Agricultural Engineering · Land & Water Resources Engineering · also Pressurised Irrigation, Drip and Sprinkler Design, Landscape Irrigation
Designing and evaluating sprinkler and drip systems, from emitter hydraulics to pump selection.
In the exams: GATE AG Section 5 (sprinkler in 4 of 8, drip in 3 of 8 papers 2019–26); ICAR AICE subject 43 Unit 4; NCEES PE irrigation; IA CID (General Ag, Drip/Micro and Sprinkler exams)
Sprinkler and micro-irrigation are the two families of pressurised irrigation. In both, a pump or an elevated source pushes water through closed pipes, and the water leaves through a device (a sprinkler nozzle, an emitter, a micro-sprinkler) whose discharge depends on the pressure at that device. That single fact explains almost every design rule in this subject: if the pressure along the pipes varies too much, the discharge varies, the crop is watered unevenly, and the farmer either over-irrigates to satisfy the driest spot or accepts yield loss where the water falls short.
The subject sits between fluid mechanics and agronomy. The engineer needs the crop water requirement and the soil's intake rate from irrigation engineering, pipe friction and pump curves from hydraulics, and then must turn them into spacings, pipe sizes, zones, filters, injection equipment and a pump that runs at a sensible efficiency. Done well, a pressurised system applies water with a uniformity no surface method can match on undulating or sandy land, allows fertiliser to be fed in small doses with the water, and saves labour. Done badly, it clogs, it applies faster than the soil can absorb, and the capital cost is never repaid.
Examiners everywhere test the same core. Postgraduate entrance, engineering aptitude and research-fellowship papers in agricultural engineering ask for numbers: the application rate from discharge and spacing, Christiansen's uniformity coefficient from catch-can data, head loss in a lateral with Christiansen's F factor, the emitter exponent from two pressure–flow readings, drip run time, and injection rates. Professional engineering licensure exams (such as the NCEES PE Agricultural and Biological Engineering exam) and the Irrigation Association's internationally taken Certified Irrigation Designer and Agricultural Irrigation Specialist exams add pump selection, filtration, water-quality hazards, chemigation safety and economics. Public service commission and engineering licensure exams in many countries, across Asia, Africa and Latin America, ask the definitions and the standard formulas.
This manual follows the design sequence a practitioner uses: devices and components, sprinkler spacing and rate, uniformity, sprinkler pipe hydraulics, then the drip side from components through emitter hydraulics, uniformity, lateral and sub-main design, filtration and fertigation, and finally layout, pumps, automation and economics. Standards and subsidy rules differ by country; where they do, the text says so rather than choosing one.
A sprinkler system sprays water into the air through nozzles so that it falls on the crop and soil as something like rain. The basic parts are a water source, a pump unit (pump, prime mover, and often a pressure gauge, a non-return valve and a flow meter), a mainline that carries water from the pump to the field, sub-mains where the field is large, laterals that carry water along the rows of sprinklers, risers that lift each sprinkler above the crop, and the sprinklers themselves. Valves, pressure regulators, air-release valves and drain valves complete the network.
Sprinkler irrigation suits sandy soils with high intake rates, shallow soils that cannot be levelled, undulating land, and places where water is too scarce for the losses of surface irrigation. It is less suited to very windy sites, to fine-textured soils with very low intake rates (unless the rate is kept low), to water with high salt content that scorches wet leaves, and to crops whose foliage diseases are favoured by wetting. Because it wets the whole surface, it also serves for frost protection, cooling, and the germination of small-seeded crops.
Systems are classified by how the laterals move. In a permanent (fixed) system the mains and laterals are buried and do not move; in a solid-set system laterals are laid on the surface for a season and removed. Both need no labour during the season but have the highest pipe cost per hectare. In a portable or hand-move system, one set of laterals is moved from position to position after each irrigation set; pipe cost is low and labour high. Semi-portable systems keep the mainline fixed and move the laterals.
Mechanised systems move continuously or in steps. A centre pivot is a single lateral carried on towers that rotate around a fixed pivot point, watering a circle; because a circle inscribed in a square covers π/4 of it (about 78.5%), corners are left dry unless a corner arm or end gun is used. The outer spans sweep far more area per revolution than the inner spans, so sprinkler discharge must increase with distance from the pivot, and the application rate is highest at the outer end. A linear-move (lateral-move) machine carries a similar lateral straight across a rectangular field, fed from a canal or a hose. A travelling gun (hose-reel or cable-tow) drags a single large gun across the field. A side-roll (wheel-line) lateral uses the pipe itself as the axle of large wheels and is rolled from set to set.
Rotating sprinklers turn a jet so that it sweeps a circle. The impact sprinkler is driven by a spring-loaded arm that is knocked aside by the jet and returns to strike the body, stepping it round; gear-driven rotors use a small turbine and gear train. Fixed spray heads and spray plates (common on centre pivots and in landscapes) have no rotating jet and throw water a short distance at a high instantaneous rate. Rain guns or big guns are large rotating sprinklers with nozzles of about 15–40 mm (sizes vary with design) operating at high pressure and throwing water tens of metres.
