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Planning for irrigation consists of identifying and collecting information on rele- vant 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 plan- ning phase certainly includes inventories of resources, problems, and the goals, priori- ties, 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 dis- cussed further in the following sections.

3.2.1 Water Requirements and Water Supply

A key point to consider in any irrigation plan is whether the water available is suf- ficient to adequately meet the water requirements for crop production. This compari- son 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 sup- ply 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 influ- ence 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 grow- ing 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), irri- gation 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 in- field 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 para- graphs. 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 effi- ciency. The application efficiency anticipated during the peak-use period should be used in this adjustment.

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

Certain management strategies may allow additional adjustments in the design ca- pacity 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 microirriga- tion 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 elec- tric 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 irri- gation 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 deliv- eries 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, agro- nomic 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. Irri- gation 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 suffi- cient 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 re- turn-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 applica- tion efficiency will increase the system’s net application rate.

The advisability of these various water management strategies is frequently af- fected by the value of the crop and the sensitivity of the crop yield to water.

3.2.2 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 artifi- cial drainage. Equipment, technology, and infrastructure for the drainage system, if

needed, should be included in the economics and evaluation of irrigation options.

3.2.3 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 cost- ing, which treats operation and maintenance costs as well as initial costs, is the pre- ferred 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 in- volve 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 mar- kets 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 op- erating costs. For example, larger pipe sizes have higher initial costs, but reduce fric- tion losses and hence may lower operating costs for pumping energy. Less equipment- intensive systems may have reduced initial costs, but higher operating costs for irriga- tion 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:

1 1 1 − + + = _n n ) i ( ) i ( i CRF (3.1)

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.

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 20 15 12

Center pivot, linear move, LEPA 25 20 15

Surface irrigation systems 30 20 15

Individual items

Farm reservoirs (heavy silting) 25 20 15

Farm reservoirs (light silting) 50 35 25

Well 40 25 20

Irrigation pump 20 15 10

Pump power units

Diesel 15 12 8

Gas, propane 13 9 7

Gasoline 8 6 5

Electric motor 30 25 15

Water conveyance

Open ditches (unlined) 8 6 4

Concrete structures 25 20 15

Concrete pipe systems 20 15 10

Wood flumes 10 8 6

Large aluminum pipe (lightweight) 10 8 6

Coated steel pipe (underground use) 20 15 10

Coated Steel pipe (surface use) 12 10 8

Galvanized steel pipe (surface use) 25 20 15

PVC pipe (underground use) 40 20 18

Sprinkler equipment

Aluminum sprinkler laterals 25 15 10

Sprinklers (medium size) 15 12 10

Sprinklers (giant, large-volume guns) 13 10 8

Microirrigation equipment

Filters 20 15 10

Emitters, hose (permanent) 15 10 8

Thin-walled tape 3 1 1

Surface irrigation

Land grading 40 25 20

[a] [b] [c] _{Assumptions regarding maintenance, operating and environmental conditions: [a] excellent mainte-}

nance, ideal operating conditions, and favorable environmental circumstances; [b] typical or ordinary con-

ditions; [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 ap- propriate figure to use in an economic analysis. Subsidies, incentives, taxes, or gov- ernmental 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 deci- sions.

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 bene- fits 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.