meet the total water volume required by the crop. During the off-peak periods, water in excess of crop needs is diverted to storage. During the peak-use period, the normal water supply is augmented from stored water. Any action that increases the attainable application efficiency, such as the use of return-flow systems to capture and re-use surface irrigation runoff water, can help to meet design peak-use needs. For a given gross application rate, increasing the application efficiency will increase the system s net application rate. The advisability of these various water management strategies is frequently affected by the value of the crop and the sensitivity of the crop yield to water Need for Drainage Irrigation water inevitably contains salt, which is left behind when plants take water from the root zone for evapotranspiration. Thus, salts tend to accumulate unless leached away by excess water passing through the root zone. In humid regions the excess may be provided by rainfall, but in arid regions additional irrigation water (in excess of the crop s consumptive use) is required for the purpose of leaching. Unless this excess or leaching water is removed, high water tables and waterlogged soils can result. High water tables can lead to soil salination due to upward movement of water and salt. Drainage, natural or artificial, is necessary to carry away excess or leaching water applied to maintain a favorable salt balance in the root zone. Irrigation planning should include an investigation of the potential need for artificial drainage. Equipment, technology, and infrastructure for the drainage system, if

8 64 Chapter 3 Planning and System Selection needed, should be included in the economics and evaluation of irrigation options Irrigation Economics An important part of irrigation planning and system selection involves economic evaluation of the alternatives. It is tempting to consider only the most obvious, initial costs, but this is too simplistic an approach for sound decision making. Life cycle costing, which treats operation and maintenance costs as well as initial costs, is the preferred approach. The basic price of the equipment or improvement (such as land grading) is not the only initial cost. To this may be added sales tax, import duties or tariffs, and initial loan fees if the purchase is to be financed with a loan. Freight and delivery charges may also apply, and can be particularly important when considering systems utilizing imported equipment. Freight charges are usually based on the weight of the material to be shipped and the volume it occupies. Equipment assembly and installation may involve additional initial expenses. Important initial costs that are sometimes overlooked are expenses for a stock of spare parts and for initial training. Particularly in remote areas or in developing markets without a fully mature maintenance and repair infrastructure, initial equipment purchases should include an inventory of spare parts. An initial effort to train irrigators and equipment operators may be necessary if the anticipated benefits of the chosen irrigation system are to be achieved. The energy provider may assess an initial charge to bring service (particularly electric power lines) to the farm site. Annual costs to operate and maintain an irrigation system include charges for labor, water, and energy. Allowances for preventative maintenance and repairs, repair parts, taxes, insurance, and ongoing training should also be included in the annual operating budget. Different irrigation options often involve trade-offs between initial and annual operating costs. For example, larger pipe sizes have higher initial costs, but reduce friction losses and hence may lower operating costs for pumping energy. Less equipmentintensive systems may have reduced initial costs, but higher operating costs for irrigation labor. In order to compare the full, life cycle cost of various irrigation options, some method is needed to place initial and annual irrigation costs on the same basis. A common approach is to convert the initial costs to an equivalent annual cost. This is done by multiplying the initial cost by a factor (often called the capital recovery factor) that depends on the interest rate and the expected economic life for the initial cost item. The capital recovery factor may be calculated according to Equation 3.1: n i( 1+ i ) CRF = (3.1) n ( 1+ i ) 1 where CRF = capital recovery factor, dimensionless i = annual interest rate, decimal n = expected economic life, years Table 3.2 lists economic lives for different irrigation items and conditions. Increasing the interest rate or reducing the economic life will increase the capital recovery factor and the annualized cost. The total annualized cost for an irrigation option is the sum of its annualized initial cost and its annual operating cost.

