2002 Volume 40 Issue 3 Pages 291-300
An accurate irrigation control system needs to be developed in order to improve the stability of high Brix tomato production under the moderate water stress by restricted root-zone hydroponics. In this paper, applicability of a simple evapolimeter devised to control irrigation based on evaporation rate (Ep) was investigated. The evaporimeter is composed of a combshaped water tank, which is assembled with polyvinyl chloride pipes (φ20 mm), and an evaporating disk made of φ200 mm non-woven fabric sheet. In this evaporimeter, Ep is evaluated as the difference of water levels in the water tank measured by an ultra sonic level sensor with the precision of 0.1 mm. The irrigation control system attached with this evaporimeter starts irrigation when the cumulative evaporation (ΣEp) exceeds the irrigation point (IP) of ΣEp during the irrigation period in the day and thus the higher IPs result in longer intervals of irrigation per ΣEp. Transpiration rate of tomato plants at the 6th leaf stage was more closely correlated with Ep than with photosynthetic photon flux density. A high positive correlation between the amount of irrigation and cumulative evapotranspiration was evident on a daily basis, except the early stages of cultivation. In tomato production with this irrigation control system, more than 9% Brix fruits were successfully harvested when the IP was adjusted to an appropriate level to optimize the water stress of plants.
