What is Subsurface Drip Irrigation?
Subsurface Drip Irrigation (SDI) is a way of irrigating crops through plastic tubes placed under the soil surface. The tubes have droplet emitters with even spacings, similar to the drip system located on the soil surface. There are different pipe arrangement variations, but the pipes are usually placed at a distance of 38" to 84" and at a depth of 6" to 10" below the soil surface. In the United States, SDI is widely used to irrigate annual crops; elsewhere, the irrigation system is popularly used for orchards and other perennials. SDI systems require intensive management and come with high initial costs of up to $1,000 /acre.
Using an SDI system provides high water-use efficiency and savings of 25-50% compared to flood irrigation. Using SDI also has a positive effect on the environment through reduced nitrate leakage in the lower soil layers, unlike other irrigation systems. Additionally, SDI systems can help increase fertilizer efficiency through fertigation, where nutrients are applied directly to the root zone. Hence, the optimizations that come along with SDI systems can end up leading to higher yields and reduced usage of herbicides.
History of Subsurface Drip Irrigation
Technological advances within the plastics industry have led to significant progress for irrigation systems. The most massive increase occurred in the period from 1981 to 1995 with the mass use of micro-irrigation methods which reached 2,471,053 acres. These methods, including SDI, have improved production and yield in scientific experiments for various crops. Phene et al. (1987), using high-frequency SDI and precise application of fertilizers, significantly increased tomatoes' yield. In 1996, Hutmacher et al. demonstrated an increase in yield in alfalfa production with an SDI system at a depth of 2.3 ft. Two studies (Smith et al., 1991; Ayars et al., 1998) demonstrated an increase in cotton yields with SDI.
SDI Systems: Materials, Costs, and Life Expectancy
On the market, you can find a large selection of tubes. The optimal amount of flow and the recommended distance between the droplet emitters largely depends on the type of crop being grown and the type of soil where the SDI system will be installed. Polyethylene tubes have built-in droppers that can be at distances of 4 to 24 inches, operating at 7-14 psi. Also, the tubes differ in the thickness of the walls. The wall thickness ranges from 0.01 inches to 0.02 inches, depending on the use of the system. In general, greater thickness means greater durability and increases the service life of the tubes. The investment to install the system can range from $2,000 to $4,000 per acre. The amount of costs largely depends on the choice of materials, the availability of a water source, water quality, filtration method, the type of soil, and the degree of automation of the system itself. The system's average lifespan is 12 to 15 years, although some systems with continuous maintenance have reached up to 20 years.
SDI Wetting pattern
SDI Related to Corn Production
With increasing interest in ethanol production from plant origin, intensive research is being done on how to increase the yield of corn production. Experience has shown that by performing irrigation operations in the correct time frame according to the plant's needs, the yield increases, and the amount of water used decreases. Smith and Riley (1992)  observed lower levels of corn earworm damage in crops with controlled irrigation. There was also lower production of aflatoxins by the plant in controlled irrigation, where drought stress was minimal.
Although irrigation costs were relatively low in the past, and the cornfields were mostly irrigated by center pivot or flood irrigation, the current situation is characterized by high irrigation costs and water shortages. For these reasons, SDI is a realistic and appropriate way to reduce the costs and amount of water used in corn production.
Research conducted in 2009 by E.D. Vories  explains that to achieve maximum corn yields using subsurface drip irrigation, the system needs to replace 60% of the estimated daily crop evapotranspiration (ET). Although the SDI system can save water and increase yields, additional observations are required to calculate the SDI system's feasibility for specific locations and whether it is the correct irrigation method to be used by the farmer.
Additionally, with the development of technology and agrometeorology, there is an improvement in the weather forecast for crop coefficient functions, which are calculated precisely for the field's location and can result in more efficient irrigation management.
 Smith, M.S., Riley, T.J., 1992. Direct and interactive effects of planting date, irrigation,
and corn earworm (Lepidoptera: Noctuidae) damage on aflatoxin production
in preharvest field corn. J. Econ. Entomol. 85, 998–1006.
 Vories E.D., Tacker P.L., Lancaster S.W., Glover R.E.
(2009) Subsurface drip irrigation of corn in the United States Mid-South
Agricultural Water Management, 96 (6), pp. 912-916.