9 Design and Operation of Farm Irrigation Systems 65 Table 3.2. Expected economic lives for irrigation equipment (years). Adapted from McCulloch et al. (1967). Item Maximum [a] Conservative [b] Difficult [c] Irrigation systems Side roll Center pivot, linear move, LEPA Surface irrigation systems Individual items Farm reservoirs (heavy silting) Farm reservoirs (light silting) Well Irrigation pump Pump power units Diesel Gas, propane Gasoline Electric motor Water conveyance Open ditches (unlined) Concrete structures Concrete pipe systems Wood flumes Large aluminum pipe (lightweight) Coated steel pipe (underground use) Coated Steel pipe (surface use) Galvanized steel pipe (surface use) PVC pipe (underground use) Sprinkler equipment Aluminum sprinkler laterals Sprinklers (medium size) Sprinklers (giant, large-volume guns) Microirrigation equipment Filters Emitters, hose (permanent) Thin-walled tape Surface irrigation Land grading [a] [b] [c] Assumptions regarding maintenance, operating and environmental conditions: [a] excellent maintenance, ideal operating conditions, and favorable environmental circumstances; [b] typical or ordinary conditions; [c] inadequate or infrequent maintenance, harsh operating or environmental conditions. The current price for an initial or annual cost item may not always be the most appropriate figure to use in an economic analysis. Subsidies, incentives, taxes, or governmental policies may result in prices that do not reflect an item s true cost or value. The correct number to use in a comparative analysis can often be determined only after considering who is paying the costs, receiving the benefits, or making the decisions.

10 66 Chapter 3 Planning and System Selection For example, consider the price and value of water. If, according to local policy, the government pays all costs of water supply development and conveyance, the price of water to the farmer may be zero and private irrigation decisions may be based on that premise. For public irrigation projects, those sponsored or funded by the government or financed by international organizations (for example, a World Bank loan), the use of a zero water cost is questionable. In this case, a more appropriate water cost might include allocations for at least the operation, and perhaps the facilities, of the water delivery system. Further, perhaps credits for the recreational and environmental benefits of water development, and charges for environmental degradation or the cost of mitigation measures to avoid it, should be included as well. Costs for other inputs to irrigated agriculture are subject to similar considerations. While private decision making may be based on local prices, full cost, world market cost, or even opportunity cost (net return possible by diverting that input to alternative uses), values should be considered for public project decision making. Note also that financial or resource constraints may prevent the selection of the most economical irrigation option. Limitations on available capital, or credit limits, may preclude some capital-intensive options, even if these would otherwise be the best option economically. Constraints on available resources, such as land or water, may have a similar effect on the decision process. In these instances, it may be helpful to consider indicators such as the maximum net return per unit of constrained resource (for example, maximum net return per unit land area, per unit water volume, or per unit of capital invested) instead of or in addition to the usual economic indicators. 3.3 IRRIGATION SYSTEM SELECTION To do a proper job of system selection, one must give careful consideration to the capabilities and limitations of potential irrigation methods. The remainder of this chapter will summarize such considerations for the most common types of irrigation systems. Labor, management, energy, and economic factors relevant to each system type are briefly addressed. This section is adapted from Solomon (1988), which was based on an unpublished study by Anderson et al. (1987). U.S. costs cited are representative of U.S. irrigation practice as of 1996, and are based on Solomon et al. (1992), Westlands Water District (1988), as updated by the study of Jorgensen (1996). Egyptian cost figures are based on studies conducted by the second author (El-Gindy) in 1995, based on a field size of 20 ha Surface Irrigation Types of surface irrigation. Basin, border strip, and furrow irrigation are common types of surface irrigation. In basin irrigation, water is applied to a completely level (sometimes called dead level ) area enclosed by dikes or borders. This method of irrigation is used successfully for both field and row crops. The floor of the basin may be flat, ridged, or shaped into beds, depending on crop and cultural practices. Basins need not be rectangular or straight sided, and the border dikes may or may not be permanent. Basin irrigation is also called by a variety of other names: check flooding, level borders, check irrigation, check-basin irrigation, dead-level irrigation, and level-basin irrigation. Factors affecting basin geometry are available water stream size, topography, soil properties, and degree of leveling required. Basins may be quite small or as large as 15 ha or so. Level basins simplify water management, since the irrigator need only

