Subsurface Drip Irrigation

Subsurface drip is a highly efficient irrigation system that uses buried drip tubes or drip tape to meet crop water needs. Since the water is applied below the soil surface (as opposed to surface irrigation or traditional drip irrigation), the effects of surface infiltration, such as crusting, saturated condition of ponding water, and water losses via evaporation and surface runoff (including soil erosion) are eliminated. With an appropriately sized and well-maintained subsurface drip irrigation system, water application is highly efficient and uniform.

Wetting occurs around the tube and water moves out in all directions. Moreover, water is applied directly to the root zone of the crop as opposed to the soil surface where most weed seeds hibernate. As a result, germination of annual weed is reduced. This lowers the pressure on valuable crops. Furthermore, some crops may benefit from the additional heat provided by dry surface conditions, and produce more biomass. When managed properly the application of fertiliser can be optimised. Fields can still be worked when irrigation systems are installed.

A subsurface drip irrigation system has a similar design as a common drip irrigation system. A typical system layout consists of a settling pond (where possible), pumping unit, pressure relief valve, check valve or back flow prevention valves, hydrocyclone separator (if a settling pond is not feasible), chemical/fertiliser injection unit (see also fertigation and nutrient requirements), filtration unit equipped with back flush valves, pressure regulators, air vent valves and PVC pipes delivering the water to the crop. The piping is 10 to 60 cm below the ground, depending on crop and soil (capillary attraction). As a water source, treated greywater or even blackwater is possible, with the risk of clogging being greater if the influent flow has not properly settled. Therefore, treatment of the water (e.g. a non-planted filter system, constructed wetlands (horizontal flow or vertical flow) or at least a septic tank) before the settling pond is necessary.

Subsurface drip irrigation is generally a high-tech, automatically operated technology applied to medium and large scale production (see also automatic irrigation). However, several low-cost and simple methods of subsurface (drip) irrigation like pitcher or bottle irrigation (see manual irrigation) exist that are equally effective for small-scale farming. There are several subsurface techniques used for secondary wastewater treatment such as leach field or evapotranspiration bed that also provide uncontrolled irrigation to fields.

REICH et al. (2009) estimates that investment costs of a subsurface drip irrigation system are high (1000 to 2000 US$ per acre). The costs vary depending on water source, quality, filtration needs, choice of material, soil characteristics and degree of automation (see also automatic irrigation). Normal life expectancy is between 12 and 15 years. With good maintenance and high water quality the system can be used even longer.

The performance and life of any system depends on how well it is designed, operated and maintained. It should either be automatically controlled or regularly inspected. Repairing the buried pipes is difficult, cumbersome and time-consuming. To prevent rodents from chewing the pipes, precautionary measures should be undertaken. The mechanical components such as pumps, valves and filters need to be maintained as well as checked and cleaned regularly.

The filter system of a large-scale subsurface drip irrigation facility is complex and its proper functioning is crucial to the whole system. The maintenance of the three main filters of the system (this contains centrifugal separators, screen and disk filters as well as sand media filters) should be carried out carefully (e.g. backwashing of sand media filter). Systems must be designed so that mainlines, sub-mains, manifolds and laterals can all be flushed. Mainlines, sub-mains and manifolds are flushed with a valve installed at the very end of each line. Lateral lines can be flushed manually or automatically. It is important to flush the lines at least every 2 weeks during the growing season (ENCISO et al. 2010).

(Adapted from TILLEY et al. 2014)

If wastewater that is not properly pre-treated (i.e. inadequate pathogen reduction) is used for irrigation health risks may arise. Appropriate pre-treatment should precede any irrigation scheme to limit health risks for those coming into contact with the water (see also waterborne diseases pathogens and contaminants). However health risks stemming from subsurface drip irrigation are considerably reduced because the irrigation water is discharged into the root zone and direct contact with crops and labourers is thus prevented.

Moreover, chemicals that are introduced into the system may contaminate the water. When effluent is used for irrigation, households and industries connected to the system should be made aware of the products that are not appropriate for discharging into the system (see also water pollution).