Pressure ranges quoted in textbooks for low-, medium- and high-pressure sprinklers differ between authors, so learn the principle rather than one table: higher pressure breaks the jet into smaller drops and throws them further; too low a pressure leaves large drops that fall in a ring well away from the sprinkler (the doughnut pattern) and can crust the soil; too high a pressure produces a mist that drifts and evaporates and a pattern that is over-watered near the sprinkler. Each nozzle size has a manufacturer's recommended pressure range, and a sprinkler's catalogue gives discharge and wetted diameter at each pressure.
The discharge of a sprinkler nozzle follows the orifice equation, q = Cd × a × √(2gh), where q is the discharge, Cd the coefficient of discharge (commonly about 0.95 for a well-shaped nozzle, but it depends on the nozzle), a the nozzle area and h the pressure head at the nozzle. Because q varies with the square root of pressure head, the exponent in the general relation q = k × h^x is x = 0.5 for sprinkler nozzles. A 20% change in pressure therefore changes discharge by only about 10% (1.2^0.5 ≈ 1.095). This square-root relation is the basis of Christiansen's rule, taken up in Chapter 4, that pressure along a lateral should not vary by more than about 20% of the design operating pressure.
Risers must be tall enough to clear the crop canopy at its tallest and long enough that turbulence from the pipe fitting has settled before the water reaches the sprinkler; manufacturers and textbooks give minimum riser lengths that increase with discharge.
Sprinklers are laid out on a grid. The spacing along the lateral is Sl (sometimes written Se) and the spacing between laterals is Sm (the move distance for a portable system). Square, rectangular and triangular patterns are used; a triangular pattern gives slightly better overlap for the same area per sprinkler, while rectangular patterns fit the pipe lengths and the direction of travel of portable laterals.
A single sprinkler's catch pattern is not uniform: depth is usually greatest near the sprinkler and tails off towards the edge of the wetted circle. Uniform application therefore depends on overlap: each point in the field receives water from several sprinklers. Spacing is set as a fraction of the wetted diameter. Textbooks and manufacturers commonly give spacings of roughly one-half to two-thirds of the wetted diameter, reducing as wind speed increases, and they often recommend the closer spacing across the wind direction. The exact fractions differ between sources and sprinkler models, so a designer uses the manufacturer's spacing tables or, better, overlaps measured single-sprinkler catch data to find the spacing that gives the required uniformity.
The average application rate (precipitation rate) of a sprinkler grid is the discharge of one sprinkler spread over the area it serves: I = q / (Sl × Sm). With q in litres per second and spacings in metres, I (mm/h) = 3600 × q / (Sl × Sm); with q in cubic metres per hour, I (mm/h) = 1000 × q / (Sl × Sm). For example, a sprinkler discharging 0.5 L/s on a 12 m × 18 m grid applies 3600 × 0.5 / 216 = 8.33 mm/h.
The application rate must not exceed the soil's basic (steady) infiltration rate, or water ponds and runs off, and the non-uniformity then becomes much worse than the sprinkler pattern alone. Typical basic intake rates fall from tens of millimetres per hour on sands to a few millimetres per hour on clays, and they fall further on bare soils that crust; field measurement is better than a table. The maximum rate is also lower on slopes. Conversely, an application rate that is very low relative to evaporation demand wastes a larger fraction of water to evaporation and drift during windy hot hours. For a centre pivot the instantaneous rate at the outer end is far higher than the average rate over the circle, which is why pivots are often matched with tillage practices that store surface water, or with low-pressure spray heads on booms that widen the wetted pattern.
The set time (the time a portable lateral stays in one position) is the gross depth to be applied divided by the application rate. If the net depth to be replaced is dn and the application efficiency is Ea, the gross depth is dg = dn / Ea, and the set time is T = dg / I. The irrigation interval is the net depth divided by the peak daily crop water use (ETc). Sets are usually fitted to convenient working hours, for instance two sets of about eleven hours each per day with an hour for moving pipe, so the depth applied is adjusted to fit the time available rather than the other way round.
The capacity of a sprinkler system is the flow the pump must deliver to irrigate the whole area within the irrigation interval, operating a given number of hours per day. In SI units, Q (L/s) = 2.78 × A × d / (F × H × E), where A is the area in hectares, d the net depth in millimetres, F the irrigation interval in days, H the operating hours per day and E the application efficiency as a fraction. The constant 2.78 converts hectare-millimetres to cubic metres (10) and hours to seconds (3600) and cubic metres to litres (1000): 10 × 1000 / 3600 = 2.78. For 10 ha, 50 mm net, a 10-day interval, 20 hours per day and 75% efficiency, Q = 2.78 × 10 × 50 / (10 × 20 × 0.75) = 9.3 L/s.
The number of sprinklers operating at once is the system capacity divided by the discharge per sprinkler, and the number of laterals running at once follows from the sprinklers per lateral. Designers round up to whole laterals and check that the pump and mainline can carry the result.
No sprinkler or drip system applies exactly the same depth everywhere. If the farmer irrigates so that the driest part of the field receives enough, the rest is over-irrigated and water and nutrients are lost to deep percolation; if the farmer irrigates to the average, part of the field is under-irrigated and yields there fall. Uniformity measures describe how even the application is, and they link to efficiency through the concept of adequacy: the fraction of the area receiving at least the required depth. Uniformity is a property of the hardware and its operation (pressure, spacing, wind, nozzle wear); efficiency also depends on how much water is applied relative to the need.