11 Design and Operation of Farm Irrigation Systems 67 supply a specified volume of water to the field. With adequate, non-erosive stream size, the water will spread quickly over the field, minimizing non-uniformities in inundation time. Basin irrigation is most effective on uniform soils, precisely leveled, when large stream sizes (relative to basin area) are available. High efficiencies are possible with low labor requirements. Border strip irrigation uses land formed into graded strips, level across the narrow dimension but sloping along the long dimension, and bounded by ridges or borders. Water is turned into the upper end of the border strip and advances down the strip. After a time, the water is turned off, and a recession front, where standing water has soaked into the soil, moves down the strip. High irrigation efficiencies are possible with this method of irrigation, but may be difficult to obtain in practice, because the balancing of advance and recession phases during water application is difficult. Border strip irrigation is one of the most complicated of all irrigation methods. The primary design factors are border length and slope, stream size per unit width of border, planned soil water deficiency at the time of irrigation, soil intake rate, and degree of flow retardance by the crop as the water flows down the strip. However, because of the large variations in field conditions that occur during the season, the irrigator can have as great an effect on irrigation efficiency as the system designer. Furrow irrigation uses furrows, which are small, sloping channels formed in the soil. Infiltration occurs laterally and vertically through the wetted perimeter of the furrow. Systems may be designed with a variety of shapes and spacings. Optimal furrow lengths are primarily determined by intake rates and stream size. Even when soils are uniform, the intake rates in furrows may be quite variable due to cultural practices. The intake rate of a newly formed furrow will be greater than a furrow that has been irrigated. Wheel-row furrows can have greatly reduced infiltration rates due to compaction. Because of the many design and management-controllable parameters, furrow irrigation systems can be utilized in many situations, within the limits of soil uniformity and topography. With runoff-return flow systems, furrow irrigation can be a very uniform and efficient method of applying water. However, the uniformity and efficiency are highly dependent on the timing of the irrigation and the skill of the irrigator. Mismanagement can severely degrade system performance Capabilities and limitations. Some form of surface irrigation is adaptable to most any crop. Basin and border strip irrigation have been successfully used on a wide variety of crops, but furrow irrigation is less well adapted to field crops if cultural practices require equipment travel across the furrows. Basin and border strip irrigations flood the soil surface and will cause some soils to form a crust, which may inhibit the emergence of seedlings. Surface irrigation systems perform better when soils are uniform, since the soil controls the intake of water. For basin irrigation, basin size should be appropriate for soil texture and infiltration rate. Basin lengths should be limited to 100 m on very coarse-textured soils, but may reach 400 m on other soils. Furrow irrigation is possible with all types of soils, but soils with extremely high or low intake rates require excessive labor or capital cost adjustments that are seldom economical. Uniform, mild slopes are best adapted to surface irrigation. Undulating topography and shallow soils do not respond well to grading to a plane. Steep slopes and irregular topography increase the cost of land leveling and reduce basin or border size. Deep cuts may expose areas of nonproductive soils, requiring special fertility management. Erosion control

12 68 Chapter 3 Planning and System Selection measures may be required if large stream sizes are used. In areas of high-intensity rainfall and low intake rate soils, surface drainage should be considered with basin irrigation, to reduce damage due to untimely inundation. It is important that irrigation stream size be properly matched to basin or border size and furrow lengths for uniform irrigation. Since intake rates may vary during the season, it is helpful if the water supply rate for border and furrow systems can be varied from one irrigation to the next. Border and furrow systems are not suitable for leaching of salts for soil reclamation, since the water cannot be held on the soil for any length of time. The basin method, however, is ideal for this purpose. Under normal operating conditions, leaching adequate for salinity control can be maintained with basin, border, or furrow irrigation. High application efficiencies are possible with all surface irrigation methods, but it is much easier to obtain these potential efficiencies with the basin method. Design application efficiencies for basin systems should be high, perhaps 