Subsurface Drip Irrigation Systems

Subsurface drip irrigation, or SDI, is ordinary drip irrigation with one difference: the emitter line is buried in the root zone instead of lying on the surface. Water reaches the roots directly and the soil surface stays dry. The result is that most surface evaporation disappears, weed growth falls, and field operations can be carried out without working around irrigation lines. In exchange, it is a system you cannot diagnose by eye — which is why its design and maintenance are held to a stricter standard.

Network structure and where the PE pipe sits

The network has three layers. The main line runs from well or reservoir to the control and filtration station and carries the highest flow and pressure. The manifold or submain distributes between field blocks. The emitter laterals are buried. Pressure-rated polyethylene pipe makes up the first two layers; the third is a separate product. The point that is often understood too late is that the durability and uniformity of the whole system are set by those first two layers, not by the emitter.

Burial depth

Lateral burial depth is determined by crop rooting, soil texture and tillage practice. On row crops a range of roughly 20 to 40 cm is usually chosen. Shallower than that exposes the line to damage from implements and brings moisture to the surface; deeper puts water below the root zone of young plants and disrupts germination and establishment. The final figure has to come from an irrigation specialist familiar with the crop and the region.

The three things that kill an SDI system

In surface drip you can see a blockage. In SDI the first symptom is usually a yellow patch in the field — by which time the damage is done. The three main failure mechanisms are:

• Root intrusion into the emitter passage — roots grow toward moisture and block it
• Soil suction when the system shuts down — as pressure falls the line tends to empty, and instead of air it draws soil and silt in through the emitters
• Chemical and biological scaling — calcium carbonate, iron and microbial slime progressively narrowing the passage

The answers belong in design, not in operation

• Root-resistant emitters, or a periodic injection programme per the emitter manufacturer's recommendation
• Air/vacuum valves on the manifold and at high points — the component that prevents soil being drawn into the emitters, and the cheapest insurance this system has
• A flush manifold at the ends of the laterals, so the line can be flushed at velocity periodically; without one, accumulated particles have nowhere to leave
• A filtration station matched to the water source: screen or disc for clean water, sand for water carrying algae and organic matter
• Pressure-compensating emitters on sloping ground or long laterals
• Pressure gauges at the head and tail of each block — a change in the differential is the first sign of clogging, and appears before the field goes yellow

Distribution uniformity is the real measure of success

In SDI you cannot assess uniformity by looking, so it has to be guaranteed in design. The conventional criterion is that pressure variation between the head and the tail of an irrigation block should not exceed about 20 per cent of emitter operating pressure. Meeting it usually makes the manifold and main larger than flow alone would have required — and this is exactly where saving on main-line diameter reappears later as an uneven crop.

Where it makes sense

SDI carries a higher capital cost and expects more disciplined maintenance. It earns its place where the planting is perennial or the crop is high-value, where water is expensive or limited, where strong wind undermines the efficiency of sprinkler irrigation, or where you want to carry out field operations and harvest without lifting irrigation lines. For a low-value annual crop with abundant water, it usually does not pay.