J.E. Christiansen proposed in 1942 a coefficient based on the mean absolute deviation of catch-can depths from their mean:
CU = 100 × [1 − Σ|xi − x̄| / (n × x̄)]
where xi are the individual catch depths (or volumes), x̄ their mean and n the number of cans. A perfectly even catch gives CU = 100%. To compute it: find the mean, subtract the mean from each reading and take the absolute value, add the absolute deviations, divide by n × mean, subtract from one and multiply by 100. For example, catches of 10, 12, 9, 11, 8, 12, 10 and 8 mm have mean 10 mm and absolute deviations summing to 10 mm, so CU = 100 × (1 − 10/80) = 87.5%.
Catch cans are set out on a regular grid between sprinklers (for a portable lateral, between two adjacent laterals, or by overlapping a single-lateral test mathematically), with cans of identical opening, run for a known time in recorded wind. For a centre pivot, cans are placed along a radius and each catch is weighted by its distance from the pivot, because it represents a ring of area proportional to radius; that weighted form is known as the Heermann and Hein coefficient.
Commonly cited targets are CU of about 85% or more for shallow-rooted, high-value crops and lower values, around 75% or so, for deep-rooted field crops whose soil redistributes water; sources differ and a design target should reflect crop value and water cost.
Distribution uniformity (DU), in the low-quarter form used by the US Soil Conservation Service (now NRCS) and many texts, is the average depth caught in the lowest one-quarter of the cans divided by the average of all cans, times 100. For the catch data above, the lowest quarter (two of eight cans) are 8 and 8 mm, so DU = 100 × 8/10 = 80%. DU is more severe than CU for the same data because it focuses on the dry part of the field.
For approximately normally distributed catch data, the two are related by DU ≈ 100 − 1.59 × (100 − CU), or equivalently CU ≈ 100 − 0.63 × (100 − DU). A CU of 87.5% thus corresponds to a DU of about 80%. The relation is an approximation and fails for strongly skewed patterns.
Application efficiency is the depth stored in the root zone divided by the depth applied. For sprinkler systems the losses between nozzle and ground (evaporation and wind drift) are separated out: the effective portion of water applied, Re, is the fraction that reaches the soil. The application efficiency of the low quarter, a term used by the Irrigation Association and USDA texts, equals DU multiplied by Re when the low quarter is just adequately irrigated. Losses from spray depend strongly on wind speed, air humidity, drop size and temperature; published values vary widely, and night operation often reduces them.
Christiansen's design rule is that the pressure difference between any two sprinklers on a lateral should not exceed about 20% of the design operating pressure, which (because q ∝ h^0.5) keeps discharge variation within about 10%. The 20% allowance covers friction loss and elevation change together. On level ground the whole 20% may be used by friction; where the lateral runs uphill, the elevation rise uses part of it; where it runs gently downhill, the gain in elevation can offset friction and allow a longer lateral or a smaller pipe. Some designers split the allowance between lateral and sub-main when both are variable; the total must still be kept within the chosen limit.
Friction loss in a pipe carrying a constant flow can be computed by the Hazen-Williams equation, which in SI units is hf = 10.67 × L × Q^1.852 / (C^1.852 × D^4.87), with hf and L in metres, Q in cubic metres per second and D in metres; C is the roughness coefficient, commonly taken near 150 for plastic pipe and lower for aluminium, galvanised and steel pipe (texts give ranges). The Darcy-Weisbach equation, hf = f (L/D) V²/2g, with f from the Moody diagram, is more general and is preferred for small plastic tubing and for water well away from 20 °C.
A lateral, however, loses flow at every sprinkler, so the flow in each successive section is smaller. Christiansen showed that the friction loss in such a lateral equals the loss in the same pipe carrying the full inlet flow over the full length, multiplied by a factor F:
F = 1/(m+1) + 1/(2N) + √(m−1)/(6N²)
where N is the number of outlets and m is the velocity (flow) exponent of the friction formula: 1.852 for Hazen-Williams, 2 for Darcy-Weisbach with constant f, and about 1.75 for the Blasius smooth-pipe form. This expression assumes the first outlet is one full spacing from the inlet. As N grows, F falls towards 1/(m+1), about 0.35 for m = 1.852. For N = 10 and m = 1.852, F = 0.3506 + 0.0500 + 0.0015 = 0.402. When the first sprinkler is only half a spacing from the inlet, the adjusted factor is F′ = (2NF − 1)/(2N − 1).
About three-quarters of a lateral's friction loss occurs in the first half of its length, because the flow there is largest. For a lateral of uniform diameter, the average sprinkler pressure is reached roughly one-quarter of the friction loss below the inlet pressure. The widely used design relation is therefore:
Hinlet = Ha + (3/4) hf ± (1/2) ΔZ + Hr
where Ha is the average (design) operating pressure head of the sprinklers, hf the lateral friction loss (with F applied), ΔZ the elevation difference between the ends of the lateral (plus when the lateral runs uphill, minus when it runs downhill) and Hr the riser height. The pressure at the distal end on level ground is Hinlet − hf − Hr. The relation is approximate and is used for laterals on uniform, modest slopes; steep or irregular profiles are analysed section by section.
