- Foggers & Misters—relatively high water pressure is needed, using very fine
nozzles located on a pipe network near to the plants. - Humidifiers—fans draw air across a greenhouse, passing through a wet membrane,
so that the whole greenhouse becomes humid. - Subsoil Irrigation—such as the Durwick or Cell Systems. An artificial field is
made with an impermeable membrane of PVC sheeting, butyl rubber or similar
material placed about one metre below the soil surface. Water—which should have
a low salt content—floods the area and is then drained away to be pumped again
and reused.
Information Required for the Design of an Irrigation System
In order to design an efficient irrigation system, as much as possible of the following
data should be collected:
- Climate and Geography—daily maximum and minimum mean temperatures,
rainfall, relative humidity and wind speed. The longitude, latitude and altitude may
also be useful information. - Soil Texture—this gives information on infiltration rates and water holding
capacity. The soil texture, or type, also influences to some extent the size of nozzles
used; smaller nozzles should be used on fine sands and silts to reduce the problem
of soil capping (crusting). - Soil Chemistry—a soil analysis to determine the following data can be made to
help decide the correct irrigation method and scheduling: pH, EC (Electrical
Conductivity), adjusted SAR (the relationship between sodium, calcium and
magnesium), ESP (Exchangeable Sodium Percentage), chlorides and sulphates. - Water Quality—pH, EC (or TDS—total dissolved salts/solids), adjusted SAR,
sodium, calcium, magnesium, chlorine, nitrogen, phosphorus, potassium,
carbonates, bicarbonates, sulphates and organic matter. If one or more of these
elements is present in excessive amounts, especially chlorides or sodium, the
balance of all nutrients in the soil may be upset and so make other essential
nutrients unavailable to plants. The Boron content in water can be a problem if it
exceeds 1ppm, but it is very expensive to analyse.
Overhead irrigation with water containing more than about 125ppm of sodium or
chloride may cause the leaves to burn and fall off the plants.
Calculating the Quantity of Water Required
As a very approximate guide, the following daily amounts are required for field crops—
figures are the maximum (“peak”) daily requirements, for the driest season:
Arid desert Semi-arid/
mediterraneanTemperate16 mm/m^2 /day* 8-10 mm/m^2 /day 3-4 mm/m^2 /day* 16 mm/m^2 /day = 16 litres/m^2 /day = 160,000 litres/ha/day