Preliminary requirements to design a suitable drip irrigation system for coconut ( Cocos nucifera L.) in gravelly soils

Field experiments were conducted in drought-prone gravelly soils of the Andigama series (Red Yellow Podzolic) to investigate the effective root zone of coconut ( Cocos nucifera L.) for water absorption, the optimum wetting pattern of soils with respect to flow rates of drippers, the number of drippers per tree, the volume of irrigation water and the frequency of application, and the placement of drippers in the effective root zone. Root distribution studies showed that 75–80% of coconut roots were concentrated at a depth ranging from 20 cm to 100 cm with about 5% roots beyond 100 cm and 15–20% of the roots in the top soil layer (0–20 cm). Moisture stored in the 20–120 cm depth range was found to be highly extractable by the coconut tree. Roots at a distance of 50–100 cm away from the base of the tree were responsible for most of the water absorption, and the highest moisture extraction was observed at 100 cm distance. Irrigation through four drippers placed equidistant in the circumference of a circle of radius 100 cm around the base of the tree and discharging water at the rate of 30 l/h for 2.5 h wetted a large volume of soil in the effective root zone. The evapotranspiration rate of 15-year-old coconut trees (cv. CRIC 60) during the dry period was 2.52±1.12 mm/day and therefore, irrigation frequency for coconut grown in this soil series was determined to be 8 days. A drip irrigation system for adult coconut trees could be accordingly designed, based on these characteristics.