Mains carry the full flow of all laterals operating at once and do not have outlets along every metre, so their friction is computed section by section without F. Pipe size is chosen to keep velocity moderate (a common working limit for plastic mains is about 1.5 m/s, to limit water-hammer surges and friction) and to minimise the sum of annualised pipe cost and pumping energy cost over the system's life (Chapter 12). Where several laterals run from a sub-main, the sub-main can itself be treated as a multiple-outlet pipe with its own F. Mains must also be checked for surge pressure against the pipe's pressure class, fitted with air-release and vacuum-relief valves at high points, and with drain valves at low points where freezing or flushing requires it. Pressure regulators at lateral or sprinkler inlets allow steep or long systems to meet the uniformity limit at the cost of some pressure loss.
Micro-irrigation (also called localised or trickle irrigation) applies water at low rates, often frequently, to a part of the soil near each plant. Drip emitters discharge small flows, typically a few litres per hour each; micro-sprinklers and micro-jets spray over a small area; bubblers release a small stream for tree basins; subsurface drip (SDI) buries the laterals below the surface. Because only part of the field surface is wetted and flows are small, evaporation, runoff and weed growth between rows are reduced and fertiliser can be delivered to the roots with the water. The price is sensitivity to clogging, dependence on filtration and a higher management demand.
The head unit (control head) is where water enters the system. In order from the source it usually contains: the pump; a non-return (check) valve; a sand separator or media filter where the water needs it; the fertiliser injection point with its backflow prevention equipment; a screen or disc filter downstream of the injector to catch undissolved fertiliser and precipitates; pressure gauges on both sides of each filter so the pressure differential can be read; a flow meter; a pressure regulator or relief valve; and the main control valves. Air-release and vacuum-relief valves are fitted at high points of the head unit and mains. Placing a secondary screen or disc filter after the injector is standard practice because the injected chemicals can carry or create particles.
The mainline carries water from the head unit to the field, commonly in rigid PVC buried below cultivation depth; sub-mains (manifolds) feed the laterals within each block and are often PVC or polyethylene; laterals are flexible polyethylene tubing, commonly 12–20 mm nominal diameter, laid along each crop row with emitters on-line (punched into the tube) or in-line (built into the tube wall during manufacture, as in drip tape and heavier dripline). Flush valves at the ends of laterals and sub-mains allow sediment to be flushed out. Control valves divide the field into blocks (zones or shifts) that are irrigated one after another so that the pump works at a steady flow.
Emitters dissipate the pipe pressure through a narrow passage so that only a small flow leaves. Types include long-path emitters (a long, narrow channel in which friction dissipates the head), labyrinth or tortuous-path emitters (a zigzag channel that creates turbulence, allowing a larger, less clog-prone passage for the same flow), orifice emitters, vortex emitters (flow spins in a chamber) and pressure-compensating (PC) emitters, in which a flexible diaphragm narrows the passage as pressure rises so that discharge stays nearly constant over a stated pressure range. Emitters can be point-source (discrete emitters) or line-source (drip tape or porous pipe with closely spaced outlets). Anti-drain (non-leakage) emitters close when pressure falls, so laterals do not drain through the lowest emitters after each cycle, which matters for short pulsed irrigations. Standards for emitters and emitting pipe exist internationally (ISO 9261) and nationally, where national standards bodies often adopt or adapt ISO 9261 and subsidy programmes may require certification to them; a buyer should check the product is certified to the standard that applies where it will be used.
An emitter's discharge is described by the power law q = k × h^x, where q is discharge, h the pressure head at the emitter, k a coefficient that depends on the units and emitter size, and x the emitter discharge exponent. The exponent describes how sensitive the emitter is to pressure. It takes the value 1.0 for fully laminar flow (as in a long, fine capillary), 0.5 for fully turbulent flow through an orifice or labyrinth, around 0.4 for vortex emitters, and close to zero for pressure-compensating emitters within their compensating range. Values between 0.5 and 1 indicate partially turbulent flow.
The exponent is found from two measurements: x = ln(q1/q2) / ln(h1/h2). For example, an emitter giving 2.0 L/h at 100 kPa and 2.2 L/h at 150 kPa has x = ln(1.1)/ln(1.5) = 0.235. Once x is known, k = q/h^x. With more than two points, x and k come from a straight-line fit of ln q on ln h.
The fractional change in flow for a given fractional change in pressure follows from the power law. If the pressure varies by a fraction hvar, the flow varies by qvar = 1 − (1 − hvar)^x; conversely, a permissible flow variation qvar allows a pressure variation hvar = 1 − (1 − qvar)^(1/x). With x = 0.5, a 10% allowable flow variation allows about 19% pressure variation; with x = 1.0 it allows only 10%; with x = 0.2 it allows about 41%. The approximate linear form, qvar ≈ x × hvar, is fine for small changes. Low-exponent and pressure-compensating emitters therefore permit longer laterals and steeper slopes. Temperature also matters for laminar-flow emitters, because the viscosity of water changes with temperature; turbulent emitters are much less affected.
