applied with the water; labour input is lower; crop damage from heat or frost can be
reduced; and the accumulation of salts in the soil is reduced.
Disadvantages—very high cost, to buy, to install and to run; a filtration system is also
normally necessary; over-watering can easily happen, leading to water runoff and
erosion; and the normal problems associated with operating and maintaining
sophisticated equipment.
- Drip (Emitter or Localised) Irrigation
A highly efficient system if the water supply is clean or can be cleaned (filtered). Small
plastic nozzles are installed at intervals, to coincide with plant spacing. Depending on
the conditions, from 1.2 to 4 litres per hour are delivered directly above the plant root
zone. The Leak hose is another form of localised irrigation, where a porous rubber hose
leaks water along its entire length. This product can withstand high pressure and ice, and
requires low pump capacity, but is usually not economical to run because it is haphazard
and has a high flow rate.
With drip irrigation the amount of soil which becomes wet is a function of the soil
texture; sandy soils become wet in a long, carrot-shaped section underground, about
The chart below shows the approximate ranges of wetted area per spot location, for
three soil types:
Soil type Area wetted (m^2 ) Diameter wetted (m)
Sandy (Coarse soil)
Loam (Medium soil)
Clay (Fine soil)0.45–1.84
1.84–2.16
2.16–3.36
0.76–1.53
1.53–2.75
2.75–4.27
The soil is wetted by a process known as capillarity, the movement of water in the soil
due to the attraction of soil for water. The rate of movement depends on the size of the
pore spaces in the soil, and the soil moisture content. The smaller the pore space, the
further (and slower) is the movement of water.
Other Irrigation Methods
For high value crops, or when the supply of water is very limited, special techniques can
be used:
- Drip Hose/Leak hose—see above.
- Bubblers—as with sprinklers, underground pipes serve a rigid pipe extending
above ground. The water comes out as it would from a tap, flowing in a continuous
stream from special brass or plastic nozzles that provide water to individual trees or
bunded areas. - Air-wetting Methods—used in greenhouses to provide a humid atmosphere. Used
for the propagation of plant cuttings, to raise young seedlings and sometimes with
soft fruit and high value vegetables.
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about 2.8–4.3 m in diameter.
0.8–1.5 m in diameter, while clay soils become wet in a shallower, onion-shaped section,