Drip and sprinkler irrigation

Approximate application efficiency — the share of the water that ends up in the root zone — from FAO (Phocaides, 2007, Table 6.7):

• Surface irrigation from earth canals 40–50 %, lined canals 50–60 %, pressure pipes 65–75 %.
• Hose irrigation 70–80 %.
• Low- to medium-pressure sprinklers 75 %.
• Micro-sprinklers and mini-sprinklers 75–85 %.
• Drip 80–90 %.

Drip suits row crops, vegetables, vines and trees, and places where water or pumping is dear. Sprinklers suit close-sown crops and pasture that drip cannot reach plant by plant, on soils that take water in quickly. Both need clean water, steady pressure and someone to maintain them.

The crop's water use is ETc = ET₀ × Kc (FAO handbook, chapter 6, which tabulates Kc for seasonal and permanent crops): ET₀ is the reference evapotranspiration for your weather (from a pan or a weather station), and Kc the crop coefficient for its stage of growth, low at planting and highest at full cover.

The gross dose is the net dose stored in the root zone divided by the application efficiency. FAO's worked example: a net dose of 19.8 mm on 1 ha is 198 m³; if 280 m³ had to be delivered, the efficiency was 198 ÷ 280 = 0.70.

Where water or soil is salty, add 10–15 % to the dose so that a little water drains below the roots and carries the salts with it (FAO, leaching requirement).

A tensiometer — a water-filled tube with a porous ceramic tip and a vacuum gauge — reads how hard the soil holds its water, in centibars (FAO, Phocaides):

• Install them in pairs next to the plants, one at 30 % and one at 60 % of the effective root depth, with good contact with the soil.
• Below 10 centibars: saturated soil. 20–40: water easily available. Above 55: danger of severe stress.
• At field capacity the reading is about 10 in sandy soil, 20 in medium soil and 30 in clay.
• Start irrigating cash crops when the shallow tensiometer reads 18–25, depending on the soil and the crop's stage. The deep one shows whether the dose reached the bottom of the roots.

Drippers release water at 1–24 litres an hour; a common on-line dripper gives 4 L/h at 1.0 bar (FAO). One bar is the pressure of about 10 m of water, so a tank raised a few metres feeds only low-pressure drippers or drip tape made for it.

Uniformity rule: the flow of any two emitters in operation should not differ by more than 10 %, so the pressure between them must not differ by more than 20 %, so the friction loss along a lateral must stay below 20 % of the operating pressure (FAO, after Christiansen). Long or thin laterals break this rule: shorten them, feed them from both ends or the middle, or use a larger pipe.

Space drippers along the line to the crop and to the soil: closer in sand, where water spreads little sideways, wider in clay.

Every emitter needs clean water; the filter is sized by the emitter (FAO):

• Drippers: 80–160 mesh, 200–100 µm.
• Mini- and micro-sprinklers: 60–100 mesh, 250–120 µm.
• Slow-rotation impact sprinklers: 16–25 mesh, 1,000–500 µm.

The head control, in order from the supply (FAO): an air release valve, a check valve that lets water flow one way only, outlets for the fertilizer injector, and the filter; a sand separator or gravel filter first where the water carries sand or algae. Clean the filter before it chokes: a rising pressure difference across it is the sign.

Flush the network at least three times in a season when the water comes from underground, and once every fourth irrigation with reservoir water: open the ends of the mains and laterals and run each line for about 2–3 minutes (FAO).

Against slime and algae, chlorinate with sodium hypochlorite, injected continuously at about 5 ppm or intermittently at about 10 ppm, so that 1.0–2.0 ppm of free chlorine remains at the end of the line; use made hypochlorite when bleach runs out. Against scale, inject hydrochloric, sulphuric or phosphoric acid to bring the water below pH 6. Algae in a reservoir: copper sulphate at no more than 2 ppm.

Set sprinklers on a rectangle or triangle no more than 60 % of their wetted diameter apart, so each patch of ground is reached by more than one (FAO). Small sprinklers stand on risers about 60 cm high.

Apply water no faster than the soil can take it in: an application rate above the soil's intake rate runs off the surface and leaves the subsoil dry. Check after an irrigation by digging: the wetted depth should reach the bottom of the roots, with no puddles or runoff.

Catch test: run the system at its working pressure, set a cup under 10 drippers spread from the near to the far end of the longest lateral, collect for a timed 5 minutes, and measure each in a graduated cylinder. Work out each flow in L/h and the average.

• All within 10 % of the average: the line is even.
• The far end low: too much friction loss; see entry 4.
• Scattered low drippers: blockage; flush and treat the line (entry 6), then replace drippers that stay low.