No two emitters are exactly alike. The manufacturing coefficient of variation, Cv, is the standard deviation of discharge divided by the mean discharge, measured on a sample of new emitters at the same pressure and temperature. ASABE engineering practice EP405 classifies point-source emitters as good for Cv below 0.05, average for 0.05–0.07, marginal for 0.07–0.11, poor for 0.11–0.15 and unacceptable above 0.15; line-source products are judged on a looser scale (below 0.10 good, 0.10–0.20 average, above 0.20 marginal to unacceptable). International standard ISO 9261 sets limits on the Cv a product may have and on how far the mean flow may depart from the nominal flow. Where a plant is served by several emitters, the effect of manufacturing variation on the plant is reduced, because random variations average out: the Cv of the total flow per plant is roughly Cv/√n for n emitters per plant.
Beyond manufacturing variation, emitter flows change in service. Partial clogging by sediment, chemical precipitates (calcium carbonate, iron oxides) or biological slime reduces flow and increases variation, and it is usually the dominant cause of poor uniformity in older drip systems. Pressure-compensating diaphragms can age and lose their compensating range. Field evaluation (Chapter 7) is the only way to know what uniformity a system actually achieves.
Keller and Karmeli defined a design emission uniformity that combines hydraulic variation and manufacturing variation:
EU = 100 × (1 − 1.27 Cv / √n) × (qmin / qavg)
where Cv is the manufacturing coefficient of variation, n the number of emitters per plant (for line-source tubing, a representative number per plant or per unit length), qmin the minimum emitter discharge computed from the minimum pressure in the block, and qavg the average or design discharge. The factor 1.27 comes from the statistics of the low quarter of a normal distribution. For Cv = 0.05, n = 2 and qmin/qavg = 0.93, EU = 100 × (1 − 0.0449) × 0.93 = 88.8%.
Recommended design EU values in ASABE EP405 and in the classic texts depend on emitter type, spacing, topography and climate; values of roughly 85–95% are typical targets for point-source emitters on uniform slopes, somewhat lower on steep or undulating ground and for line-source tape. The designer uses EU to set the allowable qmin/qavg, and from it the allowable pressure variation in the block.
For an existing system, Merriam and Keller's field emission uniformity is EU = 100 × (qlq / qavg), where qlq is the mean flow of the lowest quarter of emitters sampled and qavg the mean of all samples. Emitters are sampled at several locations along laterals at the inlet, one-third, two-thirds and far end of the sub-main, and each sample is timed into a graduated container. Bralts proposed a statistical uniformity, Us = 100 × (1 − Vq), where Vq is the coefficient of variation of the measured flows, which captures hydraulic, manufacturing and clogging effects together.
Because drip wets only part of the surface, peak transpiration of a young or widely spaced crop is less than the full-cover ETc. Keller and Karmeli's reduction factor is Kr = Gc/0.85 (not more than 1), where Gc is the fraction of ground shaded by the canopy at noon; Freeman and Garzoli used Kr = Gc + 0.5 (1 − Gc). FAO-56 treats the same idea through the fraction of soil surface wetted and shaded in the evaporation coefficient. Many practical guidelines instead multiply ETc by a canopy or wetted-area factor. The choice of method differs by country and crop; whichever is used should be stated.
The percentage of the root-zone soil area wetted, Pw, is a design choice: Keller and Karmeli suggested about one-third to two-thirds for widely spaced crops in arid areas, with lower values accepted in humid regions. More wetted area gives more buffer against a failure of supply; less keeps cost and weed growth down.
The daily volume per plant is V = ETc × Sp × Sr × (wetted or canopy factor) / Ea, in litres when ETc is in millimetres per day and the plant spacing Sp × Sr is in square metres (1 mm over 1 m² is 1 L). The run time is T = V / (n × q), where n is emitters per plant and q the emitter discharge. For ETc = 5 mm/day, 3 m × 3 m spacing, a factor of 0.7 and 90% efficiency, V = 5 × 9 × 0.7 / 0.9 = 35 L/plant/day; with four 4 L/h emitters, T = 35 / 16 = 2.19 h. Where water is saline, a leaching fraction is added. For drip in rows of close-spaced crops, the same calculation is done per metre of row or per unit area.
Drip design begins from the target emission uniformity. From EU and Cv the designer finds the permissible ratio qmin/qavg; from the emitter exponent the permissible pressure difference across the whole block (the sub-main unit, meaning the sub-main and all the laterals it feeds). A common starting point in the textbooks is a 10% allowable flow variation, which for x = 0.5 gives about 20% pressure variation. That allowance is then divided between the laterals and the sub-main, often roughly half each, though the best split depends on the relative cost of larger lateral tubing and larger sub-main pipe. Pressure-compensating emitters relax the constraint to the range over which the emitter compensates.
A drip lateral is a multiple-outlet pipe with many closely spaced outlets, so Christiansen's F approaches 1/(m+1). Friction is computed for the full inlet flow over the full length and multiplied by F; because lateral tubing is small and smooth, the Darcy-Weisbach equation with a smooth-pipe friction factor (Blasius, m about 1.75) is usually more accurate than Hazen-Williams, although exam questions often use Hazen-Williams with C near 150 for simplicity. In-line emitters add local losses at each barb or insert, commonly allowed for by an equivalent extra pipe length per emitter taken from the maker's data.
On level ground the maximum lateral length is the length at which friction just uses the lateral share of the pressure allowance. Laying laterals downhill on a gentle slope lets the elevation gain offset friction, so the pressure profile becomes flatter and the lateral can be longer; on steep downhill slopes the pressure may rise towards the end, and on uphill slopes lengths must be shorter. Where the sub-main lies across a slope, laterals are often paired: a shorter uphill lateral and a longer downhill lateral from the same sub-main point, located so that their end pressures are similar.
