Drip irrigation scheduling for tomatoes in unheated greenhouses

During the last two decades, energy-saving solar greenhouses without heating systems, about 150-800 m² (width: 5-10 m, length: 30-80 m) in size, have been widely used in the North of China for vegetable production during cold seasons. The greenhouse is also suitable for use in other countries located in the temperate or warm-temperate zones, especially in the developing countries, because it is inexpensive with a simple structure, and does not need any additional energy for heating in winter. Drip irrigation has been recognized as a sensible method of supplying water inside the greenhouses, and a simple method for drip irrigation scheduling at low cost is urgently required. Using the water-balance method, the authors studied the water consumption of tomato (Lycopersicon esculentum Mill.) and the relationship between evapotranspiration and water surface evaporation measured with 20 cm pan. Research results show that there is no significant water flux at 0.75 m depth when soil water potential is kept higher than -20 kPa at 15 cm depth, and around -20 kPa at 60 cm depth. The accumulative value of evapotranspiration is approximately equal to the accumulative value of water surface evaporation measured using 20 cm pan. Further analysis found that 20 cm pan can be used to determine the water requirement of tomato drip irrigation scheduling in this type of greenhouse.