80% to 90%, for all but very high intake rate soils. Reasonable efficiencies for border strip irrigation are from 70% to 85%, and from 65% to 75% for furrow irrigation. With either the border of furrow methods, runoff-return flow systems may be needed to achieve high application efficiencies. The system designer and operator can control many of the factors affecting irrigation efficiency, but the potential uniformity of water application with surface irrigation is limited by the variability of soil properties (primarily infiltration rate) throughout the field. Field studies indicate that even for relatively uniform soils, the uniformity of infiltration rates may be only about 80%, as calculated in a manner analogous to distribution uniformity for irrigation application amounts. Surface irrigation uniformity estimates based on infiltration time differences need to be decreased somewhat to account for soil variability. Basin irrigation involves the least labor of the three surface methods, particularly if the system is automated. Border and furrow systems may also be automated to some degree to reduce labor requirements. The complicated art of border irrigation (and to a lesser extent furrow irrigation) requires skilled irrigators to obtain high efficiencies. The labor skill needed for setting border or furrow flows can be decreased with costly equipment (gated pipe or automated valves/gates). The setting of siphons or slide openings to obtain the desired flow rate is a required skill, but one that can be learned. With surface irrigation, little or no energy is required to distribute the water throughout the field, but some energy may be expended in bringing the water to the field, especially when water is pumped from groundwater. In some instances these energy costs can be substantial, particularly with low application efficiencies. Some labor and energy will be necessary for land grading and preparation Economics. A major cost in surface irrigation is that of land grading or leveling. The cost is directly related to the volume of earth that must be moved, the area to be graded, and the length and size of farm ditches. Typical earth moving volumes are on the order of 800 m 3 /ha, but have on occasion exceeded 2500 m 3 /ha. Volumes greater than 1500 m 3 /ha are generally considered excessive, suggesting a design review may be needed. For the U.S., typical earth moving costs are $0.50/m 3. For basin irrigation, final finishing with laser-controlled drag scrapers after primary earth moving will cost about $110/ha. Touch-up leveling (at about $50/ha) may be required every two to three years, although some farmers choose to touch-up each year. In Egypt, land leveling costs for surface irrigation are about $170/ha.

13 Design and Operation of Farm Irrigation Systems 69 Ditch construction can cost from $3/m for earthen ditches to $50/m for large, concrete-lined ditches. Buried, low-pressure plastic or concrete pipelines for low flows can cost about double the cost of concrete-lined ditches, and may cost 5 to 10 times as much for higher flows. They are generally uneconomical on flat terrain where pumping is not required. They may be desirable on steeper slopes (over 1%) or on undulating terrain. Lined canals for a 20-ha Egyptian farm can cost about $1300/ha. U.S. estimates for furrow irrigation labor requirements range from 1.9 to 2.5 h/ha per irrigation or 10 to 13 h/ha per year (based on about 5 irrigations per season). In Egypt, labor costs for surface irrigation range from $80/ha to $170/ha per year. In the U.S., siphon tubes may cost about $30/ha, while gated pipe may cost $370/ha to $735/ha, depending on pipe size, material and length of furrow run. In Egypt, gated pipe for a 20-ha field costs about $340/ha. An operational storage reservoir (i.e., one for short-term storage of water) may be advisable to permit use of a large stream size accumulated from a smaller steady flow. A medium-sized, compacted-earth reservoir capable of storing 24 h water volume would cost about $250/ha for a small (to 16 ha) farm in the U.S. For larger U.S. farms, the cost can drop to about $125/ha. A lined reservoir may cost two to five times as much. In California s Westlands Water District (1988), a typical runoff recovery and reuse system (3700 m 3 sump capacity, serving four fields of 65 ha each) costs about $170/ha served Sprinkler Irrigation Types of sprinkler irrigation. In sprinkler irrigation, water is delivered through a pressurized pipe network to sprinklers, nozzles, or jets, which spray the water into the air, to fall to the soil in an artificial rain. The basic components of any sprinkler system are a water source, a pump to pressurize the water, a pipe network to distribute the water throughout the field, sprinklers to spray the water over the land, and valves to control the flow of water. The sprinklers, when properly spaced, give a relatively uniform application of water over the irrigated area. Sprinkler systems are usually designed to apply water at a lower rate than the soil infiltration rate, so that the amount of water infiltrated at any point depends upon the application rate and time of application, but not the soil