The sub-main feeds laterals at every row spacing, so it is also a multiple-outlet pipe and uses F. Its inlet pressure must be high enough that the lateral inlet at the far end of the sub-main still receives the required pressure. Tapered sub-mains, in which diameter steps down as flow drops, reduce cost; the design must keep velocity below erosion and surge limits and above the flushing requirement. Pressure regulators at sub-main inlets isolate each block from the variation in the main, and are standard on sloping land.
Sediment and biological growth settle in laterals and sub-mains. ASABE EP405 recommends that flushing produce a velocity of at least about 0.3 m/s at the ends of laterals, which requires the flush valve to open and the system pressure to be maintained while flushing; the sub-main and supply main must be sized to carry the extra flush flow. Flushing is done at start-up, regularly through the season (the frequency depends on water quality) and after any repair.
Emitter passages are small, commonly well under 2 mm, so particles that would pass unnoticed in a sprinkler system block drip emitters. Clogging is physical (sand, silt, clay, debris), chemical (precipitation of calcium or magnesium carbonate, iron and manganese oxides, and some fertilisers when mixed incompatibly) or biological (algae, bacterial slime, particularly iron and sulphur bacteria). Filtration removes particles; chemical and biological clogging are prevented by water treatment (acid, chlorine) and by good fertigation practice.
A widely cited water-quality guide for drip clogging hazard (Bucks and Nakayama) classes the hazard as minor, moderate or severe by thresholds: suspended solids under 50, 50–100 and over 100 mg/L; pH under 7, 7–8 and over 8; dissolved solids under 500, 500–2000 and over 2000 mg/L; manganese and iron each under 0.1, 0.1–1.5 and over 1.5 mg/L; hydrogen sulphide under 0.5, 0.5–2 and over 2 mg/L; and bacterial populations under 10,000, 10,000–50,000 and over 50,000 per millilitre. A water analysis before design is essential.
A hydrocyclone (centrifugal sand separator) spins the water so that dense particles such as sand are thrown to the wall and settle into a collection chamber; it removes sand heavier than water but is ineffective against organic matter and fine clay, and its head loss depends on flow, so it must be sized to its design flow. Media filters (sand or gravel filters) pass water down through a bed of graded crushed silica or basalt and trap particles in depth, making them the preferred filter for water with algae and organic load, typical of open canals and ponds; they are cleaned by backflushing, reversing the flow to lift and scour the bed. Screen filters trap particles on a woven or perforated screen and suit water with light inorganic loads or serve as secondary filters; they clog quickly with algae. Disc filters stack grooved plastic discs; water passes through the grooves, trapping particles on the surfaces and in depth, so they handle a mix of organic and inorganic load and can be backflushed automatically.
Filtration fineness is expressed as mesh (openings per linear inch) or in micrometres; for example, 120 mesh corresponds to openings of about 125 µm and 200 mesh to about 75 µm. A common rule of thumb is that the filter should remove particles several times smaller than the smallest emitter passage (fractions from about one-quarter to one-tenth of the passage are quoted in different sources), but the emitter manufacturer's filtration requirement governs. Filters are sized by flow per unit filtering area; media filters by surface loading rate, which varies with water quality. Undersized filters clog quickly, cause high head loss and need constant backflushing.
Filters are cleaned when the pressure difference across them rises by a set amount above the clean value; automatic systems use a differential pressure switch or a timer to trigger backflush. Media filters are installed in pairs or more so that one can be backflushed with filtered water from the others. Maintenance also includes periodic acid treatment to dissolve carbonate precipitates (lowering the water pH for a period, the target depending on the water chemistry), chlorination to control bacteria and algae (continuous low-concentration or periodic higher-concentration doses are both used, and rates vary with water quality and crop tolerance), and routine flushing of laterals and sub-mains. Chlorine is less effective at high pH, so acid and chlorine programmes are designed together, and chlorine and fertilisers must be injected separately where they react.
Fertigation is the application of dissolved fertiliser through the irrigation water. Under drip it places nutrients directly in the wetted root zone and lets the dose be split into many small applications matched to crop uptake, which reduces leaching and improves fertiliser use efficiency. Nitrogen and potassium are the nutrients most commonly fertigated because common sources are highly soluble. Phosphorus can be fertigated but reacts with calcium and magnesium in hard water to form precipitates; acid phosphorus sources are used and water chemistry is checked. Chemigation is the wider term that also covers pesticides and other chemicals; under many national laws it is permitted only for products whose label allows it and only with prescribed safety equipment.
Three families of injector are used. A fertiliser tank (bypass or pressure-differential tank) is connected across a small pressure drop in the main line; water flows through the tank and carries the dissolved fertiliser into the line. It is cheap and needs no power, but the concentration falls continuously as the tank is diluted: for a well-mixed tank the relative concentration after a volume V has passed is about e^(−V/Vt), where Vt is the tank volume, so about four tank volumes are needed to remove roughly 98% of the fertiliser. It suits quantity-based dosing, not constant concentration.
