7.7 Irrigation Methods

Various irrigation methods exist, from manual watering with a can to highly automated systems like centre-pivot irrigation. However, these methods can be categorised into five main groups based on the application method:

• Flood irrigation: Water is spread across the entire field to seep into the soil (e.g., wild flooding, contour flooding, borders, basins, etc.).
• Furrow irrigation: Water is applied between ridges, allowing it to reach the concentrated root zone through capillary action (e.g., level and graded furrows, contour furrows, corrugations, etc.).
• Sprinkler irrigation: Water is sprayed, mimicking rain, and reaches the soil (e.g., portable and solid set sprinklers, travelling sprinklers, spray guns, centre-pivot systems, etc.). The application rate is controlled to prevent surface water pooling.
• Sub-irrigation: Water is applied beneath the root zone, wetting it through capillary rise (e.g., subsurface irrigation canals, buried pipes, etc., using deep surface canals or buried pipes).
• Localised irrigation: Water is applied around individual plants or groups to moisten the root zone (e.g., drip irrigation, bubblers, micro-sprinklers, etc.). Application rates are adjusted to minimise percolation losses while meeting evapotranspiration needs.

Flood Irrigation: Widely-spaced Border Irrigation

Widely-spaced border irrigation is particularly suitable for areas with favourable topographical conditions. It works best on land that can be graded to slopes of less than 1%, preferably around 0.2%. Minimal grade changes are preferred, and reverse grades should be avoided. This technique is especially well-suited for deep-rooted, closely spaced crops, and orchards. Typically, the irrigation techniques achieve a water application efficiency of 45% to 60%.

Furrow Irrigation: Graded Contour Furrows

Graded contour furrows are particularly well-suited for row crops on steep terrain, although erosion is risky during heavy rainfall. They are unsuitable for fields with rodent infestations or soils that crack excessively. The optimal grade in the direction of irrigation is between 0.5% and 1.5%. No additional grading is needed beyond filling gullies and smoothing abrupt ridges. This irrigation method typically achieves 50% to 65% water application efficiency. It is suitable for both row crops and fruit cultivation, and it can be applied on varying land slopes ranging from 2% to 25%, although slopes less are preferable.

Furrow Irrigation: Rectangular Level Irrigation

Rectangular level irrigation is another type of furrow irrigation well-suited for orchards. This technique requires land that can be graded to allow single or multiple tree basins to be levelled within 6 cm. It benefits soils with varying water intake rates, relatively high or low. However, this method may require significant grading efforts. Water application efficiency typically ranges from 40% to 60%.

Sprinkler Irrigation

Sprinkler irrigation is suitable for undulating slopes ranging from 1% to 35%. It can be used for all types of crops. However, it is associated with high operation and maintenance costs and is only recommended for regions with low power costs and strong markets. It’s also well-suited for high rainfall areas where a small additional water supply is required. The water application efficiency for sprinkler irrigation typically ranges from 60% to 70%. It also brings potential hazards from aerosols during the spray irrigation process.

Sub-irrigation

Sub-irrigation is best suited for areas with minimal slopes, highly permeable subsoil conditions, and a sufficiently high groundwater table. The system requires precise levelling. Therefore, only a limited number of locations are well-suited for this type of irrigation. Sub-irrigation typically achieves a water application efficiency of 50% to 70%. The irrigation technique is suitable for shallow-rooted plants.

Localised Irrigation

Localised irrigation is suitable for a wide range of topographic conditions. The method involves applying wastewater through perforated pipes on the soil surface or subsurface, enabling precise delivery at the base of individual plants or around fruit trees. It typically achieves a high-water application efficiency of 70% to 85%. This irrigation technique is well-suited for row crops and fruit trees.

As shown in the preceding module, wastewater quality requirements are influenced by the chosen irrigation method. When using localised irrigation for category B crops mentioned in topic 7.5 (crops eaten cooked, cereals, industrial crops, fodder, pasture, and trees), particularly for high-growing crops where the edible part is not in contact with the soil, and there is no exposure to workers or the public, there may be no need to meet specified quality targets for helminths and E. coli according to WHO and therefore no need for frequent monitoring.

However, because of the high risk of clogging, certain effluent quality standards are required to be achieved, as can be seen in the table presented below.

Parameter	Clogging hazard for drip irrigation
	Minor	Moderate	Severe
TSS (mg/L)	< 50	50 – 100	>100
pH	< 7	7 – 8	>8
TDS (mg/L)	<500	500 – 2,000	>2,000
Mn (mg/L)	<0.1	0.1 – 1.5	>1.5
Fe (mg/L)	<0.2	0.2 – 1.5	>1.5
H2S (mg/L)	<0.2	0.2 – 2.0	>2.0

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