infiltration rate. Hand move or portable sprinkler systems employ a lateral pipeline with sprinklers installed at regular intervals. The lateral pipe is often made of aluminum, with 6-, 9-, or 12-m sections, and special quick-coupling connections at each pipe joint. The sprinkler is installed on a pipe riser so that it may operate above the crop being grown (in orchards, the riser may be short, so that the sprinklers operate under the tree canopy). The risers are connected to the lateral at the pipe coupling, with the length of pipe section chosen to correspond to the desired sprinkler spacing. The sprinkler lateral is placed in one location and operated until the desired water application has been made. Then the lateral line is disassembled and moved to the next position to be irrigated. This type of sprinkler system has a relatively low initial cost, but a high labor requirement. It can be used on most crops, though with some, such as corn or sugar cane, the laterals become difficult to move as the crop reaches maturity. On bare sticky soils, moving the lateral lines is very difficult, and an extra line (a dry line) is used to give the soil under the wet line some time to dry out before that particular line is moved. The side roll sprinkler system is a variation on the hand move lateral sprinkler line. The lateral line is mounted on wheels, with the pipe forming the axle (specially

14 70 Chapter 3 Planning and System Selection strengthened pipe and couplers are used). The wheel diameter is selected so that the axle clears the crop as the lateral is moved. A drive unit, usually powered by an aircooled gasoline-powered engine located near the center of the lateral, is used to move the system from one irrigation position to another by rolling the wheels. Gun type sprinkler systems utilize a high-capacity, high-pressure sprinkler (the gun ) mounted on a trailer, with water being supplied through a flexible hose or from an open ditch along which the trailer passes. The gun may be operated in a stationary position for the desired time, and then moved to the next location. However, the most common use is as a continuous move system, where the gun sprinkles as it moves, in a traveling gun system. The trailer may be moved through the field by a winch and cable, or it may be pulled along as the hose is wound up on a reel at the edge of the field. The gun used is usually a part-circle sprinkler, operating through 80% to 90% of the circle for best uniformity, and allowing the trailer to move ahead on dry ground. These systems can be used on most crops, though due to the large droplets and high application rates produced, they are best suited to coarse soils having high intake rates and to crops providing good ground cover. The center pivot system consists of a single sprinkler lateral supported by a series of towers. The towers are self-propelled so that the lateral rotates around a pivot point in the center of the irrigated area. The time for the system to revolve through one complete circle can range from a half a day to many days. To keep the lateral aligned as it moves around the circle, the outer sections of the lateral must travel faster, and cover larger areas per rotation, than the inner sections. Thus, the instantaneous water application rate must increase with distance from the pivot to deliver an even application amount. The high application rate at the outer end of the system may cause runoff on some soils. A variety of sprinkler products have been developed specifically for use on these machines to better match water requirements, water application rates, and soil characteristics. Since the center pivot irrigates a circle, it leaves the corners of the field unirrigated (unless additions of special equipment are made to the system). Center pivots are capable of irrigating most field crops, and have on occasion been used on tree and vine crops. Linear move (or lateral move) systems are similar to center pivot systems in construction, except that neither end of the lateral pipeline is fixed. The entire line moves down the field in a direction perpendicular to the lateral. Water delivery to the continuously moving lateral is by flexible hose or open ditch pickup. The system is designed to irrigate rectangular fields free of tall obstructions. Both the center pivot and the linear move systems are capable of very highly controlled and efficient water applications. They require moderately high capital investments, but have low irrigation labor requirements. Low energy precision application (LEPA) systems are similar to center pivot and linear move irrigation systems, but are different enough to deserve separate mention. The lateral line is equipped with drop tubes and very low-pressure orifice emission devices discharging water just above the ground surface into furrows. This distribution system is often combined with micro-basin land preparation for improved runoff control (and to retain rainfall which might fall during the season). High efficiency irrigation is possible, but requires either very high soil intake rates or adequate surface storage in the furrow micro-basins to prevent runoff or non-uniformity along a furrow.