A venturi injector creates suction at a constriction and draws solution from an open tank. It has no moving parts but needs a substantial pressure difference across it, often obtained by placing it in a bypass with a booster or a valve; its suction rate is read from the maker's performance chart and varies with inlet and outlet pressure. Injection pumps (electric diaphragm or piston pumps, or hydraulically driven proportional pumps) deliver a controlled rate or a constant ratio to the irrigation flow and suit precise and automated fertigation.
The key calculation links the fertiliser dose to the injection rate. The mass of product per application is the nutrient rate × area ÷ nutrient fraction in the product. If that product is dissolved in a stock solution of known concentration, the stock volume is the mass divided by concentration, and the injection rate is that volume divided by the injection time. For example, 5 kg N/ha on 4 ha is 20 kg N; as urea (46% N) this is 43.5 kg of urea; dissolved at 0.3 kg urea per litre of stock, it makes 145 L; injected over 2 hours, the injection rate is 72.5 L/h. The resulting concentration in the irrigation water is the nutrient mass rate divided by the system flow: 10 kg N/h into 40 m³/h is 250 g/m³, i.e. 250 mg/L (250 ppm) of N.
Injection is normally started after the system has filled and pressurised, and stopped early enough that clean water flushes the fertiliser out of the laterals before the system shuts down; the flush time is at least the travel time from injector to the farthest emitter.
Calcium-containing fertilisers must not be mixed in the same stock tank with sulphates or phosphates, because calcium sulphate and calcium phosphate precipitate; separate tanks (an A tank and a B tank) are standard in greenhouse fertigation. A jar test (mixing the intended products at the intended concentration in the source water) reveals incompatibility. Stock solutions should not exceed the product's solubility at the lowest expected temperature, and dissolving some fertilisers, such as urea, is endothermic and cools the solution.
Backflow of fertiliser or chemicals into a well or public supply is a serious hazard. Standard chemigation safety equipment, which is legally required in some countries for chemicals whose labels permit chemigation, includes a check valve in the irrigation line upstream of the injection point, a vacuum-relief valve and a low-pressure drain between the check valve and the pump, a check valve on the injection line, a normally closed solenoid valve on the injection line, and an interlock that stops injection if the irrigation pump stops or pressure falls. Local rules differ; the designer must follow the regulations where the system is built.
Layout starts with a survey: field boundary, contours, crop rows, water source, power supply, and obstacles. Laterals are laid along the crop rows and, where possible, along the contour or gently downhill; sub-mains run across the slope or down the slope depending on the lateral direction; the mainline links the head unit to the sub-main inlets by the shortest practical route. The field is divided into blocks (zones, shifts) so that each block needs about the same flow and the pump operates near its best-efficiency point for every shift. The number of shifts is set by the ratio of total emitter flow to the available source or pump flow, and by the hours available per day: the total operating time per day is the run time multiplied by the number of shifts, and it must fit inside the available pumping hours with a margin for maintenance and power cuts.
The pump must deliver the system flow at the total dynamic head (TDH), the sum of: the static lift from the pumping water level to the pump (including drawdown for a well); the elevation difference from the pump to the highest or critical block; friction losses in suction and delivery pipes, fittings, mainline and sub-main to the critical block; losses in filters (at their dirty-condition limit, not clean), fertiliser injector, flow meter and valves; the required inlet pressure of the critical sub-main or lateral; and velocity head, usually small. The critical block is the one that needs the highest pressure at the pump; it is not always the farthest one.
Hydraulic power is P = ρ g Q H, which in convenient units is P (kW) = 9.81 × Q (m³/s) × H (m). Shaft power is hydraulic power divided by pump efficiency, and input power divides further by motor or drive efficiency. For 20 L/s at 45 m and 70% pump efficiency, shaft power is 9.81 × 0.020 × 45 / 0.70 = 12.6 kW.
A centrifugal pump is selected by plotting the system curve (required head versus flow, rising with the square of flow because friction varies roughly with flow squared) on the pump's head–flow curve; the operating point is their intersection. It should lie close to the best-efficiency point for all the shifts, which is easier when shifts have equal flows. For suction lift, the net positive suction head available at the site must exceed the pump's required NPSH with a margin, or cavitation damages the impeller. The affinity laws describe a change of speed: flow varies with speed, head with speed squared and power with speed cubed. Variable-frequency drives exploit these laws to match pump output to varying zone demands and to hold a constant pressure. Solar-powered pumps are now common in many countries; their output varies through the day, so drip scheduling and storage are designed around the solar profile.
Automation ranges from simple timers opening solenoid valves in sequence to controllers that measure applied volume, soil water status, weather or plant response. Time-based controllers open each valve for a set duration; volumetric controllers close the valve when a set volume has passed the meter, which compensates for pressure variation and partial clogging of filters. Hydraulically operated valves are opened and closed by small command tubes; electrically operated solenoid valves are usually normally closed so that the system shuts if power fails. Soil water sensors (tensiometers, capacitance and resistance sensors) or evapotranspiration estimates from a weather station can start or adjust irrigation. Automation adds alarms for low or high pressure, high flow (a burst pipe) and low flow (a closed valve or clogged filter), and it can command backflushing and fertiliser injection. Remote monitoring through mobile networks is now widespread. Whatever the level of automation, the controller is only as good as the calibration of sensors and the reliability of valves, and a field check of flow and pressure remains essential.
