Sprinkler and drip, explained

Sprinkler and micro-irrigation are the two families of pressurised irrigation. In both, a pump or a raised tank pushes water through closed pipes, and the water leaves through a device, a sprinkler nozzle, a dripper or a micro-sprinkler, whose flow depends on the pressure at that device. That single fact explains almost every design rule in the subject. If pressure varies too much along the pipes, flow varies, the crop is watered unevenly, and the farmer must either over-irrigate to satisfy the driest spot or accept lost yield where water falls short.

The subject sits between fluid mechanics and agronomy. The engineer starts from the crop's water need and the soil's intake rate, adds pipe friction and pump curves, and turns them into spacings, pipe sizes, zones, filters, fertiliser injection and a pump that runs efficiently.

Sprinklers: types and spacing

A sprinkler system has a pump, a mainline, sub-mains, laterals, risers and sprinklers, with valves and regulators between. Permanent and solid-set systems cost more in pipe but little in labour; hand-move systems are the reverse. A centre pivot waters a circle covering about 78.5 per cent of the square around it, and because the outer spans travel furthest, discharge and application rate rise towards the end. Pressure matters at the nozzle: too low gives big drops and a doughnut-shaped pattern, too high gives mist, drift and evaporation. Sprinklers are usually spaced at about one-half to two-thirds of their wetted diameter, closer in wind.

How even is even?

Uniformity is measured by setting out a grid of catch cans and comparing what each collects. Christiansen's uniformity coefficient, from 1942, compares each catch with the average; the newer distribution uniformity compares the driest quarter with the average. Wind, wrong pressure, worn nozzles and bent risers are the common culprits when uniformity is poor.

Designing the pipes

Christiansen's classic rule keeps pressure variation along a sprinkler lateral within 20 per cent of operating pressure, which holds discharge variation to about 10 per cent. Friction in a pipe with many outlets is less than in a plain pipe, and the manual shows how the Christiansen F factor accounts for that, and how slope adds to or subtracts from the pressure needed at the inlet.

Drip: water near each plant

Micro-irrigation applies water slowly to part of the soil near each plant: drippers, micro-sprinklers, bubblers and subsurface drip. The head unit runs in a set order, and a screen or disc filter always sits downstream of the fertiliser injector. Laterals are polyethylene, with flush valves at the ends. Emitters range from long-path and labyrinth types to pressure-compensating ones, which hold their flow nearly constant as pressure changes.

Every emitter also has a small manufacturing variation, so designers combine that variation with pressure differences to set a target emission uniformity for each block. Run time then follows simply: the daily water each plant needs, divided by the number of emitters per plant and their discharge.

In pressurised irrigation, the filter decides success.

Filtration and water quality

Clogging can be physical, chemical or biological. Filters deal with the physical part; water treatment deals with the rest. Hydrocyclones remove sand, media filters handle algae and organic loads, screens handle light inorganic dirt, and disc filters handle a mix. The filter should be several times finer than the smallest emitter passage, and filters are backflushed when the pressure drop across them reaches a set value. Acid dissolves carbonate scale; chlorine controls slime and algae.

Fertigation, safely

Fertigation feeds dissolved nutrients, mainly nitrogen and potassium, through the water in small doses. Injectors range from simple fertiliser tanks to venturis with no moving parts and precise injection pumps. Two safety rules stand out: never mix calcium with sulphates or phosphates in one stock tank (do a jar test), and always fit backflow protection so chemicals can never flow back into the water source.

Pumps, automation and money

The field is split into shifts of roughly equal flow so the pump runs near its best efficiency. By the affinity laws, flow rises with pump speed, head with its square and power with its cube, so a small change in speed makes a big difference to energy use. Normally closed solenoid valves fail safe. Finally, the economics: each component is annualised over its own life, pipe size is chosen to minimise pipe plus energy cost, and the manual adds an honest warning: water saved in the field is not always water saved in the river basin.