Factors influencing the irrigation efficiency of flue-cured tobacco grown in north Queensland, Australia
A three-year study was begun in 1988 using commercial size (10-15 ha) flue-cured tobacco ( Nicotiana tabacum L.) plantings within the Mareeba-Dimbulah Irrigation Area (MDIA) of north Queensland, Australia. This study aimed to determine the water requirements of tobacco, and to evaluate and increase the efficiency of commercial irrigation systems. A survey of commercial farms showed a large variation in water use. Fifty percent of the systems had sub-optimal distribution patterns, resulting in uneven crop development, uneven maturity, and variable leaf quality. Efficiency was enhanced when irrigation systems were operated at or near 310 kPa, with pipe runs less than 240 m, and with sprinklers arranged opposite rather than staggered from one line to the next. Tobacco plants extracted moisture down to at least one meter. The majority of soils used for tobacco in the MDIA hold 100-120 mm of available water in the top meter of soil. Although the Neutron Moisture Meter was the most accurate method for determining crop water use and water uptake patterns, the water budget procedure was found to be a less expensive and easier method of scheduling irrigation. The crop factors (Kc) determined in this study can be used in conjunction with daily evaporation data to estimate water requirements at different stages of development. Kc varied with planting time and with the age of the crop, and in early plantings it can be influenced by weather conditions affecting plant growth. In early plantings, Kc started at 0.20 in week one, increased to 0.59 by week five, and peaked at 0.85 in week 12. In later plantings it reached a maximum of 0.95. Crop canopy measurements can be used to adjust the crop factor in seasons when crop development is slowed by adverse weather conditions. Guidelines for using the water budget method are outlined, together with a recommended irrigation program for growing tobacco in winter and spring in the MDIA.