The cost of a pressurised system has a capital part (pump and power unit, head unit and filters, fertigation equipment, mains, sub-mains, laterals and emitters or sprinklers, valves, controllers, trenching and installation, and design) and an annual part (energy, labour, maintenance, replacement of short-lived items such as drip tape, filter media and seals, water charges, and chemicals for water treatment). Different components have different lives: buried PVC mains may last decades, heavy-walled dripline several seasons to many years, and thin drip tape often one or a few seasons. Micro-irrigation's capital cost per hectare is highest for wide-spaced crops that still need a lateral per row, lower where laterals serve several rows, and it rises steeply as crop row spacing narrows.
To compare alternatives on a yearly basis, each capital item is converted to an equivalent annual cost using the capital recovery factor, CRF = i(1 + i)^n / [(1 + i)^n − 1], where i is the interest (discount) rate and n the life in years. At 10% and 10 years, CRF = 0.1627, so a 100,000-unit investment costs 16,275 units a year. Items with different lives are annualised separately and added to the annual operating cost. Salvage value, if significant, is subtracted as a discounted amount.
Larger pipes cost more to buy but lose less head to friction, so the pump uses less energy for the whole life of the system. The economic pipe size minimises the sum of annualised pipe cost and annual energy cost. Because friction varies with about the 4.87th power of diameter in Hazen-Williams, stepping up one pipe size sharply reduces energy cost; the optimum shifts to larger pipes when energy is expensive, operating hours are long and interest rates are low. Sizing only by a velocity limit ignores this trade-off; a designer should at least compare two or three candidate sizes on annual cost.
Benefits include yield and quality increases, water savings (which may allow more area to be irrigated from the same source), energy savings from pumping less water, fertiliser savings through fertigation, and labour savings. Their size varies widely with crop, climate, water price and the method being replaced, so appraisal uses local data. Projects are compared by net present value, benefit–cost ratio, internal rate of return and payback period; farmers often focus on payback. Water savings at the field do not always become water savings for the basin, because part of what surface irrigation 'loses' returns to aquifers and streams; if saved water is used to expand irrigated area, total consumption may rise. Many governments subsidise micro-irrigation, in South and East Asia, North Africa and the Middle East and parts of Latin America among others; schemes commonly pay a share of the capital cost, often at higher rates for small and marginal farmers, with requirements such as certified components and approved designs; rates, ceilings and rules change and must be checked in the current guidelines of the country concerned.
A student must be able to draw a sprinkler and a drip layout with every component in the right order, and work the core numbers without hesitation: application rate from discharge and spacing, Christiansen's CU and low-quarter DU from a set of catch depths, lateral friction using Hazen-Williams with the F factor, inlet pressure with elevation and riser, the emitter exponent from two readings, emission uniformity, drip volume and run time per plant, and fertiliser injection rate. Practise unit conversions until they are automatic, because most lost marks in these papers are unit slips rather than wrong methods. Know which filter suits which water and why the secondary filter sits after the injector.
A designer or dealer engineer with five years' work spends most days on quotations and designs: taking the field survey, water analysis and crop data, choosing emitter or sprinkler and spacing, dividing the field into shifts, sizing laterals, sub-mains and mains, selecting filters and injector, computing TDH at the critical block and choosing a pump near its best-efficiency point, then producing a bill of quantities and subsidy paperwork. In the field, the same engineer commissions systems (flushing, pressure checks, flow checks) and diagnoses complaints, which are usually clogging, wrong pressure, undersized filters, or a pump working far from its design point. Measuring field EU or CU turns a complaint into a fix.
A senior engineer with twenty-five years' experience reviews designs for large projects and public schemes, sets company or agency design standards, and decides trade-offs: pressure-compensating versus cheaper emitters on sloping land, surface versus subsurface drip, pivots versus drip for field crops, grid versus solar pumping, and pipe sizes on life-cycle cost. The work includes quality control of components against national and ISO standards, specifying water-treatment programmes for difficult water, writing chemigation safety requirements, training installers, and auditing schemes in which systems have been installed but are not operated. Judgment about which systems farmers will actually maintain matters as much as hydraulics.
A researcher with long field experience works on the questions design rules leave open: emitter clogging mechanisms and treatment under specific water chemistry, the hydraulics of long laterals on complex terrain and their optimisation, uniformity–yield relations and the economic uniformity target, wetting pattern and root-zone modelling for drip and SDI, salinity management under micro-irrigation, fertigation scheduling against measured uptake, variable-rate sprinkler and pivot control, and whether field-level water savings translate into basin-level savings. The researcher designs replicated field and laboratory tests, uses standard test methods so results are comparable, and publishes the data needed to update the design tables practitioners rely on.
This manual does not yet give sprinkler spacing tables by wind speed, basic intake rate tables by soil, emitter clogging treatment doses (acid and chlorine), or head-loss tables for specific pipe classes, because those values differ between sources, products and countries and must come from the local standard or manufacturer. Subsurface drip design, landscape and turf irrigation, frost-protection design, centre-pivot nozzle packaging and end-gun hydraulics, water hammer calculation and detailed solar pump sizing are only mentioned. The lateral design uses the classic uniform-slope approximations; step-by-step numerical analysis of laterals on irregular terrain is not covered. Subsidy rules are described in principle only.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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