15 Design and Operation of Farm Irrigation Systems 71 Solid set systems are similar in concept to the hand move lateral sprinkler system, except that enough laterals are placed in the field that it is not necessary to move pipes during the season. The laterals are controlled by valves that direct the water into the laterals irrigating at any particular moment. The pipe laterals for the solid set system are moved into the field at the beginning of the season (after planting and perhaps the first cultivation), and are not removed until the end of the irrigation season (prior to harvest). The solid set system utilizes labor available at the beginning and ends of the irrigation season, but minimizes labor needs during the irrigation season. A permanent system is a solid set system where the main supply lines and the sprinkler laterals are buried and left in place permanently (this is usually done with PVC plastic pipe) Capabilities and limitations. Nearly all crops can be irrigated with some type of sprinkler system, though the characteristics of the crop, especially the height, must be considered in system selection. Sprinklers are sometimes used to germinate seed and establish ground cover for crops like lettuce, alfalfa, and sod. The light frequent applications that are desirable for this purpose are easily achieved with some sprinkler systems. Most soils can be irrigated with the sprinkler method, although soils with an intake rate below 5 mm/h may require special measures. Sprinklers can be used on soils that are too shallow to permit surface grading or too variable for efficient surface irrigation. In general, sprinklers can be used on any topography that can be farmed. Land leveling is not normally required. Leaching salts from the soil for reclamation can be done with sprinklers using much less water than is required by flooding methods, although a longer time is required to accomplish the reclamation. This can be particularly important in areas with a high water table. A disadvantage of sprinkler irrigation is that many crops (citrus, for example) are sensitive to foliar damage when sprinkled with saline waters. In contrast, other crops (apples, peas) react favorably to sprinkler irrigation at harvest, actually producing higher-quality fruit. Attainable irrigation efficiencies for different sprinkler systems are given in Table 3.3. Labor requirements vary depending on the degree of automation and mechanization of the equipment used. Hand move systems require the least degree of skill, but the greatest amount of labor. At the other extreme, center pivot, linear move, and LEPA systems require considerable skill in operation and maintenance, but the overall amount of labor needed is low. Energy consumption relates to operating pressure requirements (at the inlet to the system), which vary considerably among sprinkler systems. At the extremes, the LEPA systems may require only 100 kpa or so, while the traveling gun system may require 700 kpa or more. Other systems may require 200 to 400 kpa, depending on the design of the sprinklers and nozzles chosen, spacing, crop, climate, and other factors. Table 3.3. Attainable application efficiencies with sprinkler irrigation. System Type Efficiency (%) Hand move or portable Side roll Traveling gun Center pivot Linear move LEPA Solid set or permanent 70-80

16 72 Chapter 3 Planning and System Selection Economics. Table 3.4 summarizes cost factors for sprinkler irrigation systems based on U.S. prices and conditions. Capital costs depend on the type of system and size of the irrigated area. These capital costs assume that water is available at ground level at the side of the field, and include mainline and pumping plant. Table 3.5 summarizes costs for pressurized irrigation systems in Egypt. Energy costs are highly variable from place to place. The energy requirements listed in Table 3.4 may be used to estimate costs by applying the locally appropriate unit energy cost. A pumping plant efficiency of 75% has been assumed. The energy figures cited are in terms of kwh per 1000 m 3 (gross) of water applied. Table 3.4. Sprinkler irrigation system costs, U.S. irrigation practice. Typical Field Size (ha) Capital Cost ($/ha) Energy Use (kwh per 1000 m 3 ) Labor Required (hours per 1000 m 3 ) Maintenance Cost Factor [a] (%) System Type Hand move or portable Side roll Traveling gun Center pivot: without corner system with corner system Linear move (ditch fed) Linear move (hose fed) Solid set aluminum Permanent [a] Annual maintenance costs are expressed as a percentage of the system capital cost. Table 3.5. Irrigation costs, Egyptian irrigation practice, assuming a 20-ha field size. Source of Water Supply Initial Capital Cost ($/ha) Labor ($/ha) Energy & Consumables ($/ha) Maintenance & Spare Parts ($/ha) Solid set sprinkler Canal Well Hand move sprinkler Canal Well Center pivot, Canal low pressure type Well Minisprinkler Canal Well Bubbler Canal Well Drip on orchards Canal Well Drip on vegetables Canal or row crops Well